Fuelly Forums - Actual volume of exhaust gas

The thing to remember here is that air and exhaust gases have mass. You exhaust valve opens and the exhaust gases have to get whatever volume/mass of exhaust moving as is sitting dead in the pipe. Then if it's slow to get moving, due to having large volume/mass relative to the exhaust gas, the exhaust gas and the stack of it along the pipe might present resistance to the upcoming piston. It may eventually gain the same momentum per unit length as a smaller pipe, due to more mass moving slower vs less mass moving faster, but it's somewhat "stretchy" as it were, and cylinder scavenging will not be as efficient. You get it moving at high enough velocity down the pipe and it can actually pull on the piston slightly and as the intake valve cracks, transfer significant momentum to the intake charge. However, there's a sweet spot between the resistance due to viscosity and surface friction in the pipe and the resistance due to stagnant gases in the pipe from previous strokes. This sweet spot compromise is often tuned towards high RPM often higher than stock redline, or to forced induction motors, which do not have valve overlap and don't need scavenging. Ignore the "biggest exhaust you can afford" advice from forced induction folks, it ain't like that for N/A.