There is a lot of confusion about how exhaust systems work so I’m going to try to explain an ideal exhaust system and its affects on the engine. First I would like you to disregard any rumors that you have every heard about exhaust systems. There is a nasty rumor that has been around for years that you need backpressure in an exhaust to give your engine more torque and keep from burning valves. This is completely false.

First, I’m going to talk about the design of an exhaust manifold, also referred to as a header. Exhaust travels out of the cylinder head in pulses each time the piston pushes it out of the cylinder bore. Naturally when there is a pulse of gas, there exists a high pressure and a low pressure behind it. If you arrange the header pipes such that each pipe connects to the other pipes in such a way that it creates a vacuum and pulls the exhaust out of the cylinder that is currently firing. One part of the design that most effectively utilizes the vacuum created by the prior exhaust pulse is the use of equal length tubes. If the tubes are equal in length, then it takes an equal amount of time for the exhaust pulses to reach that point so that the pulses are separated equally in time.

Another method of increasing the vacuum created from the pulse is by using a tube that increases in diameter down the length of the tube. This is referred to as a stepped header. In general there is a great deal of time manufacturing this type of header so they are almost always going to also be equal length and probably relatively expensive.

An anti-reversion header design is also another feature that increases power by blocking reflected waves from reaching the exhaust valves at the incorrect times. If a reflected wave were sent back toward the exhaust valve just as it opened, it would slow down the flow of exhaust out of the cylinder, which is not good. Anti-reversion pockets can be placed at calculated lengths down the primary exhaust pipes that reflect waves to the valve when it is closed so it does not hinder the flow. Another method of achieving this is by changing the shape of the primary exhaust pipes to some shape that minimizes this effect. Some headers made with D-shaped primaries have shown some success in the past.

All of these same principles apply to the exhaust system after the header also. The ideal exhaust system has a very high delta pressure between exhaust pulses, which maximizes exhaust scavenging. If the piping diameter is too large the delta pressure is reduced and that also reduces the ability of the exhaust pulses to assist each other. Ideal exhaust piping diameter varies greatly between cars. The diameter of the piping should be chosen based on the amount of exhaust flow and the desired power band of the engine.