REF: Engine Mechanicals

Evo Crankcase Pressure and Engine Breathing

See also in the REF section of the Sportsterpedia:

Crankcase Pressure

In the crankcase air system;
Crankcase air pressure is mainly generated by the up and down movement of the pistons.
Additional air pressure is created by blowby from the combustion chambers past the rings and into the crankcase.

It helps to push sump oil up and out the scavenge passage to the return side of the oil pump.
(the scavenge side of the pump also pulls a vacuum on the sealed passage from the sump outlet to the pump) 1)
It also initiates the splash and mist process as the compressed air above the oil is ready to spring up when the piston rises.

  • Using a 1000cc motor: 2)
  • Consider the pistons as they stroke up and down in the cylinders.
    • It's the down action that draws fresh intake charge into the cylinders, where it burns propelling you down the road.
    • Then the pistons expel the waste as exhaust when they travel up.
    • We all know this. 500cc is what each piston 'sweeps'.
    • This all happens on the topside of the pistons.
  • That same 500 each (1000 total) is also getting displaced on the underside of the pistons, compressing it into the crankcase.
    Instead of into the cylinder head.

Intake and exhaust valves in the cases:

  • The piston up and down movement in the cases get pressurized and then 'relaxed'. 3)
    (like if you blow into a balloon it pressurizes then breathe in and it relaxes)
  • If you take the timing plug out of a running engine, this opens the crankcase to the atmosphere.
    • Then you'll get 1000cc of air/oil mixture blowing out of it followed by 1000cc getting sucked back in.
  • On 77 and up motors, there are holes through thru the wall separating the crank case from the cam case.
    • These holes are the same as 'opening the timing plug'.
  • It's the blowing out oil-air that we should address 1st. 4)
    • The oil that's getting blown out is oil that got pumped to the big end bearings through the pinion shaft.
      (and from the piston squirters on 04 and up models)
    • The oil did it's thing for the crankpin bearings (and piston lowers) and then flowed out into the case where it mixes with the air in case.
    • Mixing isn't really the right term.
      Suspended is.
    • The oil is suspended in the air just like rain water is suspended in air in the wake of a tractor trailer in the rain.
      The faster the tractor trailer drives, the better the suspension.
      Or, the higher the engine rpm the better the oil suspension.
  • The drain down oil from the top end (and oil squirters respectively) is also introduced into the case air but its only a fraction of what came from the crankpin.
    (and piston squirters respectively).
  • This used oil needs to be evacuated (to the oil tank) so that the case doesn't fill up with oil.
  • The downstroke of the pistons causes the volume underneath the pistons to decrease which puts positive pressure in the lower end chambers. 5)
    • This also puts positive pressure on the oil in the sump.
    • This pressure is multi use;
  • Then the upstroke of the piston creates an upward negative (vacuum) pressure bringing some of the oil from the sump with it (suspended).
  • With little to no (piston ring) blow-by and a check valve on the breather system;
    • Crankcase pressure is essentially cycling between (positive and negative pressures) as the pistons go down and back up.
      (remember, due to the common crankpin 45 degree design, a Harley motor is a variable volume crankcase, unlike most motors) 6)

This creates splash oil which is bounced about in the crankcase.
This also creates an air / oil mix when tiny particles intertwine with the oil in suspension.
The two don't actually mix as does sugar and water.
So separating them back apart is fairly easy if you add an obstacle for that 'mix' to collide into.
The obstacle is widely known as the breather or umbrella valve although anything the mix touches in the motor could accomplish the same thing in theory.

  • Instead of the oil-air getting blown into the garage, HD blows & sucks it into the cam chest where it accumulates on the walls of this smallish area. 7)
    • Imagine the tractor trailer going into a tunnel.
      That suspended rain water will collect on the walls of the tunnel and flow down into the storm drain.
    • Or, in our case, to the return side of oil pump and then back to oil tank.

The piston motions create a pulsating blast of air pressure (push pull condition as each piston rises and falls).
Static oil pump pressure has already been dissipated by the time it reaches the crankcase.
(although it takes static oil pressure to get the oil from the pump to the crankcase)

Likewise, crankcase (CC) pressure will have a constant change in velocity.
Oil in the crankcase adds resistance to the air pressure generated (raising the pressure).
The movement of the pistons and flywheels splash oil around in the engine.
Gravity oil (from the drain ports in the heads) returns to the crankcase or gearcase (respective to year model);

  • On 86-03 engines, gravity oil falls into the crankcase sump area.
  • On 04 and up engines, gravity oil falls into the gearcase.

CC pressure both pushes to and sucks from the breather valve.
So testing full CC pressure with a PSI gauge would probably just destroy it.

The volume between positive and negative pressure decreases as RPM goes up.

Affects of the 45° Rod / Piston Arrangement

Piston / rod positions are relative to positive or negative pressure in the crankcase. Forget about valve opening for a second.

  1. With the piston up,
    • The top of the piston is not pulling vacuum in the cylinder.
    • The crankcase is pulling vacuum.
  2. With the piston down,
    • The piston is pulling vacuum in the cylinder.
    • The crankcase is exerting positive pressure.

These two conditions create what we call crankcase pressure.
How crankcase pressure moves inside the engine does or doesn't help during wet sumping.

Intake vacuum is relative to piston / rod positions.
On carbed models with VOES / MAP:
Vacuum is created in the cylinders which pulls vacuum from the carb.
This vacuum is tapped into and used to operate ignition timing through the VOES / Map.
So a vacuum gauge plugged into the vacuum line at the carb does reflect the position of the two pistons.
I.E. the gauge moves when the pistons are on the way down and goes toward the resting position when the pistons are coming up.
(of course, you'd then have to take in accountancy if the intake or exhaust valve was open at the time)

Some time and tune older V-8s with a vacuum gauge instead of a timing light.
For each piston going up, there is a piston going down.
Intake vacuum stays (more) constant in the middle of the push / pull.
A vacuum gauge shows a more steady dynamic condition.
Likewise are the forces in the crankcase (crankcase pressure).

The rods \ piston action in the Sportster engine are close together (45° apart).
There are 360° around the flywheels.
If the rods were 180° apart instead, one piston would be going up at the same time the other is going down.
Just as in a V-8, crankcase pressure would be more equalized between them.

Since Sportster piston movement is not equalized, we get the potato, potato sound we all love but the equilibrium in the crankcase is off by design.
Take the PCV off the valve cover on the V-8 while running and it just smokes a little.
Take the breather valve off the Sportster and oil pukes out.

Likewise, a vacuum gauge on the carb line will be erratic and not a useful test. Hook up a vacuum gauge to it and the gauge bounces from vacuum to no vacuum pretty wildly. Crankcase pressure is doing the same thing.

Differential Pressure

Vacuum and (positive) pressure are the terms that describe the amount of molecules of a gas in a given unit of space. 10)
More molecules inside the engine than outside = inside air pressure.
Less molecules inside the engine than outside = inside is vacuum pressure.

Image two scenarios:

  1. The case is sealed (closed to atmosphere).
    • The pistons just compress and relax the fluid in the case.
  2. The case has a huge passage that allows the fluid to pass into and from the atmosphere.

Which takes more power to cycle?
Possibly the second.
Because the 1st, as the pistons use power to compress the fluid;
That power is returned as the pistons rise from the stored energy in the compressed fluid pushing pistons up.

That may be what's really happening for the most part in our bikes.
Obviously, the breather opening is too restrictive to allow so much flow that the case pressure stays more constant.
That restriction is on purpose to lessen the pumping losses by being closer to #1 than to #2.

As the vacuum increases, the pumping losses decrease and the fluid is less dense. 11)
On the flip side, less dense fluid can't suspend as much liquid (possibly resulting in more liquid drag).

Below is some terminology for vacuum measurements. 12)
PSIG - (pounds per square inch (gauge):
Gauge pressure is pressure measured relative to ambient atmospheric pressure (approximately 14.7 PSIA).
PSIA - pounds per square inch (absolute):
Absolute pressure is measured relative to high vacuum (0 PSIA).
PSIV - pounds per square inch (vacuum):
Vacuum pressure is measured relative to ambient atmospheric pressure.
PSID - pounds per square inch (differential):
Differential pressure is pressure measured relative to a reference pressure.
If the reference pressure is one atmosphere the differential pressure range is equal to gauge pressure range.

The earth's atmosphere exerts a pressure upon us, known as the atmospheric pressure, which can be measured in a number of ways. 13)
At sea level, the standard pressure is 14.7 psia or 29.92“ of mercury (Hg) or 760 mm of mercury (Torr).
Because the barometric pressure varies, the above “sea level” pressures are used as a reference point.
There is 14.7 psia pressure being exerted on us by the atmosphere, but there is also 14.7 psia inside of us pushing out.
(given the fact that for every action there is an equal but opposite reaction)
Thus, we do not feel discomfort from the atmospheric pressure.
Another way to state this is that there is no differential pressure between the inside and outside of our bodies.

The term “vacuum” is used to describe the zone of pressure below atmospheric pressure.
Vacuum is a negative gauge pressure, usually referenced to the existing standard barometric pressure where the equipment will operate.
This means vacuum is a differential reading between the surrounding atmospheric pressure and the pressure in the system evacuated.
In all instances when given a vacuum condition, the question should be asked, at what elevation the pump will operate.
(since the barometric pressure varies with altitude above or below sea level)

Example of differential pressures (or Vacuum): 14)

Differential pressure as it applies to the Sportster breathing system:

Applying the same principles, you can see the relation of positive and negative crankcase pressure in the Sportster engine.
Every piston upstroke and downstroke reverses crankcase pressure from positive to negative forces.
The pistons act as an air pump and then a suction pump respectively.
Below are drawings exampling positive and negative (vacuum) forces inside the engine.

86-90 models with cam chest breather vents:
16) 17)

91-97 models with head breather vents:

98-03 models with head breather vents:

04 and Up models with head breather vents:
18) 19)

The breather valve is necessary to keep the imbalance in the crankcase (from the 45° arrangement) at bay.
The umbrella closes when the upstroke happens. It's not mechanical. Vacuum pulls it closed.
The forces are simple in that during upstroke, a negative pressure is pulled inside.
The outside air has a higher pressure than the inside air does.
So the outside air tries to enter the engine.
If the umbrella closes, it keeps outside air from entering.
If the umbrella stays open or doesn't seal fully, the higher pressure from atmosphere enters the crankcase.

In most engines negative crankcase pressure allows less ring pressure and the combination of both means more hp. 20)
Over the years folks have used exhaust system energy to pull pressure from the case for this reason.
Guys have won championships with an engine that had an electric pump to reduce crankcase pressure.
Crankcase pressure in these engines fluctuate wildly from positive to negative. 21)
At some point, it can have a dramatic affect on scavenging.

Oil scavenging:
Positive crankcase pressure aids scavenging. 22)
Negative pressure makes the pump's job harder, because the pump is fighting the crankcase vacuum.
(with little to no blow-by and a check valve on the breather system)

Symptoms of High Crankcase Pressure

Picture a balloon inside the engine being blown up.
It puts internal pressure against the weakest structural points (gaskets and seals).

Symptoms include: 23)
Sweating oil from the cylinder base gaskets and rocker boxes.
As well as the push rod tubes and lifter blocs on the other side.


Blowby pertains to the condition of ring seal at the cylinders / pistons.
Combustion above the piston is pushed past the rings and into the crankcase.
It is fundamentally just exhaust that went past the rings instead of out the exhaust port. 24)
It's depleted of oxygen, it's hot, and it picks up moisture and oil in it's travels through the crankcase and into your intake tract.
So it's displacing oxygen in the intake charge (through the air cleaner) that would otherwise contribute to combustion.
It also heats the intake charge when the breather vent is piped back to the air cleaner which contributes to detonation.
It's just a bad thing all the way around, robbing power, causing detonation, and contributing to carbon build-up.
It's well worth it to remove it from the intake tract and vent it to the atmosphere or exhaust instead.

What actually 'blows by' the rings and into the crankcase is a mix of unburnt fuel, water, soot, acids etc. 25)

Regarding blowby at high revs, the key factor is ‘ring-flutter’ or ‘ring-seal’. 26)
Starting with a dead cold engine, there’s no blow-by measurable for a time after start-up.
Then at warm idle, it starts. Above idle it either drops off or holds constant, as revs go up.
Ring seal improves above idle.
As you get near peak revs, ring-seal again fails and gusts of blow-by pass down into the crankcase.

Combustion chamber blowby adds positive pressure to the crankcase at the same time it's being lowered by the vacuum condition on piston upstroke.
The greater the blowby, the more it pressurizes the crankcase during this period.
By design, we have a little blowby bridging the gap between positive and negative pressure on piston upstroke also.

27) 28)

However, without the addition of blowby during this vacuum period, seals and gaskets could be in an imploding type position.
That could stress the seals enough to pull outside atmosphere through the gasket / seal areas and into the engine during upstroke.
(adding more air into the system only to regenerate higher on downstroke)

If you put your finger on the end of a syringe and pull the plunger up, you get an indent in your finger from the suction on the end.
If you were to drill a small hole in the rubber cap on the plunger, you would not get the indent (or suction) on your finger.
The air between the rubber cap and your finger would be equalized by the air coming into that area from the outside (through the small hole).


The hole in the rubber cap on the plunger referenced above represents ring seal.
So a certain amount of blowby is needed to fill the air volume in the crankcase to just below atmospheric pressure, but not greater.
If blowby charged the crankcase with positive pressure on upstroke, the breather would open prematurely while splash and oil mist is being pulled up.
This positive air / oil charge would be amplified upon the next downstroke which normally opens the breather.
Then it darts toward the low pressure in the system which is the breather opening = oil puking out the A/C.

Ring construction

Gapless rings:

We've just historically had trouble with wet sumping when using gapless rings. 30)
Basically, the better your ring seal, the more likely you are to have a wet sumping issue.

A lot of guys who work on a lot of gas/diesel/car/truck/bike/old/new performance stuff have moved away from the gapless second ring. 31)
It traps pressure under the top ring.
In fact the trend has become to run the gaps looser on the second ring to allow what gets thru the top to not get stuck.

New ring / cylinder install

Neither new rings nor cylinder bores are perfectly round. 32)
As a result, when you put a new ring on a piston and into a new bore, there's actually very little surface area of contact between the two.
The honing process puts a texture on the cylinder wall that allows the ring to machine the wall into the same shape as the ring during operation.
This increases the surface area of the contact patch.
The ring is said to be “seated” when it has carved the cylinder into it's shape and there's contact all the way around.

Many, many times I've pulled bikes apart and looked at the cylinder walls and spotted places where the ring was never touching it. 33)
It has a huge amount to do with the accuracy of the boring job as well as the accuracy of the rings used.
If it was bored perfectly and the ring was perfectly round, there would be no seating even needed.

Ring seating during break-in

There is a risk of ring microwelding by getting too aggressive in the break-in. 34)
HD's break-in procedure, and S&S's, and others are designed to minimize heat build-up.
Be gentle on the motor, don't put it in a situation that makes it hot.
The reason is simple.
With very little contact area between the rings and the cylinder walls;
Ring tension is concentrated and those areas that do make contact get very hot.
That localized heat can and will damage the piston, and remember, the ring land in the piston is a sealing surface.
Damage it and you'll never get a good ring seal.
This scenario happens more than you might think.
Good lubrication and a gentle break-in consisting of several heat cycles to begin with are absolutely mandatory on an Evo engine.
Ring seat depends on how good the machine work is.
If everything is perfect, it'll be seated when it's put together (it won't be though).
If the bore and/or rings are out of round badly enough they may never seat.

Normal blowby

In the absence of any blow-by getting past the rings, the crankcase alternates from atmospheric (pistons down) to a vacuum (pistons up). 35)
But in the real world, a little gets past the rings, so there's a net outflow equal to that.

Conventional rings have a ring gap and the combustion pressure is very great. 36)
So you can bet some of this tremendous pressure is entering into your crankcase instead of 100% of it exiting your exhaust pipes.

Excess blowby

The ringlands on the pistons 'should be' sealing but sometimes are not.
You can end up with 'out of round' or scratched cylinders from different conditions.

Testing ring seal

A leak down test is the best way to check ring seal. 37)
The tester is not expensive and it's handy as hell.
Listen for where the air is coming out during the test: intake port, exhaust port, or breathers.
See also Performing a cylinder leak down test in the REF section of the Sportsterpedia.

Crankcase Pressure Testing

Dyno testing using the timing plug location for an additional crankcase vent by aswracing

The following is to share some dyno testing by aswracing of using the timing plug location for an additional crankcase vent. 38)

First, a little background.
In the stock configuration, the crankcase vents through a pair of “umbrella” valves, which are essentially check valves.
There is a slight air inlet into the motor from a tiny hole near the umbrella. 39)
It acts as both an oil drain for anything that gets past the umbrella and an air intake to keep negative pressure from getting too high. The pistons come down the first time and the crankcase air is expelled with the air being forced out through the umbrella valves.

But when the pistons go back up, the umbrella valves block the inflow of air, causing a slight vacuum in the crankcase.
The next time the pistons come down, crankcase pressure will return to atmospheric at BDC before the upstroke.
If no air is allowed into the motor, the crankcase will cycle between a vacuum (pistons up) and atmospheric (pistons down).

However, some air is actually allowed to enter, primarily in the form of blow-by that escapes the combustion chamber past the rings.
Therefore, in the stock design, there is a small net outflow.
The amount will vary with the condition of the motor.

Properly functioning umbrella valves therefore serve the purpose of significantly reducing the breather capacity requirement while also minimizing crankcase pressure.
Excessive airflow & oil discharge through the breathers can be caused by malfunctioning umbrella valves that are allowing more air into the motor.

For this test, an additional vent was added at the timing plug hole.
No check valve was installed on this vent.
Therefore, the crankcase is being allowed to pull in air as the pistons go up.
This fundamentally changes the engine's venting design.

Some people feel that allowing the engine to both inhale and exhale in this manner reduces crankcase pressure.
Several people cite a “seat of the pants” improvement in performance.
The purpose of this test was to determine if there actually is a performance improvement from this change to the venting system.

The fitting arrangement used is in the pic below.
The threads on the flare match the timing plug hole threads.
There are two fittings threaded together and app. two feet of 3/8” I.D. hose was attached to the hose barb.

3/8“ flare to 1/4” FPT fitting
with a 1/4“ MPT to 3/8” hose barb. 40)
Fitting and hose as installed on the bike.41)Test bike (near stock 1999 M2). 42)

Lots and lots of dyno pulls were performed in each configuration and the configuration was switched back and forth a few times.
Dyno results are not 100% repeatable, and as such, below is a range of results for each configuration, as well as a comparison of best pulls.
Click on a chart to enlarge:

10 best pulls from the stock configuration. 43)10 best pulls from timing plug vent configuration. 44)Best stock pull and the best timing plug vent pull. 45)

As you can see, the difference is within the repeatability of the measurement.
If a person *had* to declare a winner, the results with the stock setup would seem to have a little edge.
(both in the “best” results and just looking at the average of the 10 best results)
But I'd be careful doing that, you could be looking at normal variation.

I was surprised at how little air movement there was at the end of the hose.
When a motor is started with nothing screwed into the timing plug, there's a massive inhalation and exhalation evident.
But apparently, necking it down to a 3/8“ hole and connecting 2 feet of hose adds a pretty significant restriction.
Air flow was nowhere near what I expected.
Unfortunately, getting a 7/16” or 1/2“ hose into that area would be problematic, space is tight.
Plus, a fitting with the correct thread and a 7/16” or 1/2“ hole may not be available.

Another surprise was just how easy it was to plug the hose with my finger, and how it felt when I did.
The pressure was not great.

Breather System Air Volume Test by DK Custom

The full article is on the DK Custom web site.
This testing was done to find out:
How much air was passed out the breather vents at idle, under a load, at cruising speeds and on throttle let-off.
And also the differences between a variety of HD engines, along with engines that had been hopped up with cams and or higher compression pistons/heads.

In this test, air was captured and measured as to how much water volume was displaced in a fixed period of time.
Taking the liquid ounce displacement, you can convert that to Cubic Feet Per Minute (CFM) There are 957.50649350649 U.S. fluid oz. in 1 Cubic Foot.

Sportsters move the least amount of air through the breathers.
Twin Cams move the most amount of air through the breathers, with little difference between air cooled and Twin Cooled.
Milwaukee-Eights move more air than Sportsters, but little more than half as much as the Twin Cams through the breathers.
Even more surprising is the least amount of air is moved on all bikes while at cruising RPM.

The only way to get a significant amount of air to move through the Sportsters was to get the RPM up around redline.
(and that crankcase pressure was probably because the valves were beginning to float) The most amount of air is moved through the breathers at idle, during hard acceleration and during deceleration.
A visual of this can be seen in this video: DK Custom Breather System Air Volume Testing of Harley-Davidsons

The actual numbers are in the chart below.
(engines warmed up / oil level on midway mark of dipstick before testing)

DK Customs Products Breather Report (from different throttle / riding conditions)
Bike testedCFM
(hard acceleration)
Air Cooled Twin Cam (103).2172.08.1952.2504
Twin Cooled Twin Cam (103).2548.0972.2231.2874
M8 Air Cooled (107).1211.06.1059.1127
Sportster (1250 with high compression).0125.0125.0125.0125
At redline (6200 RPM) with no load (.3326 CFM)
Sportster (1200).0626.0626.0626.0626

CFM: Cubic Feet per Minute.
Deceleration Test: Measured by chopping the throttle to 0% with the clutch in.
Cruise (low load test): Typical RPM most riding takes place in (2500-3000 RPM).

Testing with a Slack Tube (Manometer) by bustert

Sub Documents

Testing was done from the timing plug hole and then from the oil tank with a slack tube on a 2001 XL1200S (with no load) by bustert of the XLFORUM. 46)

On the left (from timing hole plug), the engine begins at high vacuum (green liquid line on scales in pics below).
Notice that there is a transition to a positive pressure above the 5k mark.
On the right (from oil tank), there is an equalization on positive and negative at 5K.

One could speculate anything from over-running vent capacity to time factors.
The numbers are subjective to ambient temps and elevation.
However, we could use it as a tool to determine engine wear like they do on a diesel engine.
All-in-all, the subject engine operated as HD intended (within the intended most used rpm range).

The results are in (inches of water) and you can convert to psig but remember, you have to add both sides.
So a 15 on one side with a 15 on the other would be 30”.

Slack Tube testing from timing hole plug. 47)Slack Tube testing from oil tank

The testing showed that the test bike acted as intended with head breathers (venting through lines bypassing the A/C to atmosphere).

Testing Notes

Each of the tests above do basically support each other given the different variables.
But the results have to be taken in context as each have different criteria for testing.

  • Testing from DK Custom:
    • The criteria for their testing was to see how much air was passed out the breather vents (outside the engine) at idle, under a load, at cruising speeds and on throttle let-off' for different model engines. They sell modified breather venting configurations and was doing some R&D presumably in the interest of same.
    • Their testing supports bustert's slack tube testing as normally at most of the RPM range, there is more vacuum than positive pressure. And it's the positive pressure that leaves the engine. Therefore, their results for the Sportster are equaled out more. Even though there is normal blowby throughout the RPM range, the vacuum created buffers that.
    • In example, 15“ of vacuum at idle that all of a sudden is hit by 5” of positive pressure rolling the throttle still yields 10“ of vacuum at the time. So there would be no air moving into the balloon or container at that point. In theory and during that transition from 15” to 10“ vacuum, more oil is pushed toward the scavenge hole in the sump, the pump gets a fatter supply of oil to send to the tank, pressure goes up in the air space in the tank due to the restriction size of the vent.
  • Testing from bustert:
    • This was a test of the differential pressure changes (inside the engine) through the RPM range up to 6000 RPM.
    • You may have read and heard from many sources that the Sportster requires a 'slight vacuum'. But the slack tube testing puts a visual to the process showing that the 'slight vacuum' is not really a stagnant number but a constantly moving range.
  • Testing from aswracing:
    • Dyno testing was with the normal head breather vents in place (with and without the timing hole plug removed) to see if either would show increased HP over the other.
      The dyno sheets show the affects (HP changes) between the stock setup and with addition of air induced into the engine through the RPM range.
      However, it does not show internal pressures during the testing.
    • The testing revealed a dip in performance starting around 5700 RPM which coincides with bustert's slack tube testing showing positive and negative pressure equaling out up in that range. But the Dyno test is a load test as where the slack tube was done with no load on the engine… more variables.

What does all this mean?

The testing shows that there is more potential for crankcase pressure problems in the high RPM range.
It also shows that Sportster engine breathing was designed for peaks in the upper range but not for sustained use there.
There will be a normal amount of air passing the rings by design.
As the rings heat up and expand, there will be less air passed by them until you run up into the 5000 RPM range.
Then, the rings will flutter and pass more combustion air into the crankcase which creates more positive pressure in the crankcase.
Normal blowby increases with engine speed.
Couple that with the increasing speed of the pistons which helps to equalize positive and negative pressure during operation.
As engine speed increases, there is not as much time to build vacuum on upstroke or positive pressure on downstroke due to the faster changing piston positions.
Just as you can inhale air slowly and fill up your lungs but faster breathing will not allow you to fill them due to the faster time that you exhale.
This would make for a shorter range of (both vacuum and positive) pressure that would be able to build in the crankcase.
So the internal pressure is more stable until extra air (blowby from ring flutter or other) is induced into the crankcase.

What causes extra air in the crankcase?

Ring flutter around 5000 RPM and higher is thought of as the main culprit on a healthy engine.
Gasket / air leaks react the same as ring flutter but at a lower RPM.
They allow more air into the engine that add to the positive and take away some of the negative (vacuum).
So the introduction of air leaks into the crankcase lowers the RPM at which pressure changes affect the system.
Worn / stiff breather valves that do not fully close will allow more air at atmosphere into the crankcase.
This lowers the vacuum and contributes to higher positive pressure on downstroke changing the ratio at a lower RPM.
The timing of the breather valve opening and closing can also bring in air to the crankcase during upstroke.
Guess we could call that umbrella flutter.
The faster it closes, the more vacuum is kept in the crankcase on upstroke.
The slower it closes, the more air is allowed to enter and lower residual vacuum.

Why is the ratio of positive and negative pressure important?

It takes a balance of the two to run a Sportster engine.

Piston upstroke creates negative pressure and suction of oil from the sump.
It pulls oil up in the form of oil mist to be tossed around on the moving metal parts.
So it is important for lubrication and it keeps down aeration in the oil.
But without the reciprocating piston downstroke, there wouldn't be a lot of force to help splash it around other than the spinning wheels.
The upstroke pulls oil into suspension (air/oil mist) so the downstroke can help blow the mist around working in conjunction with flywheel and cam rotation.
Negative pressure is also important for ring seal as it allows the rings to fully seat on the bottom of the ringlands during upstroke decreasing blowby.
Too much negative pressure is detrimental to oil scavenging as it allows more oil to be pulled up into suspension instead of moving toward the scavenge port in the sump.
The bulk of gravity oil on the sump floor is heavier than the moving air.
But the spinning action of the flywheels can pull that oil up to be slung around the wheels creating more drag as it does.
So it's important to get the excess oil in the bottom out of the engine as fast as possible to keep down flywheel drag.
That's where the positive pressure comes in.

Positive pressure is important for oil scavenging as it works in conjunction with splash lubrication as well as the suction of the oil pump.
The positive pressure generated by the downstroke pushes oil toward the scavenge pump to be sucked vertically into the oil passage to the pump.
So there is a balance of positive and negative pressure that has to be maintained for overall engine operation.

The role of positive and negative pressure can be confusing.
Even though there is a positive 'push' on internal pressure through piston downstroke, the overall internal pressure is still negative.
It's just less negative than it was before the downstroke. This creates a pulsing effect on oil in the sump which helps shift the oil toward the scavenge port.
Even though there is normal blowby throughout the RPM range, the vacuum created buffers that.

Engine Breathing

Typical automobile V-8 engine breathing: 48)

Engine ventilation is connected to the rockerbox, crankcase, cam gear case and the oil tank. 49)
If you blow down the oil tank line…air comes out the rockerbox vents (or cam breather hose).
There are airways linking these compartments together and in looking at these airways.
If one pressurizes, they all pressurize and air can pass between them;

  • Rocker Box:
    • Pushrods connect the rockerbox to the gearcase.
    • Gravity oil:
      • 03 and prior: Heads drain oil to the crankcase sump (through passages in the cylinders).
      • 04 and Up: Heads drain oil to the gearcase (through passages in the cylinders and gearcase wall).
      • All: Pushrod tubes drain oil into the gearcase.
  • Gearcase / cam chest:
    • Rockerbox oil return connects to the gearcase.
    • Airways thru the wall joins the sump and the scavenge side of pump.
      • Piston downforce pumps air and oil from the sump to the gearcase compartment.
    • This would equalise air pressures in the two chambers.
  • Crankcase:
    • Piston downstroke creates a positive pressure against the oil in the sump.
      • This forward pressure is connected to the oil tank thru the oil pump.
        • It also helps to push oil into the scavenge chamber from the sump upward into the scavenger side of the oil pump.
    • Piston upstroke creates a negative pressure (noted as vacuum for this article).
      • Some of the oil either draining to the sump or collected from the sump is picked up by the vacuum in the form of oil mist.
      • Also some of the oil is picked up in the form of oil droplets (or splash oil) and is moved around by the next positive pressure condition.
        Splash oil is further moved by the action of the flywheels, connecting rods, cam gears, air pressure and gravity.
    • The pinion gear shaft is hollow and connects the crankcase to the gearcase (but would only pass air with the engine off).
    • 03 and prior engines connect rockerbox oil to the crankcase.
      04 and up do not.
  • Oil Pump:
    • The pressure side of the oil pump is fed from the oil tank and is connected to the rocker box and the crankpin.
      • Gravity from the oil tank initially feeds the oil pump.
      • But once the engine starts, the motion of the gerotors creates a suction in the feed line from the tank.
      • Pressure in the oil tank also adds pressure on the gravity feed to the pump.
    • The pump creates non pressurized oil flow from the feed gerotors.
      • Restrictions (oil line / feed passage sizes) to the oil filter pad and through the engine create back pressure on the pump.
      • This pressure builds and is sent to the lifters and rocker box as well as the crankpin through the hollow pinion shaft.
      • The pressure is increased at the pump as oil flows through more restrictions to get to these places.
        (strictly as a non tested example, 10 psi on the feed side of the pump may equate to 4 psi or lower once it reaches the crankpin)
        Pressure is restricted in the cam cover,
        Less restricted with the wider opening at the pinion shaft bushing,
        Then restricted again thru the shaft hole and the turns in the flywheel to the crankpin.
    • Once the pressurized oil reaches the rocker arms and crankpin, the pressure is released into the wider openings in the oil path.
      From there it is added to and becomes a part of crankcase pressure and is used and vented as such.

OEM oil paths and engine breathing drawings:

86-90 engine breathing paths. 50) 91 engine breathing paths. 51)
92-97 engine breathing paths. 52) 98-03 engine breathing paths. 53) 04 and up engine breathing paths. 54)
Testing CC pressure on the dyno. 55)

Revised crankcase breathing is an area where you have huge potential to create unintended consequences.
Few if any really understand the ifs, ands and buts of all the factors the factory took into consideration when they designed the system.

Gappless rings while great for ring seal are another area where you can get in over your head if you are not careful. 56)
Wet sumping is only one potential problem you may encounter with them. 57)
In the right application they can not be beat but you better have your ducks in a row.

Buell crankcase breathing:

This is a Buell XBRR with reed valves through the cam chest wall. 58)

Symptoms of Breather problems

The most noticeable signs of breather valve problems are weeping gaskets or oil excessively leaking out the breather or puking oil out of it.
When the umbrella(s) gets hard, it doesn't flex well to allow the engine to breath out on exhale.
Then the trapped in air is compounded on the next stroke. The excess air contributes to more vacuum created and implodes the gaskets.
Or it contributes to too high of oil density and slings excess oil out.
Other factors are involved so results will vary.

Why Oil Pukes Out the Air Cleaner

Wet sumping is a term usually used when oil spits out the breather vent / air cleaner. 59)
However, engine breathing and wet sumping are two different ideas. But they are tied together as much as air and water.
See Wet Sumping below.
Oil puking and oil seepage out the air cleaner are different conditions.
You'll know it if it pukes as there will be a good puddle of oil on the shop floor or gobs of oil running rearward of the A/C all over the bike.
Oil seepage (or the oil filter getting saturated with oil over time) may be perfectly normal.

The real reason these things like to spit oil out the breather is because they're a common crankpin 45° design. 60)
Since the pistons are only 45° crankshaft degrees apart, they arrive at BDC 45° apart and again at TDC 45° apart.
So the volume of the crankcase is constantly changing.
If the check valve isn't functioning properly, it'll cause this issue.
There are usually two main concerns when this happens.

  1. The breather valve(s) have to function properly. 61)
    Often the original breathers / umbrella valves will no longer work or will become less effective. 62)
    • This piston design causes a variable volume crankcase:
      • Pistons come down and the volume is smaller, pistons go up and the volume gets larger.
        Most engines don't work this way, they have a piston going up for every piston going down.
      • The variable volume design causes it to want to inhale and exhale air into and out of it's crankcase constantly.
        For a graphic illustration of this, take your timing plug out and start your motor.
      • If it's allowed to suck air in, it'll have an inhalation & exhalation effect going.
        Whenever air goes out, it'll carry some oil with it and deposit it.
        So by allowing it to inhale & exhale, you've basically created an oil pump.
  2. Likewise if the motor has excessive blowby (poor ring seal) it'll cause it.
    Read more about blowby above.
    • Bad ring seal helps evacuate the sump from oil. 63)
      But on the other hand it increases the flow rate through the crank vent system to such levels that a lot of oil droplets join in.

Helpers for oil puking out the engine breather:

  1. Wet Sumping:
    • During engine down time.
    • During high revs.
  2. Over filling the oil tank: 64)
    • If the puking starts after you top you oil tank, this is probably the problem.
      If you fill the oil tank to the full mark while some oil has wet-sumped down into the engine, you have too much oil in the system.
      The oil from the sump will be pumped back up to the tank, dribble down the vent tube to the timing cover, from where it is fired out the engine breather.
      This puking will continue after initial start up until all the excess oil has been fired out, which can take a while.
    • The cure is to drain a quart or so out of the oil tank, run the engine for five minutes until the puking stops, then top up the oil tank to the full mark.
    • On 57-76 engines, do not be tempted to drain oil out of the sump by taking out the threaded drain plug under the front of the engine.
      These are notorious for stripping the threads and are very difficult to repair properly.
      See Sportster Drain Plugs Explained for more information on that.
  3. Worn engine / rings: 65)
    • If your engine breather continues to puke oil or blow smoke after the above two things have been eliminated, your problem is most likely wear in the cylinders and heads.
      Worn rings and even valve guides, can allow blowby of combustion gasses into the crankcase area, which then comes out the breather.
      Usually this will be accompanied by smoke or oil coming out the exhaust pipes too.
      A compression test will give some indication of top-end condition.
      Anything below 120psi is suspect, according to the factory manual.
    • These bikes will still run OK at even 100psi, but they will be down on power and consume oil, and blow fog out the breather pipe.

The OEM system is spec'd with a certain pressure on downstroke in mind.
If atmospheric pressure is also present then, the total positive pressure will be higher at the breather vent when it opens next.
The extra air is combined with what the downstroke exerts (including normal blowby).
Likewise, increased blowby adds more air in the crankcase which increases positive pressure.
The higher the positive pressure is when vented, the greater the oil that is carried out with it.
It's a balance.
There are very small amounts of oil mist that will normally go back into the carb.
You may not see it, but it is there.
When the balance is off, you see it though.
When we modify the system, these balances have to be maintained.
That may be in the form of a new or better breather valve.
Or that may require a better breather valve in addition to something else.

Worn rings / leaking oil out the umbrellas is only one cause of oil loss, however; 66)

  • The one way breather valve is designed to only allow air out.
    • This helps restore the OEM aim of a crankcase vacuum.
    • It also cuts down on oil leaks.
  • Once the umbrella valve(s) fail, it changes to an open breathing system that allows air to suck in and blow out.
    • Violent air pressure changes and power losses follow.

The fluctuations in air pressure suck oil up into the air in the engine.
The fluctuations are driven by the pistons.

So to keep oil out of the A/C, means keeping engine breathing And wet sumping in balance. 67)

Breather Catch Can Test For Oil Leaks Out the Vent

A test was done by XLFORUM member cjburr. Test apparatus and results are below. 68)
This testing was done to help diagnose an oil leak that couldn't be seen or otherwise detected.
However, this may be a good exercise if you are concerned about how much oil you may be losing from the crankcase puking.
It should show what, how much and when oil accumulates in the hose / catch can.
The length / size of tubing will change crankcase pressure to an extent so the results may not reflect exactly what's going on inside your engine.

The vent to air cleaner was inspected and then removed for testing. The factory system routes the breathers to the front of the carb.
There was no evidence of the amount of oil that was being lost in the intake tract.
With the Forcewinder A/C removed, there was no evidence of oil. A homemade catch can with a vent hole and a clear tube was installed to inspect while riding.

A/C removed. 69) Homemade catch can. 70)

The A/C was re-installed with the venting ran to the catch can.
The oil tank was filled to just under half way up the stick with SYN3.
The bike was ridden hard (10 miles of city driving then 50 miles of elevated speeds).
RPM was ran up to 6000 in 5th (8 times) and up to 100 MPH for 2 miles twice.
Total miles run were 75 and all but 20 miles of that were at 80 MPH or more.
The only thing that showed up in the tube running to the catch can was just some moisture but no oil.

The catch can was clean but 1/4 quart of oil had been lost as viewed from the dipstick. 71)

Breather Valves

See also in the Sportsterpedia.
………….* IH Crankcase Ventilation
………….* Evo Crankcase Ventilation

Positive and negative crankcase pressure is an intended balance:

76< engines have rotary breather valve on the oil pump that is timed to open on the downstroke of the pistons. 72)
This allows crankcase exhaust air pressure to expel the scavenge oil from the crankcase breather oil trap into the gearcase.
The breather valve closes on piston upstroke, creating vacuum in the crankcase.

77> engines do not have a breather gear on the oil pump.
Without the breather gear that means you have no control of the oil/air density of the crankcase.

1957-1976 models: 73)
There is a timed breather valve built into the oil pump drive, which vents crankcase pressure into the cam timing chest.
A six-inch metal tube hanging down from the timing cover near the generator drive, at the 6 o’clock position vents that controlled pressure to atmosphere.
A metal disc on the end of the generator drive gear centrifugally separates oil from the air as it is discharged overboard.
1977-78 models only: 74)
The timed breather on the oil pump drive was dropped.
An external non-return valve was plumbed into that vent tube sticking down from the timing cover at the generator drive.
This allows air out, but not in. It is sometimes referred to as the foo-foo valve.

1979-Early 1982 models: 75)
The external foo-foo valve and the six-inch metal vent tube at the front of the timing cover were done away with.
Instead, a one-way foo-foo valve was built inside the timing cover.
(reed valve assembly 26909-79A) A rubber breather hose then ran from the generator drive area of the timing cover, at the 9 o’clock position.
It connected to the stock air filter so that any oil mist was fed back through the engine.
This made the EPA more happier than they were with the idea of engine oil spraying out into the atmosphere.
Many of these bikes with custom air filters simply run that hose down to the bottom of the frame.
(and let the oil mist blow out in the time honored manner)

Late 1982-Early 1984 models:
The internal crankcase breather valve was redesigned to incorporate a rubber umbrella valve.
This is attached to the base plate along with a larger diameter (1-3/4” O.D.) oil separator washer on the generator armature. 76)

Late 1984-1990 models:
A breather baffle tube system was incorporated into the cam cover in the area behind the oil filter.
The baffle tube has a one way umbrella valve mounted into it.
There is a plug in the 6 o-clock position and a tube in the 11 o-clock position going to the air cleaner.
On each piston downstroke, crankcase pressure (air and oil mist) is routed to the breather baffle at the front of the gearcase.
Oil is separated from the air pressure by the one-way umbrella valve.
The oil then drains into the gearcase through a drain hole in the breather valve.
Exhaust air escapes past the one way umbrella valve in the baffle tube and into an outlet fitting on the cam cover.
The air is then routed into the rear of the air cleaner via an oil hose to the gearcase outlet.

1991-2003 models:
In 1991, along with the 5-speed transmission, the MoCo moved the crankcase breathing from the cam box cover outlet to breather bolts in the heads.
Previously, these bolts were just used to mount the carb and air cleaner to the heads.
The new breather system uses one-way umbrella valves in the rocker boxes.
These exit crankcase vapors through vents in the top of the cylinder head into the carb mouth to be burnt. 77)
On each piston downstroke, crankcase pressure (air and oil mist) is routed up the pushrod tubes into the rocker box.
Collected air pressure and oil mist in each rocker box is routed up into a sealed cavity in the lower portion of the box.
This mixture passes up from underneath a rubber one-way valve (umbrella valve) sitting over the cavity inlet.
The oil is designed to separate from the air by hitting the underside of the umbrella valve.
Then dropping back down into a recessed area behind the umbrella valve in the cavity.
From there it should drain back into the main rocker box through a tiny hole behind the umbrella valve and then back to the lower end.
Air pressure is designed to continue up past the umbrella valve and exit a hole in each head on the intake valve side.
Air pressure escapes the head through the hollow bolts (one in each head) that hold the air cleaner mount.

The 91-03 umbrella is sitting on the middle box spacer inside an enclosed cavity when it's buttoned up.
But this cavity has a tiny oil drainback hole on the left side of the umbrella.
So, if you blow into one head vent (carb bracket off and umbrella in good shape), the air comes out the other head vent.
There is the restriction of the tiny hole you're blowing air into.
So you will have to pucker up to blow hard through it.
That being said, that tiny hole will allow a small amount of outside air back into the crankcase on piston upstroke.
As soon as the sealed cavity is drained from separated oil, the only thing left is outside air to pull into it.
(for a small amount of time until the piston falls again)
This, in affect, will add a slight atmosheric aide to the 'then' negative crankcase pressure.

Just as you can't pump into a container with no vent, you can't suck from a container with no back vent.
Gravity drain oil (heads / pushrod tubes) also has the force of suction to help pull it down on piston upstroke.
Likewise, that suction also helps to drain the tiny hole in the sealed rocker box cavity.
So the head vent also doubles as a back vent to help CC vacuum drain the sealed cavity on piston upstroke.

78) 79)

In 2004 the MoCo made some changes to the umbrella valve configuration.
The umbrella was retained but now inside a plastic housing with a pre-umbrella oil separating screen (fiber mesh).
On 03< models, the oil is actually separated by the umbrella. The umbrella doubles as a one way air valve.
On 04> models, the oil is separated first by the mesh below the umbrella which frees the umbrella to be more of a one way air valve.
There is an extra oil chamber between the in and out of the breather assembly.
So it could be said that the 04-up breather valve has 2 stage separation with a small hole in the middle chamber to drain what gets past the mesh.

Each 'breather valve' is assembled into a plastic fitting that is sealed over the outlet hole in the lower rocker box to the head breather bolt.
The new breather valves were also accompanied with new style hollow air cleaner mounting bolts for the pressure to escape.
The new bolts are the same thread size as previous.
But instead of a simple hex, it also has a shoulder past the hex for an O-ring to be fitted between the hex head and the air cleaner.
The breathing system is functionally the same as 91-03 with the one-way umbrella valves in the rocker boxes.
These exit crankcase vapors through vents in the top of the cylinder head and into the carb mouth to be burnt. 80)
On each piston downstroke, crankcase pressure (air and oil mist) is routed up the pushrod tubes into the rocker box.
Collected air pressure and oil mist in each rocker box is routed up into the breather valve unit in the lower portion of the box.
This mixture passes up from underneath the breather unit.
The oil is designed to separate from the air by hitting the underside of the screen / umbrella valve and dropping back down into the rocker box.

From there it is routed back to the lower end.
Air pressure is designed to continue up past the breather unit and exit a hole in each head on the intake valve side.
Air pressure escapes the head through the hollow bolts (one in each head) that hold the air cleaner mount.

To keep the engine from pulling in air from the outside, HD fits the breather vent(s) with a check valve.
This way, the motor can exhale, but not inhale.
The reason for the check valve is not to keep it from pulling in dirt.
Any inhaled air is going to get pushed out again and take oil with it.

  • We have check valves in the rocker boxes (or cam chest respectively) that are *supposed* to eliminate this.
    If they aren't functioning properly, you'll get lots of oil out the breathers.
  • The valves themselves rarely fail (unless they get hard), but the casting they sit in is, well, a casting…not very accurate.
  • Buell had a service bulletin years ago that talked about chamfering the umbrella valve hole to make it seal better.
    (as a fix for excess oil in the air cleaner)

The breather valve not only has to work but it has to operate in sync with the action of the pistons.
It has to open with downstrokes and close (completely) with upstrokes.

Breather Valves 81) View of one cut open. 82)
The mesh can become saturated with oil. 83)Oil drain hole. 84)Installed upright as shown. 85)

Aftermarket Breathers / PCV Valves

Sub Documents

There is a product called a Krankvent that can be plumbed into the lower, 6 o’clock position as an alternative to a stock foo-foo valve. 86)
But they are not cheap. Automotive PCV valves are not really made to handle the revs or air volumes of a Harley.
While a car engine is bigger, it has one piston coming down while one goes up.
So there's not much change in internal crankcase volume, so not as much breathing to be done.
A Harley has two pistons and rods on one crankpin, so is one giant air compressor.

Some guys have found that plumbing in a 77-78 foo-foo valve on the later model engines improves breathing.

Finding a better breather valve: 87)
The breather valves that work best on crankcase breathers have several features:

  • Low-inertia, i.e. they don't take more than a breath to open and close.
  • They operate at low pressures ~1-30psi.
  • They preferably have a floating type seal.
  • They should be transparent, so you can see if they block up with blowby, solids, bugs etc.
  • They should be easy to open and clean occasionally.
  • They should push-fit into your breather line.

Metal diaphragm valves with e.g. springs are hopeless.
Also avoid other types of valves designed to work at high pressures e.g. plumbing valves.
Avoid car PCV valves as they are metering as well as non-return valves and are unsuitable.
Automotive PCV valves are designed to let some air back in.
The ball and spring valves will not be able to work fast enough to keep all the air out.
When air is allowed to be drawn back into the engine, it creates a mass that must be compressed by the pistons on downstroke as it is being vented out.
This robs some power and will cause the engine to work harder (creating more internal heat at higher RPM). 88)

Aftermarket PCV's: 89)

  • Mercedes Benz P/N# 271-018-00-29 ~$13.
    • Reed type PCV valve. Fast acting. Used for their C 230 Kompressor engine.
    • Dims: 2-1/2” long by 1-1/8“ with an I.D of 1/4”.
  • McCaster Carr P/N# 4610K17 ~$6.
    • Umbrella valve. Vacuum check valve.
  • Autozone P/N# PCV1174 (PV272) ~$5.
  • Ball/spring based PCV valve.
    • Makes a clicking noise during operation.
Here is a 2-stroke reed valve. 90)
Mercedes Benz P/N# 271-018-00-29 91) Reed Valve 92) Doherty Power Vent 93)

Regarding horsepower gains from different breathers

From aswracing on testing breather vents for HP gain: 94)

Many years ago, I spent most of a day dyno testing breather check valves.
I was writing for Battle2win magazine at the time and published my findings there.
The article is here at Do Crank Vents work?

I spent several hours on the dyno testing whether or not various breather arrangements affected the power of a motor. 95)
Mainly it was a test of these breather check valve devices, but I also tested the recycling of the blowby versus venting to the atmosphere.


The result labeled “kuryakyn” is the one that's recycling the blowby.
So named because I used a kuryakyn adapter to send the blowby back into the intake tract.
All the others are vented to the atmosphere.

The bottom line was that none of them added a single iota of measurable horsepower, despite the grandiose claims of up to 7% from the manufacturers.
The only thing I could get to show up on the dyno at all was the removal of the blowby from the intake tract.
You can read all about the results in the Nightrider article above.
One of the companies involved, ET products (maker of the Spyke Krank vent) took exception.
I very carefully retested using their suggestions, but I got the exact same answer.

Head Vents vs Cam Chest Vent

CC pressure (while venting thru the heads) was tested with a slack tube as in above.
Results of the testing showed that the test bike did perform as designed using breathers in the rocker boxes.
Sustained RPM on a street bike can be up in the 4000-5000 RPM range at highway speeds.
The test bike showed a positive charge forming in the crankcase around 5000 RPM.
This positive charge is responsible, in part, for oil puking out the vent(s) if the pressure gets too high.
So the farther past 5000 RPM, the more positive pressure is created.


91-up Breather Bolts

The MoCo manipulated crankcase pressure with the different size holes in the breather bolts.
The crankcase splash holes were restricted in 2000 to keep more pressure in the crankcase.
Then the holes in the wall was closed up in 2004 to accommodate the addition of piston jets to cool the pistons.
The piston squirters (feed pressure from the pump) added the same amount of oil into the crankcase at a faster rate.
But the scavenger gerotors were not redesigned larger until 2007 to remove the oil faster from the crankcase.
So on 04-06 models, they got the same scavenge return rate (as previous models without the added oil).
The holes in the 04 Up bolts are stepped. They are smaller on the head side as above but bigger on the A/C side.
First, that creates more of a restriction than 91-03 models. The restriction can do a couple things:
It will create more backpressure inside the engine until a stronger force is applied from inside.
That stronger force would be the air created on downstroke. In a perfect world, the downstroke won't be impeded so the force won't slow down.
The higher positive pressure assists in oil scavenging (the pressure inside builds a little more from that restriction). 98)
But it only bottles up pressure on downstroke and only to a point. That little more positive pressure is an extra push on the lower end toward the sump scavenge port.
Another result would be air moving out at a faster rate thru the smaller hole. The smaller hole creates higher pressure. Higher pressure equates to faster flow.

99) 100) 101)

Head Drainage

The debated question on head breathing systems is;
When the engine is running at high speed, is there any kind of pressure being created that might slow down the drainage from the top? 102)
The oil feed to the top end is pressurized and the return is gravity fed. 103)
The drain for the top end is at least 2-3 times larger than the hole in the pushrods.

It's been said that CC pressure moving up the pushrod tubes interferes with drain oil traveling down the tubes. 104)
However, drain oil mainly goes into the head / cylinder drain holes from the rocker arms spraying the valves.
And liquid oil is much heavier than air moving up the passage.
Separated oil from mist falls back into the tubes. But that oil is also pulled down on piston upstroke and air/mist once again goes up on piston downstroke.
The higher venting from the heads also gives any oil that may be being carried along with the gas time to “drop out”.
(which returns the oil back to the cam chest vis the pushrod / lifter block drain holes via gravity)
Obviously, there is less time for oil to drop out of suspension while breathing out the cam chest instead.

The slack tube testing shows there is a predominate vacuum in a running engine until after 5000-6000 RPM.
Any output pressure in the head venting system has to travel up through separate pushrod tube passages to get out of the engine.
Any separated oil gravity falling from the rocker arms would overpower the positive upward push of crankcase pressure until the oil hit bottom.
An exception would be in high sustained, high RPM where there is excess ring flutter adding to equalized pressure in the crankcase.
(as in racing conditions).

Testing the Head Drains

Test performed by cjburr of the XLFORUM 105)

For this to be more accurate, the oil flow rate of the engine when at high RPM would need to be evaluated.
This test was done with engine shut off.
However, the engine was able to get a quart of oil poured into it straight from the bottle with no problem with drainage.
The test can be made scientific with a little ingenuity. \\

This was a test to get an idea of the level the oil would need to get to in the heads to submerge the valve seals.
(before it went to the drain on the exhaust side and if it would pool up enough to submerge the exhaust side)
The head was installed on the bike with a jack under it so everything would be level.
Oil was poured into the head to watch it drain and see if the seal became submerged.

The seals shouldn't become submerged from watching how fast the head drained.
However, there will be oil around the spring seats at all times.
To be 100% certain on this, the bike needs to be running but was beyond capabilities at the time of testing.
The springs and valves normally only receive splash oiling.
Oil was poured into the head from the bottle and monitored with very fast drainage in the ports.
CJ was 95% sure of his conclusion upon testing.
A whole quart of oil at room temperature flows slower that oil at operating temp.
That's pretty conclusive (at this level anyway).

Testing continued to see if perceived normal oil flow would flood the valve seals.
Oil was introduced into the head with a with a turkey baster.
(which is probably a higher volume of oil than it normally sees from the splash oiling)
However, if the drains were plugged, oil would overfill to the point of spewing oil at the seams and submerge the seals. 106)

Head ready for testing. 107) Oil poured in and draining. 108) Oil level at the intake valve when it starts to drain to
the exhaust side. 109)

The oil you see in the first pic below is held there by the spring seat.
At no time did the level become so great as to submerge the exhaust valve seal.
You might be able to dress up the entrance to the exhaust drain with a burr to enhance flow to the drain.
But it may not be necessary as it does drain really well and the spring seat above it might negate any advantage you got from doing so.

Oil level at the exhaust valve when draining stops. 110) This is as high as the level got at the intake valve. 111) This is the drain in the exhaust pocket. 112)

Engine Venting Mods

See also Breather Venting / Relocation for a listing of breather mods from the XLFORUM.

In addition to the above, I also did a bunch of testing of the aftermarket breather check valves from Spyke and Hayden. 113)
(and even did some experiments with vacuum pumps and the like)
Did some magazine articles here and there at the time. The motor was remarkably insensitive to anything I did with the breathers.
Like I said, the only thing I could get to show up on the Dyno sheet at all was the removing of the blow-by from the intake tract.

Engine breathers control when CC pressure exits the engine.
So when you're discussing engine breathing mods, you're also discussing changing crankcase pressure.
Revised crankcase breathing is an area where you have huge potential to create unintended consequences. 114)
Few really understand the ifs, ands and buts of all the factors the factory took into consideration when they designed the system.

The stock vent system doesn't keep up with the increased pressures and volume of air from modified engines very well.
The MoCo somehow balanced the engine design factors to come up with a compromise that worked.
Once you change CC pressure / compression ratio and etc, that equilibrium is disturbed.

aswracing on venting mods: 115)

I've induced scavenging issues mostly from using gapless rings. But not from venting mods.
However, I've dyno tested venting mods until I'm blue in the face and never found a single horsepower there.
(except for pulling the blow by out of the intake tract, which is good for a small across the board improvement)
S&S cases have no scavenging issues due to the strategic placement of the reed valve (in the sump).
The scavenge inlet sees pressure but is isolated from the vacuum when the pistons go back up.


Click to read the full article on wetsumping in the REF section of the Sportsterpedia.
See also the Sportsterpedia page on Engine and Primary Oil System Modifications

Wetsumping during shutdown periods is a condition of bad oil pump sealing, bad check valve or regulator (if equipped) sealing.
The foregoing addresses wet sumping affect on horsepower.
Later engines are not competition engines. Maybe the earlier Sportsters were, but those days are long gone. 116)

Wetsumping (at sustained high RPM) is a condition when the oil pump isn't removing the oil as fast as it's feeding it.
If the cam box fills with oil, it comes out the breather and right to your air cleaner. 117)
It's been a chronic issue on XL's for years, happens on the 5-speed bikes as well as the 4-speeds and the ironheads.
But often on the head breather models (91-up), you never know like you do on earlier bikes with the breather on the cam box (pre 91).

  • Wetsumping can also be attributed to the Density of the air / oil mix in the crankcase. 118)
  • The higher the density (not volume) of the fluid (air / oil mix), the more it drags on the rotating parts it contacts.
    • As the density increases so does the fluid drag it imposes on the rotating parts (read flywheel assembly).
    • This drag robs power. That's why we mess with it, to reduce the power loss.
  • Example: If they are the same size (volume), what takes less power?
    • Stirring a cup of coffee or stirring a coffee milkshake? It's the one that's less dense.
    • So now we know that less drag = more horsepower and the air is the medium that gets the oil out of the cases.
      The 'leaner' (less oil in the air), the less the drag (air / oil is less dense). 119)

The oil pump was updated in '98 and then in '07 and you rarely see this anymore (while putting around town).
But it still happens on high rpm and race motors from time to time.
It's always the best sealed motors that have the issue, especially gapless ring motors.
Vacuum (45° configuration as mentioned above) in the crankcase interferes big time with scavenging.
The 98-up style pump can be fitted to the older bikes (they've even been fitted to ironheads).

The late model bikes can easily wet sump if ridden aggressively on the street, however. 120)
And they've been known to wet sump with only 3 back to back dyno pulls.
The results are dramatic when it happens. It is not anything like a barely noticeable loss in performance.
The scavenging gets behind to some degree in a single drag strip run.

Beyond that, about all you can do is lower the oil pressure.
But don't go down that path unless you establish for sure you are wetsumping and other measures won't fix the situation.
Read more here on a Homemade Oil System Bypass for reducing the oil pressure in the REF section of the Sportsterpedia.
This mod will send a small amount of pressure side oil back to the tank instead of into the engine.

Liquid Drag

This is an this example of 'liquid drag' (as opposed to fluid drag, our real life medium). 121)
Consider a 5 gal pail of latex & paint mixer that gets powered from your electric hand drill.

What's the difference between liquid and a fluid?
In this example, it's that a liquid is non-compressible (oil).
A fluid is compressible(air or air-oil).

Stick the mixer in the middle of the pail about 1/2 way to the bottom in the center of the paint mass.
Hit the trigger and the drill wants to twist out of your hand (liquid drag on the mixer).
As the mixer accelerates the paint, the drag reaction at the drill gets less.
And you can see the paint moving fast around the mixer and slow at the pail wall.

Eventually you steer the mixer near the wall to get that stuff mixed and an important change happens.
The reaction at the drill gets less, the drill speeds up and the paint near the mixer speeds up with it.
But the rest of the paint away from the mixer stops moving (as if its hanging in it's own 'miniature sump' away from all the commotion.

That explains the less reaction force on the drill.
You're moving less than the full 5 gal now (and moving that small amount better with less drag) even though the amount in the pail is unchanged.
This is important to understand.

Summary so far:
You're mixing the dickens out of 1/2 gal and cutting 4-1/2 gal out of the picture.
And that 1/2 is really moving and it's taken less force to move it because your moving less.
(less volume don't jive with the density-not volume- as in above)
It's exactly the same if now you change to a 55 gal drum.
1/2 gal going fast but 54-1/2 not moving.

So the addition of a sump (containment area) allows a greater quantity of oil (paint) to be present in the case (pail) without any extra drag.

Some of that oil is able to drop out of suspension so it can separate into the sump.
Once the used oil gets sump trapped things are going good.
But there are drag losses geting it to the sump as it flies outward off the rods.
Some will land on the inside of the case near & on the parting seam.
Some will travel down the inner walls of the wheels then fly off to the case wall.
Some will fly up under the pistons where it needs to eventually find its way to the case wall also.

In this chaotic environment, gravity isn't going to do much to drain it down to sump when there are giant flywheels whizzing 1/8“ from this case walls.
The wheels are going to set up a following flow on the walls.
The better the following flow, the less oil in commotion. That's good.

But good movement is because of good dragging.
But drag is bad.
Good dragging sucks power.
So does not dragging because the oil is slow moving.
Oil is now making the fluid more dense.

And what if you got no sump like 99.99% of 76< motors?
This kind of drag is the main liquid drag.
Its a 'no win' situation.
A robbing Peter to pay Paul situation.

Fluid Drag

Above, we've touched on the idea that oil in the flywheel cavity of the cases probably creates a drag on the rotating lower end, robbing power.
And the amount of oil probably affects the amount of drag.
More oil = more drag and causes it to increase the density of the fluid.
Fluid, not liquid.

This drag is like the drag that makes running in a swimming pool so difficult.
This drag is sometimes the only drag that gets considered.
The idea that 'the dryer the better' don't paint the whole picture.
That drag is smaller than the power used up to physically 'pump' the oil-air fluid as the motor spins.
The more dense the fluid, the more power is lost to pumping it.

No matter the density of this fluid, its the action of the pistons that moves it from the flywheel cavity.
On 76< motors, logically the way to accomplish this is to open the breather valve as the pistons fall so the max amount of 'fluid exhaust' occurs.
Then close the valve as pistons rise.
77> motors have a reed valve or umbrella flapper that accomplishes the automatic opening and closing of the 'fluid exhaust port'.
And it is not adjustable.

In this example when the valve closes, then the piston rise creates a giant vacuum in the case.
(with the vacuum being greatest at the highest point of piston travel)
Just after this highest point the pistons start to fall again.
This is when the valve opens again. (max exhaust right?)
This vacuum sucks the previous expelled fluid back into the case resulting in the crankcase not actually getting dry.

77> engines deleted the timed open and closed breather valve as is in 76< motors.
The camchest is always open to the flywheel cavity with a one way valve between the motor and the outside environment.
The slang term “FooFoo” comes from the annoying sound that it makes when it gets clogged up with oil residue. 122)

Bad Ring Seal vs Wet sumping

Wet sumping and oil spatters from the crankcase are two different things. 123)
Daily driver owners with oily air cleaners can get these two ideas confused.

Bad ring seal helps evaquate the sump from oil.
But on the other hand, it increases the flow rate through the crank vent system to such levels that a lot of oil droplets join in.
Apart from leaking cylinder base gaskets and push rod tubes, large blowby and high crank pressure also contributes to even larger ring leakage.
It's a vicious circle.

Sometimes owners who struggle with an oily air cleaner problem seem more concerned with wet sumping.
When they should concentrate in evaluating and improving ring seal.
And ideally also route the crankcase breather lines to a catch tank instead of the A/C.
Re-routing the vent(s) from the breather to a catchtank has many advantages.
Not least it is a powerful tool to monitor the state of the engine.
There should be more water than oil in the catch tank.
At least if air temp is 20C or below.

Oil Pump's Role vs Wetsumping

Scavenge oil flow to the pump is accomplished by the scavenging effect of the pump and by the pressure created by the downward stroke of the piston. 124)

Scavenge pumps need some means of getting the oil to the pumps. 125)
Or, rather, having a pressure differential between the pump gears and crankcase to blow/suck the oil/air mix from the crankcase to the pump mechanism.
With too high a crankcase vacuum scavenging properly can be a problem.
For best results, sufficent airflow to carry the oil as a mist or droplets is much better than trying to pump oil alone.
As in a vacuum cleaner:
It uses the airflow to carry the dust, it cannot just suck dust alone.
Using a container of dry sand:
If you hold the tip a little above the surface, the air being sucked into the vacuum carries the sand with it.
But, if you stick the tip deep into the sand, it will cease working.

Therefore, there is both a suction created by the pump and a pushing effect created by the downward stroke of the piston.

Ideally, the oil pump's scavenge side should remove more oil then the feed side can deliver into the engine. This prevents wet sumping.
However, there are times when it does not which contributes to wet sumping.
The downward stroke helps but it is the suction from the scavenge section that does the most work.

You would have suction from the scavenge pump if the oil was up to the level of the scavenge pump inlet (in the sump). 126)
But unfortunately there's sometimes air in there, foamy oil, etc. and the pump is working against a vacuum (condition) in the crankcase.
And the pump just can't move the oil out fast enough.

Looking at the bottom of the flywheel compartment compared to the inlet of the scavenge pump;
The bottom of the crankcase is below the inlet of the scavenge pump 0n 86-03 engines.
The inlet is a closer to the bottom on 04 Up engines.
Because of that the scavenge pump doesn't remain submerged in oil all the time.
There's air (which is carried by the oil in the form of bubbles) in that passage.
And the air & oil in the entire area, including that passage, are constantly subjected to the rapidly changing pressure of the crankcase.
You could say the pump cavitates momentarily and is working against a vacuum so it has a hard time recovering.

Scavenge inlet on 91-03 engines. 127) 04 Up scavenge inlet is lower to the bottom. 128)

Oil Tank's Role vs Wetsumping

See the full article, Oil Tank Pressure, in the REF section of the Sportsterpedia.
There should not be any pressure difference in the oil tank than the engine although it does transfer pressure.
The oil tank vent line to the cam chest allows the pump to send oil and CC pressure to the tank without over pressurizing the inside.
It is not there to vent the cam chest, the breathers do that.

During normal operation;
With the tank cap / dipstick removed, tank pressure is vented to atmosphere from the top of the tank.
With the tank cap / dipstick installed, tank pressure is vented to the cam chest.

So if you have pressure in your oil tank and the vent to the cam chest is not blocked then the cam chest is also pressurized.
If the cam chest is holding pressure, then your breather valve can not be venting properly.

Bottom line is that if the vent system is working properly, you shouldn't have excessive pressure build up in the oil tank. 130)

It has been said by many that lowering the oil level in the oil tank will stop oil puking out the breather.
While this may work in application, by design, you should not have to lower the oil level.
This practice is not just restricted to rubbermounts although due to the CC pressure change in 04, it is a more accepted practice.
The tank acts as an oil / air separator like the breather valve but the air only expels the engine from the engine breather vent. 131)

Lowering the oil level in the tank

*When you lower the oil level in the tank, you are just masking the real problem* .
Below are some arguments for lowering the oil level with responses below them.
Basically myths and truths.

  1. The oil vent line in the oil tank is too close in height to the full mark.
    • The vent line is actually higher than the full mark.
    • If the engine breathers are working properly, the oil level in the tank should never get high enough to enter the vent line.
    • If there is an engine breathing problem,
      The tank vent will not flow separated air back into the cam chest fast enough to keep up with the incoming flow of oil in the tank.
    • The oil / air backs up in the tank, but pressure is steadily coming up the scavenge line from the pump.
    • Aerated oil will take up more space volume than non-aerated oil.
    • Then the result could be oil (backed up) running back down this vent line into the cam chest during operation (acceleration and braking).
    • Below pic below is of an 04-09 oil tank with the top and bottom separated.
      • You can see how high the oil has to rise to get into the vent tube. It basically has to fill up the tank first. 132)

  2. The extra oil in the bottom end is thrown around more inside the engine and finds it's way up into the heads during the breathing operation.
    This excess oil is then purged out the head breathers.

    • Any oil that leaves the vent in the oil tank flows back into the cam chest then gets scavenged via the cam chest port in the pump.
    • All oil in the cam chest can be subject to being picked up into suspension no matter of it's origin.
  3. Having too much oil in the tank will cause more blowby.
    • Too much oil in the tank does not cause more blowby. 133)
    • Oil in the engine must:
      • 1. Lube
      • 2. Clean
      • 3. Cool
    • If you take away what the factory says to run then you take away from one or all of those 3 properties.
    • Blowby is a condition when air pressure blows by the rings or valves.
    • Blowby has some to do with oil puking out the breathers, but it doesn't cause it by itself.
    • Oil puking out the breathers is an imbalance in the breathing system which may or may not have to do with the rings or valves.
    • The oil level only matters if the tank has been overfilled past the top mark on the dipstick. 134)
    • This is easy to do if you check the oil level and fill the tank BEFORE heating up the engine to operating temperature.
      See more about Over filling the oil tank in the REF section of the Sportsterpedia.
    • Overfilling the oil tank can bring the level up over the vent to the crankcase.
      This would stop up the vent to the cam chest, over-pressurize the tank and hinder scavenging from the sump.
      Increased sump oil (not able to be scavenged fast enough) aides in oil suspension or air /oil density increasing crankcase pressure.
      Increased crankcase pressure aides in oil puking out the vents.
  4. Running the oil level on the bottom dipstick line helps curb blowby.
    Running the oil level at the middle to high side of the dipstick causes more blowby.

    • Running the oil level on the low mark of the stick effectually lowers the amount of oil that would collect in the sump.
    • It also lowers the amount of oil circulating which could raise oil temps and starve mechanical parts of oil.
      (I.E. lifters, rockers, bearings etc.)
    • Lowering the amount of oil in the sump also leaves less oil there to get pulled up into suspension which thins out air / oil mist.
    • It's the more dense air / oil mist that is the problem, not the oil level in the tank.
    • Many people say that it is important to leave plenty of empty space in your oil tank. 135)
      • If you don't have excess crankcase pressure, you don't have to do that. You can fill the oil tank to the full mark on the dipstick without any issue.
      • The oil tank is a sealed space with a seal / O-ring on the dipstick.
        The oil tank space is connected to the crankcase space via the vent hose that runs from the oil tank to the cam cover.
        By filling the oil tank only half full, you create a larger air volume in the crankcase space itself.
        This increased air volume can help buffer the pressure in an engine with only a slight problem with blow-by gasses combined with the restricted head vents.
      • However, if you either;
        • Restore your engine to like new low levels of blow-by.
        • Or if you improve the engine's ability to vent the pressure.
          Then you don't need the extra buffer space provided by half filling your oil tank.
          You can go ahead and fill your tank with the proper amount of oil.
          Which means you have more oil to circulate through the engine which is a good thing.

So the answer is to address the air/oil density, not the oil level.

Removing the oil cap - engine running

With the oil cap off the tank on a hot idling engine (so far rigid S models only), the engine speed can drop app. 1,000 RPM at 1050 idle. 136)

Another example with oil temp 210, raised idle to 1200 and lost 700 RPM.
So the exact power loss is variable but true.
Barely opening the cap or fully removing it had no affect on RPM.

This is the same as having a bad breather valve opening but not closing.
Oil doesn't puke out the tank since higher density suspension oil comes up the return line from the sump.
The suspension fluid and return oil together in the line helps to separate the oil back out of suspension by the time it reaches the tank.
The bulk of air / oil mist is generated in the crankcase, not the cam chest where the vent line to the tank is.
(leaving most of what comes out the top of the oil tank to be air).
The higher air/oil density drags the flywheels and more oil is picked up in suspension with the engine breathing both in and out.
The engine responds at idle from the higher load on the wheels.
Read more on Fluid Drag above.

Venting the oil tank

Neither the oil cap nor the oil tank should be vented without a one way check valve / pcv.
The tank is vented to the cam chest on purpose to remove condensate (that gets pumped in) from the oil tank.
Venting the oil tank \ cap also will negatively influence crankcase venting by letting in extra air.
1/2 of the return flow is air because the return pump is twice as big as the feed pump. 137)
That air is full of condensate.
A slightly negative (to atmospheric) pressure in the tank facilitates vaporization of the condensate.

Air Leaks

The breathing system is designed for a one-way valve venting system.
Air goes out but doesn't come back in.
Air leaks (into the engine) will increase positive pressure and air / oil density = oil puking out the breathers.
Some potential air leak areas are in the pic below.
If these areas allow air to be pulled in the engine on upstroke, the added air will compound any other existing breathing problems.
138) 139)

Vacuum Pump for Reducing Crankcase Pressure

Most of the information on the web involves the use of a vacuum pump on autos.
There's not much published on using them with motorcycle engines.
That doesn't mean anything other than speed shops may not want to divulge their secrets. Also big pumps are for big displacement autos.
You can use a vacuum pump that's too big for a Sportster engine and cause more harm than good.
They are used for compensation as well as for better ring seal, but mostly advertised for better ring seal.
There are pumps spec'd for vacuum measurements and also ones spec'd by RPM range.
But, along with the addition of a vacuum pump, there is also the addition of a performance multi-stage oil pump.
If you vented from the crankcase area:

Splash is an important element in the sump area. Too much vacuum and you lose scavenge ability of the oil pump.
You may be able to tap into the side-top with a vent line and a reed valve.
The more the vacuum, even lower the positive will begin.
The rings act as a buffer between these two conditions as excess pressure could run both directions.
But positive pressure aides in oil scavenging.
So lowering positive pressure by default also hinders scavenging.
That's why racers with vacuum pumps use multistage scavenge pumps to account for the imbalance to scavenge and improve it.

Regardless, it's evident that some racers use vacuum pumps to increase vacuum pressure in the crankcase of a Sportster engine. 140)
It's been said that positive crankcase pressure upon piston upstroke gets between the rings and basically causes bad sealing at the ringlands.
This is also in the high RPM range when ring flutter is present. So there are several things happening then.
But racers record higher power when using a vacuum pump.
However, it has also been said that inducing higher vacuum in a street engine may do more harm than good. Lower RPM may suffer from the imbalance.

A vacuum pump, in general, is an added benefit to any engine that is high performance enough to create a significant amount of blow-by. 141)
(that's high performance enough…. not worn enough)
It will, in general, add some horse power, increase engine life and keep oil cleaner for longer (in a high performance engine).

The first thing that happens on downstroke is that positive pressure (greater than atmosphere) is generated due to the restrictions of:

  • Case volume
  • Path to the vents
  • Vent hole size
  • Vent line (if applicable) length
  • Any induced air from the breather valve(s) not closing properly

These things will bring the pressure inside to higher than atmosphere, else there would be nothing to expel.
As the air is pushed out of the vent, at BDC, the air returns to atmospheric.
That is the problematic condition, the higher pressure before returning to atmosphere.
Blowby adds to positive pressure which throws out the balance.

  • Using X (+1) as positive and Y (-1) as negative pressure.
  • In a perfect world, X goes down and Y comes up —— X+Y=0.
  • Add ring blowby and you get —— X+1>Y—— or the real result.
    Balance is off by nature of the moving parts.
    The pistons move up and down almost together.
    That makes the push/pull environment more violent.
  • Now add a vacuum pump with Z (-1) amount of pull.
    Now you get X+1=Y-Z… seems the balance (to zero) is restored even though positive pressure is compiled of blowby.
    But the lower the negative pressure is at the beginning of the downstroke, the lower the next positive pressure will be.
  • If you are generating 2 psi of positive pressure on downstroke but reduce it's beginning surge to -1 psi (Z),
    the result will be only 1 psi of positive pressure during downstroke.
    -1 (+) + 2 equals +1.

Another example: 142)

Considerations for running a vacuum pump:

  1. A vacuum pump will create negative pressure in the crankcase. 143)
    And that negative pressure will remove air mass and create less atmosphere on the bottom side of the rings.
    (which creates a more stable environment)
    So positive pressure is removed around the rings / ringlands.
    There is no opposing force to keep the rings from seating on the ringland bottom.
    On piston upstroke, the rings sit against the bottom of the ringlands.
    On piston downstroke, the rings sit against the top of the ringlands.
  2. The more stable environment change has allowed engine builders to reduce the size and tension of the ring stack; 144)
    creating less friction, less heat, and less power being robbed from the combustion process.
    Better ring seal is a nice advantage too.
  3. The major gain with vacuum pumps comes from sealing the top ring better on the intake stroke; 145)
    Pulling more air and fuel in with better ring seal, which allows for a bigger charge to be burned and making more power.
  4. It’s not just the rings, it’s the whole package; pistons, rings, quality machine work to get the right cylinder finish, and a proper tuneup. 146)
  5. Piston downstroke (positive pressure) aides in sump oil scavenging.
    And unfortunately, vacuum pressure fights the oil pump.
    You need more positive pressure in the crankcase to help force the oil out of the sump as the oil pumps pulls vacuum on the sump.
    However, if the vacuum on upstroke is lower, there will be more than normal negative head pressure by the time the downstroke happens.
    Less pressure on downstroke means less force pushing against sump oil to scavenge.
    Too low of vacuum head pressure when downstroke begins and you end up with more oil left in the sump.
    (which marches toward a wet sumping condition)
    So a multi-stage oil pump that doesn't depend on crankcase pressure assist will be better suited with this setup.
  6. The updraft on sump oil is lower, creating more loose suspended oil.
    The updraft is what aides in bringing oil into suspension with the air.
    Once suspended, the 'mix' is able to 'float' and it will move in the same fashion that air will move (wherever it's pushed or pulled).
    The mix separates on impact when it hits the crankcase / cam chest walls, cams / bushings etc.
    Loose suspension (lower pressure) drops the air /oil mix ratio faster upon impact.
    Tight suspension (higher pressure) drops the mix slower.
    During high CC pressure, more oil is left into suspension by the time it reaches the breather valve.
    When it hits the breather valve (on impact) more oil stays in suspension past the valve and out the vent.
    Lower pressure hitting the valve drops the ratio fast enough that less oil is left in suspension by the time it reaches the vent.
  7. Lower vacuum in the crankcase also hinders splash oil due to the lower updraft.
    So it is possible to starve splash lubrication in the interest of lowering crankcase pressure.
    Windage is also lowered and this is the propellant for splash oil.
  8. Crankcase pressure is lowered even more below atmospheric pressure.
    However, combustion chamber blowby (thru the rings) adds positive pressure to the crankcase at the same time it's being lowered.
    So there is a balance there like when you turn on a single water faucet.
    • 1/2 a turn cold, half a turn hot gives you warm water.
    • 1/2 a turn cold, full turn hot makes the water hotter.
  9. Gapless rings allow less blowby during upstroke which creates less fill pressure in the crankcase.
    (thus, lowering vacuum head pressure at piston downstroke)
    This may be the reason gapless rings increase wet sumping.

In summary:

Racing bikes can pump enough vacuum into the engine to create better ring seal = more power at high RPM.
This is fine as long as the oil pump system is modded to return more oil to the tank by itself, without the need for CC pressure assist.

The use of a vacuum pump on a Sportster street engine can easily create wet sumping issues.
Street bikes will only occasionally see high enough RPM to warrant a vacuum pump but even then still running on the OEM oil pump.
So the possibility for wet sumping goes up on them.

If you want to run a high level of crankcase vacuum (18 inches HG or more); 147)
There must be provisions in the engine to supplement the lubrication loss (splash oil through windage).
There can be problems with at least wristpin lubrication also.

Running a vacuum pump also would require scheduled diagnostics.
The amount of vacuum pulled depends on the general status of crankcase pressure at the time of use.
(I.E. current conditions such as; ring seal, breather valve wear, vacuum leaks, head valve leaks etc.)
You can't just install one and forget about it else you've defeated the purpose of installing it.

So it is possible to run a vacuum pump on a street engine.
But there are more considerations than just hooking one up.

8) , 14) , 16) , 17) , 18) , 19) , 27) , 28) , 48) , 50) , 51) , 52) , 53) , 54) , 97) , 129) , 142)
drawing by Hippysmack
9) , 15) , 29) , 79) , 99) , 139)
photo by Hippysmack
photos by aswracing of the XLFORUM
1960 HD FSM pg 3a-15
HD Service Bulletin #M-848 dated April 9, 1982
photo by Phillober of the XLFORUM, labeled by Hippysmack
photos by Bored now of the XLFORUM, annotated by Hippysmack
photo by WestJC7745 of the XLFORUM
photo by dagger rider of the XLFORUM
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