It may seem intuitive that bigger, more open exhaust would be the best for engine performance. Some believe that there is a proper amount of exhaust back-pressure for optimum performance. Both of these theories are wrong. There are a few key things that make exhaust flow different than other fluid flows. First of all, the exhaust is coming out in pulses. When each cylinder exhausts, a large surge of flow enters the exhaust system. Then it stops again until the next exhaust stroke. True, larger exhaust ports and tubes will flow better but flow is not the only thing to consider here. If you size the exhaust tubing properly, you can take advantage of the inertia of the exhaust pulses similar to what can be done on the intake side. When the cylinder exhausts, the pulse builds a pressure in the exhaust tube. Once the valve closes, this exhaust pulse is traveling down the tube with velocity. Because of this, a slight vacuum is created at the valve. This pressure-then-vacuum pulse will travel all the way through the exhaust system assuming the restriction is low and the tube diameter does not change much.
The tubes for each cylinder to the collector are called primary tubes. These primary tubes can be sized, both diameter and length, to optimize the exhaust inertia while keeping minimum restriction. It is a trade-off and will only be optimum at a specific RPM and load. This is very similar to optimizing the intake system and the same key things apply. For low RPM performance, a small diameter, long tube will give the best port velocity, and exhaust inertia. For midrange performance, a shorter length, larger diameter runner is best. For very high RPM, a very short, large runner is needed because restriction is the biggest problem at high RPM.
By grouping primary exhaust tubes into a collector, the vacuum pulse from one cylinder can be used to create a vacuum on the other exhaust ports. This vacuum will cause the exhaust to begin flowing more quickly as the exhaust valve opens. The collector must be long enough and small enough to keep the inertia of the exhaust pulse optimized while keeping back-pressure low.
Back-pressure is something we are stuck with, not something we want. By optimizing the inertia affect, some back-pressure will be realized during the peak of the exhaust pressure pulse. It is not something you want but rather simple physics.
Once you get past the end of the collector, you want to release the exhaust with as little restriction as possible. The end of the exhaust header is not really the end of the collector. The collector area in the exhaust should usually be about 18" to 24". If you have 2 banks of cylinders with each bank having it’s own collector, you will also have alternating pulses from each bank. In order to reduce restriction, you should place a tube between the banks at the end of the collector area. This will allow pressure pulses to swap back and forth which will even them out and allow smoother flow through the rest of the exhaust system.
At very high RPM, the restriction becomes a much bigger problem. Because of this, attempts to utilize exhaust inertia are generally abandoned in favor of reducing restriction.
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