[Christ]

4 stroke engines don't need back pressure to make torque, that part is a myth. It comes from the fact that people notice that by using smaller diameter pipe, you can increase low-RPM torque.

The reason this actually happens (as supported by fluid dynamics) is that when the pipe's diameter is too large to maintain attached flow inside the pipe, the fluid in the pipe will begin to swirl back on itself, creating it's own form of restriction. A smaller pipe will have higher velocity, and since, when a fluid is moving faster, it is under less pressure, the smaller pipe will have LESS backpressure (provided it is properly sized to allow the fluid to pass efficiently) than the larger pipe.

The term back pressure is used incorrectly in most applications - back pressure is never a good thing, except (there is always an exception) when flow restriction is required to prevent over flow, as in a 2 cycle engine.

Exhaust temperature and volume are the major contributing factors when considering a given diameter. You always tune for slightly larger than the median volume/temperature's required diameter, then tune length for the required RPM, which produces harmonic scavening (by length) and heat/pressure scavenging (by temperature/volume).