From all of my engineering courses, engine power (given a static displacement) has always been limited by the amount of air you can "cram" into the cylinders. That is why cars generally run stronger on those cool, fall mornings (cold air means it is more dense and therefore more can fit into the cylinders). Superchargers and turbos are another means of "forcing" more air in. Now, another way to help air in, is to move the spent air (exhaust) out of the way...modified exhaust. The modified exhaust usually has less restriction as mentioned above. Hope this helps