How Do Motorcycle Exhaust Pipes Work?

Instead of talking about stress concentration leading to cracks and failure, I'm going to talk about how things work when things are just fine. In this case, exhaust pipes. What we hope to accomplish with a correct design is to convert some of the otherwise-wasted exhaust energy coming out the exhaust port or exhaust valve into something useful for making power.

Years ago, the late Gordon Jennings in his little book, Two-stroke Tuner's Handbook, explained how a two-stroke pipe works, and it still works the same way. When the piston begins to expose the exhaust port, a pulse of high-pressure gas—80 to 100 psi—enters the pipe. It travels at the local speed of sound, which, because the exhaust gas is very hot, is very high, like 2,700 feet per second.

When it encounters the widening part of the pipe, the exhaust gas can expand somewhat. When it does, it sends a wave of expansion back to the cylinder. So the whole time the exhaust pulse is traveling down this widening part of the pipe, it is continually sending back a low-pressure signal. Upon reaching the cylinder, it increases the pressure difference across the transfer ports and helps pump fresh charge into the cylinder from the crankcase.

During the time no action is required, the pressure wave travels down a center section of constant diameter. Meanwhile, back at the engine, the piston has reached bottom center and is starting back up on compression. It closes the transfer ports but the exhaust port is still open. The rising piston may push fresh charge out the exhaust, thus wasting it. But a converging or reverse-cone pipe sends back a wave of positive pressure.

That wave arrives in the cylinder just as the piston is about to close the exhaust port, and it pushes any fresh charge lost into the exhaust port back into the cylinder, the piston closes the port, and compression continues as the piston rises. So a two-stroke pipe is an air pump; it helps the pumping of the engine. Normally we think the crankcase in a two-stroke pumps fresh charge into the cylinder, but it gets a lot of help from the pipe.

In the four-stroke case, the pulse of pressure—again, maybe 100 psi at full throttle—travels at the local speed of sound down the header pipe until it reaches a junction, where it joins one of the other two headers. That’s a point of expansion. Wherever there is a point of expansion, a negative wave of pressure is reflected back up the pipe. If the engine is operating at the designed rpm for this effect that low pressure will arrive in the cylinder during overlap.

This negative wave arrives just in time to draw out the exhaust gas that is sitting above the piston at top dead center. That low pressure propagates across to the intake valve and starts the intake process early, before the piston has even started to move down. So, in effect, this pipe is advancing the occurrence of some intake by sending back a negative wave. In the rpm range at which this effect is designed to operate, it boosts torque.

At some lower speed, instead of being a negative pressure wave with its beneficial result, the wave is positive, which blows more exhaust gas back into the cylinder, into the intake pipe, and even into the airbox. This produces the dreaded “flat spot.” All you can do is convert from one type of pipe, a 4-into-1, into another, a 4-into-2-into-1. That provides a second negative point of expansion, which can cancel part of the positive wave.

So the flat spot is filled in, maybe not completely, but it’s not a place of no return, thereby increasing acceleration. When I realized this is how a 4-into-2-into-1 worked, I walked around the garage area in Daytona looking for a 4-into-1. I didn’t find a single one. Every four-stroke racing at Daytona that year was using a 4-into-2-into-1 because it improves acceleration by keeping torque high most of the time.

Kevin Cameron has been writing about motorcycles for nearly 50 years, first for Cycle magazine and, since 1992, for Cycle World. Kevin’s unparalleled experience and knowledge of the sport were—and continue to be—prompted by a lifetime of curiosity. His willingness to share that information with anyone who is willing to listen is likewise unique.

Kevin’s greatest strength lies in his ability to present complex subjects in simple terms with clarity and, often, humor. In this video series, shot in his home shop, Kevin draws upon his vast historical references to address modern-day questions. As Kevin has written, “Emotions bring us to engineering, but engineering then becomes a special way of confronting reality.”