[gone-ot]

...yes, there are really two distinct "movements" within the exhaust gas: (1) the actual mass movement of the exhaust-gas "slug," at about 300 fps, and (2) the acoustic speed-of-sound wavefront/shockwave within the gas mass, at about 1700 fps.

...for HP, pipe diameter is set by mass velocity, while pipe length is set by acoustic velocity (Helmholtz tuning).

...with Helmholtz tuning, the goal is to have the "reflected" rarefacation wave arrive back at the just closing exhaust value, so that it can literally "suck" the last vestiges of exhaust gas from the cylinder as the exhaust valve closes, which, if done correctly, will then actually create a slight vacuum within the cylinder that helps it "suck-in" more air when the intake value opens.

...the speed of sound in normal air is about 1,100 fps, but in hot exhaust it's about 1,700 fps (there's an approximation equation available if you're interested).