scavenging amplified at a certain rpm range by taking advantage of exhaust pulses. When you have a well designed header, at the collector the pulse from one cylinder blows past the opening of another (adjacent in the collector) pipe and helps "pull" exhaust out because of venturi effect in the collector. Different designs allow tuning for different rpm ranges etc. and every engine designhas different requirements. Some respond well to long primaries going into one collector 6-1, while some work better with shorter primaries, and maybe even an intermediate collector to make 6-2-1. And the further away from the engine you look, the less exhaust pipe diameter you need. The gases cool off and shrink, so what came out of two or three pipes, will soon fit in one pipe. Also increase in diameter increases flow quite dramatically, you don't need twice the diameter for twice the flow.. And TOO MUCH flow is bad too, unless the engine was designed to run with no exhaust
