Not completely accurate on the N/A exhaust dynamics there, I think...in an N/A engine thats been properly designed, the headers will be of a tuned length so as to employ resonance frequency return pulses to scavenge gas away from the exhaust valves and speed up the removal of gas along the exhaust. Short version; valve belches out a pulse of exhaust gas that wafts along the pipe until it hits a restriction, then returns along the pipe, just like a bum-gas ripple in yer bath when it hits the edge :Giggle.
This creates a high-pressure/low-pressure cycle at the valve area, high as the pulse returns, low as it recedes, just like waves on the beach (ahh, the poetry:)). If you tune the exhaust length, ie put the first restriction that causes the wave return in the exact spot you want it, it can time the low-pressure wave with the valve opening at a certain rpm, and thus scavenge the gasses away faster.
This is one of the major problems with N/A tuning, because along with cam profile efficiency, this effect is narrowing the rev range at which the engine is at its most efficient. Especially noticeable on bikes that rev to ridiculous cielings but are only efficient really for a couple of thousand rpm out of what can be a 17,000 rpm scale....
None of which really bothers turbo\'d engines, which push the exhaust gas out pretty much regardless of what restrictions may be in the way...up to a point. Obviously, the less restriction, the easier it is for the turbo to pump gas past it, which is why the less backpressure, the better.
None of this is especially interesting, but it is true :Giggle