[niky]

Backpressure may be a myth, but the need for laminar flow is not. Simply put, too big of a pipe introduces too much flow turbulence, which robs you of power and efficiency at low rpms. Eventually, you get to a point where too much pipe robs you of power at higher rpms, too.

The only time running a 3" system will be the same as dumping to atmo is when that 3" system dumps out right after the exhaust manifold. For low powered engines, keeping flow even through the system is important.

Instead of thinking of it as backpressure, think of it as drinking through a straw. If the straw is too big, you can't generate enough suction to pull the water up into the straw. If it's too small or too bendy, you exert more effort for less fluid intake. Basically the same with exhaust. You want the exhaust to flow out as fast as possible. If the straw is too small, you have too much backpressure, and waste power pushing air out. If the straw is too big (and too long), flow will stagnate, causing power loss.

As for intake tuning... everything matters. You CAN do pulse tuning before the intake manifold... which is part of why CAIs are actually effective (it's not just the cold air). We were playing around with a WAI on the dyno and just a simple tube extension of 8", with the filter in the same place, increased horsepower by 10 hp. A non-inconsequential thing on an engine with just 140-150 whp. Even keeping the tubing the same and fiddling with the shape and position of the airbox itself can net a 3-5 hp gain in power.

The only reason power gains from intake tuning are lower with newer cars is because of increasingly adaptive engine computers... which is why some newer CAIs and intakes now feature MAF signal modulators.