EVO: Carburetor, Intake Manifold & Exhaust

Exhaust

  • 1998 Swiss model Sportsters were equipped with catalytic converters. 1)


Studs

“Use the factory studs - They're the only ones out there
with interference fit threads on the head side. 2)


Gaskets

Cometic (bottom) versus HD (top) exhaust gasket comparison 3)


Exhaust Pipes & Cams - Power or Reversion

All exhaust systems have harmonic pressure waves passing down and up the pipes. These waves are initiated by the operation of the exhaust valves. The waves move back and forth, with new waves combining or competing with the existing waves in the exhaust system, to either help move the exhausted mixture out of the pipes or back toward the cylinder. In a complicated way, the rpm of the engine is creating a certain frequency of new waves while the length and shape of the exhaust system is reflecting those waves at its own frequency.

This complex combination of frequencies causes each exhaust system design to have it's own characteristics of helping the combustion cycle at certain rpms (making more power) and hindering that cycle operation at other rpms (creating reversion and less power). The trick is to match the right exhaust system and cams so they are working together to efficienctly create the most power.

In the following quotations, of an XLForum discussion about stock 'D' cams versus Andrews 'N4' cams, Aaron Wilson provides a very good explanation:

Quoting Aaron Wilson from the XLForum (aswracing):4)

Well, contrary to what you might hear, the N4's are quite capable of delivering more bottom end than the stock D cams. The reason is that they have overlap, where the “D” cams have very little.

Overlap is the period in time when the exhaust valve is almost closed and the intake valve starts to open. There's a window of time when both valves are open. The piston passes through TDC during this window, finishing it's exhaust stroke and starting it's intake stroke.

Imagine what's going on right then. A whole bunch of hot, high pressure exhaust has just left the cylinder. Sitting in the intake tract is a fresh, relatively cool, air/fuel charge sitting at low pressure. Both valves are open.

The piston starts down, and starts pulling. Where is it going to pull from? The answer is that it wants to pull from the exhaust. That's where all that hot, high pressure is sitting.

So what makes it pull from the intake instead?

It's extremely critical that the exhaust system creates a suction wave right then. If it does, the suction wave will actually pull on the intake tract (remember, both valves are open) and get that fresh charge moving before the piston even starts pulling on it. If the exhaust system does this, it greatly helps cylinder fill.

On the other hand, if the exhaust has high pressure right then, it'll actually push the intake charge back out the intake tract, really screwing up the cylinder fill. What's more, the intake charge will go back out the carb. Then when the piston goes down and the exhaust valve closes, the piston will pull on the intake again. The intake charge ends up passing through the carb three times: in, out, and back in again, picking up fuel each time.

This phenomenon is called a “reversion” and it's readily identified by a dip in the torque curve (a torque curve is essentially a map of the cylinder fill) and a rich area in the a/f ratio. Also, you generally see a fog just outside the carb opening (called a “stand-off”).

The bottom line is that overlap connects the intake to the exhaust, and as such, gives the pipe a lot of control over the cylinder fill. When you put those N4's in your bike, you're going to be making the exhaust system really, really critical.

From 96-02, Buell made a lot of bikes with N4's in them from the factory, and also a lot of bikes with “D” cams factory. The N4 equipped bikes (S1's, X1's, S3's) had a huge hole in the middle of their powerband, because the factory pipe reverted like a bitch through the midrange. The M2 didn't have this hole, even though it had the same exhaust, because it's “D” cams had so little overlap. So as delivered, it was the torque bike, and the others were high horsepower bikes (the pipe pulled at high rpm).

HOWEVER, the Buelling world quickly figured out that by replacing the factory muffler on their S1/X1/S3 with a good aftermarket piece, they could fill in that hole in the powerband, and when they did, it was actually stronger through the midrange than an M2 with the stock pipe. The fact of the matter is that overlap with a pipe that's pulling is a good thing; it helps cylinder fill greatly. Even though the intake close timing of the N4 is biased up the rpm range, it was making more torque than the “D” cam equipped M2 because the extra overlap coupled with a pipe that pulled through the midrange more than made up for it.

So in summary, I'd say yes, the N4's are an excellent choice, but be sure your pipe works for you, and not against you. I really like Supertrapp 2:1's myself, for street use.
(end quote)

Quoting Aaron Wilson from the XLForum (aswracing): 5)

With respect to your question on the powerband, it has an enormous amount to do with the pipe.
The N4's have massively more overlap than the D's. So the pipe is going to be in control.

If your pipe is pulling hard at low and mid range, you're actually going to gain power in the low and mid range. Basically, wherever your pipe pulls, that's where you'll have power. If the pipe pushes back at some rpm, though, these cams are going to cost you a bunch of power there.

The shape of the torque curve tells the story. It's a map of cylinder pressure, which for all intents and purposes is a map of cylinder fill. A perfect torque curve is table flat. Where it comes up from that tends to be where the pipe is working. Where it drops from that tends to be where the pipe is pushing back. When it's pushing back, the afr also tends to go rich.
(end quote)



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