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Exhaust Dynamics
by: ragtop

Contrary to common popular belief, a larger exhaust does not always result in better engine performance. There are situations where an engine could loose low-end torque from "too little" backpressure. I know it seems counter-intuitive to think that an engine actually needs a certain amount of backpressure but here's why. Most engines are set up from the factory for a certain level of backpressure. Changing the exhaust can create a situation where the cam has too much overlap for the RPM range it's being driven in. In that case, the incoming fuel/air will come in the intake valve, only to have part of it sucked straight out the exhaust valve without getting burned! Obviously, engine power will suffer if it doesn't get a full fuel charge to burn. Proper backpressure will prevent this. So will choosing a more appropriate cam for your RPM range, however.

There is another, more complex reason why 5" pipes on a normal sized naturally aspirated engine won't work. It isn't that the engine needs more backpressure, it's that static pressure is only half the equation. The equation for the "equivalent" pressure at the exhaust port is P-pv^2, the static pressure minus the exhaust density times the square of the exhaust velocity at the port. A 5" pipe may see a slight reduction in static pressure but will kill off the velocity making it harder to push the exhaust out. Some tuned header systems can make the exhaust velocity high enough that the engine effectively has a lower amount of backpressure than the atmospheric pressure! This is known as exhaust scavenging and is what separates good headers from bad ones.

In the case of a turbocharged car, everything is different. They can run a huge exhaust pipe, like the HKS 5" pipes and see a performance gain rather than loss. The reason is the turbine. The exhaust coming out of the cylinders only sees the velocity going into the turbine. The velocity drop across the turbine doesn’t effect flow. On a turbocharged engine, there is no need to worry about the exhaust velocity downstream of the turbine. The size of the header primary tubes and collector, or the exhaust manifold design, still plays a larger role in determining exhaust velocity, but the pipes downstream of the turbo are a lot less important.

As for the turbo itself, you want to maximize the pressure (and temperature) difference across the turbine for the highest efficiency. A low velocity of the gasses exiting the turbine won't make it any less efficient; in fact, it can theoretically improve the efficiency of the turbine. So the exhaust on a turbocharged car can be designed to minimize static pressure, without concern for the exhaust velocity.

So what does all of this mean? It means that while a 5" exhaust would be desirable for a 350 hp turbocharged Supra, it would be a very bad idea on a 350 hp naturally aspirated Camaro and an awful idea on a 1.8 liter naturally aspirated Honda.


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