A tuned pipe creates a resonant wave that helps evacuate the exhaust gases, and then a reflected wave helps close the port at the correct time. This is all time critical in relation to port timings, rpms, etc., and generally only works well over a certain, and often small rpm range. This is quite different from back pressure from a muffling device which only reduces power.

One of the tests when tuning an expansion chamber, or tuned pipe, is to record the open exhaust rpms. Then you can start tuning the pipe length and realizing a gain from its unique characteristics. It does not work by back pressure, it works by sonic waves. Indeed, if the outlet on a tuned pipe is too small, it will cause an engine melt down in quite a hurry.

AV8TOR