I see many references to the need for some backpressure for maximum horsepower. Some tie this to low end power. I have very considerable experience with piston aircraft engines, where the typical exhaust is basically like a set of headers. Some are even individual straight pipes as on WW2 fighter aircraft. Now I could not stand the noise from a straight pipe setup on my car, so I am not looking to go even close to that, but just for my education can someone explain why backpressure can be beneficial?

 

If the car has a hot camshaft, some of the intake charge gets blown out of the exhaust during valve overlap period (both intake & exhaust valves open at the same time). This effect disappears at higher revs. So some back pressure will help the engine produce more torque at low RPM by cutting down on the amount of blow trough. On the other hand, at higher RPM, back pressure will prevent the cylinder from getting a full charge because some of the burnt gases did not have time to get fully exhausted. This means you need to find a happy medium. A way to get that, is with long tube headers which are tuned for low end power, but can flow enough to also reduce back pressure at high RPM.

 

Pete, thank you for that very clear explanation. In an aircraft engine, you are at low RPM's only while taxiing. At take off, you reach full RPM almost right away, hence the lack of backpressure is not a problem. Makes sense!