All of this works in theory on a single cylinder with a single pipe terminating into the atmosphere at a fixed point. With multiple cylinders exhausting into a common manifold, which later join a common collector pipe, I would think that the physics might get a little more complex. For example, an exhaust charge from one of the front cylinders on my transverse-mounted V6 would leave the cylinder into the manifold. The Pipe expands at the point where the other two pipes converge into a single pipe, which bolts to the crossover (and I don't even know how that flows into the rear manifold). From there, the pipe expands again where the rear manifold attaches to the collector pipe. Farther down the line there is a resonator, I think (big coffee-can-looking-thing before the cat), where the pipe expands greatly, then reduces again to 2". It then flows into the cat, expands again, and reduces again back to 2". Front there it moves into the muffler, where the pipe this time reduces slightly, expands inside the muffler, then reduces again to go out the tailpipe. That's a lot of "biggers and smallers" which I think pretty much get rid of any pressure wave charging effects beyond the resonator.
Back on the subject of tailpipe restriction, my thinking is that reducing the pipe diameter at the point that it terminates into the atmosphere increases the velocity of the exiting exhaust gases and reduces the amount of open area that atmospheric pressure can exert directly to the end of the pipe. If that is the case, then a butterfly valve would be ineffective to create this type of effect. Instead, one would want to simply reduce the tailpipe diameter. OR, perhaps a flapper or check valve with a fixed spring pressure which could effectively allow only enough open area to permit exhaust to flow out at a set velocity over a broad range of engine speeds.
Just my thinking out loud here, I might be totally wrong.
