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techtalk:ref:engmech07e [2020/10/02 21:38] hippysmack |
techtalk:ref:engmech07e [2023/12/26 21:28] (current) hippysmack |
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| ====== Further Study of Internal Engine Pressure ====== | ====== Further Study of Internal Engine Pressure ====== | ||
| - | All HD cases already pull a vacuum especially highest at idle. That vacuum just can't get too high. How high is the question. \\ | + | It's important to note there are 2 different but similar breathing systems in ironheads; (1957-1976) and (1977-1985). \\ |
| + | However different, the 2 breathing systems are more similar than different. \\ | ||
| + | When they spilt the two, 77-up still uses the same principles as 76<. They both rely on vacuum/pressure cycles to scavenge oil and make the engine breath. \\ | ||
| + | Downstroke still pushes oil to the pump in both. Upstroke still creates splash lube in both. \\ | ||
| + | 76<, the crankcase breaths out the timed breather into the gearcase then spills out the vent. \\ | ||
| + | 77>, the crankcase breaths to the gearcase thru holes in the cam wall then out the one way valve in the cover. \\ | ||
| + | If anything they freed up the crankcase by allowing oil and air to expel to different holes. \\ | ||
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| + | All HD cases already pull a vacuum especially highest at idle. That vacuum just can't get too high. How high is the question. ((Hippysmack of the XLFORUM https://www.xlforum.net/forum/sportster-motorcycle-forum/sportster-motorcycle-era-specific-and-model-specific/ironhead-sportster-motorcycle-talk-1957-1985/200736-79-85-breather-vent-mod-adding-a-secondary-vent-hose-at-6-00-position/page6?t=2077771&page=6)) \\ | ||
| At present there is little information known regarding how much vacuum Ironhead Sportster engines create. \\ | At present there is little information known regarding how much vacuum Ironhead Sportster engines create. \\ | ||
| The only way to know for sure on any specific engine is to test it manometer and see where it's at. \\ | The only way to know for sure on any specific engine is to test it manometer and see where it's at. \\ | ||
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| * Better oil scavenging and keeping down pumping loss at high RPM. \\ 76< engines breath and scavenge from the same hole. \\ They may be more sensitive to wetsumping than later models because of this. The air can't jump ahead of the oil to the rotary breather. \\ Oil is thicker and doesn't compress. What does that do to air pressure behind it? \\ 77> engines breath and scavenge from different holes. \\ Makes for a more free flowing internal system. | * Better oil scavenging and keeping down pumping loss at high RPM. \\ 76< engines breath and scavenge from the same hole. \\ They may be more sensitive to wetsumping than later models because of this. The air can't jump ahead of the oil to the rotary breather. \\ Oil is thicker and doesn't compress. What does that do to air pressure behind it? \\ 77> engines breath and scavenge from different holes. \\ Makes for a more free flowing internal system. | ||
| - | We know downstroke pushes oil toward the oil pump. \\ | + | We know downstroke pushes oil toward the oil pump. Excess vacuum slows that process, can back up oil and cause wetsumping. \\ |
| - | Excess vacuum slows that process, can back up oil and cause wetsumping. \\ | + | But, if the underside of the pistons are slight negative, it helps the rings seat without as much vibration and turbulence. \\ |
| - | But, if the underside of the pistons are slight negative when downstroke begins, it helps the rings seat without as much vibration and turbulence. \\ | + | As a rule, rings sit on their own springiness when pistons go up and are forced against the wall by the combustion pressure that is allowed to pass behind them. ((thefrenchowl of the XLFORUM https://www.xlforum.net/forum/sportster-motorcycle-forum/sportster-motorcycle-era-specific-and-model-specific/ironhead-sportster-motorcycle-talk-1957-1985/200736-79-85-breather-vent-mod-adding-a-secondary-vent-hose-at-6-00-position/page6?t=2077771&page=6)) \\ |
| + | Hence, a very small or inexistent amount of blowby should occur on combustion stroke. \\ | ||
| + | A small amount of blow by is only possible on the way up on exhaust stroke if vacuum is too high underneath pistons. \\ | ||
| + | If vacuum underneath the pistons is too low, worn rings, too much end gap, reverse happens, top end will suck up oil from underneath the pistons. \\ | ||
| So a slight vacuum is a good thing. \\ | So a slight vacuum is a good thing. \\ | ||
| Downstroke is relieved somewhat underneath due to the breather opening and beginning vacuum on decent. \\ | Downstroke is relieved somewhat underneath due to the breather opening and beginning vacuum on decent. \\ | ||
| Upstroke is not affected as much until after vacuum starts to build. \\ | Upstroke is not affected as much until after vacuum starts to build. \\ | ||
| - | The 57-76 timed breather doesn't close on BDC but rather farther upstroke. \\ | + | The 57-76 timed breather doesn't close on BDC but rather farther upstroke. \\ |
| + | It presets how much vacuum the pistons are allowed to create and how long they have to expel air pressure.\\ | ||
| So the rising piston isn't hindered by vacuum until after mechanical and perpetual motion has been exerted on the it. \\ | So the rising piston isn't hindered by vacuum until after mechanical and perpetual motion has been exerted on the it. \\ | ||
| In in 76< engines, you are limited to what air you can expel thru the breather gear before upstroke happens with the gear still open. \\ | In in 76< engines, you are limited to what air you can expel thru the breather gear before upstroke happens with the gear still open. \\ | ||
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| Once the breather valve closes, the gearcase returns to atmosphere. \\ | Once the breather valve closes, the gearcase returns to atmosphere. \\ | ||
| In regard to modding the breathing system; \\ | In regard to modding the breathing system; \\ | ||
| - | **Note**: \\ | + | **Note**: Adding a one way valve to the atmosphere vent with a timed breather should create more vacuum. \\ |
| - | Adding a one way valve to the atmosphere vent with a timed breather should create more vacuum. \\ | + | |
| Atmosphere is normally pulled back into the timed breather until it closes up the piston travel. \\ | Atmosphere is normally pulled back into the timed breather until it closes up the piston travel. \\ | ||
| The one way valve blocks the air drainback function starting from BDC instead and does tax the pistons (more than OEM) by making them work harder on the way up. \\ | The one way valve blocks the air drainback function starting from BDC instead and does tax the pistons (more than OEM) by making them work harder on the way up. \\ | ||
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| In contrast, the 77-up breather closes when the last piston is at bottom dead center (BDC). \\ | In contrast, the 77-up breather closes when the last piston is at bottom dead center (BDC). \\ | ||
| - | Is that extra vacuum taxed to the piston? | + | Is that extra vacuum taxed to the piston? \\ |
| The better the gaskets and seals, the more vacuum you can run. \\ | The better the gaskets and seals, the more vacuum you can run. \\ | ||
