Hey,
I just wanna say that I wish ppl wouldnt post B.S about single exhuast giving you more torque than duals. This opinion has no engineering basis at all. Torque is simply a matter engine size(bore x stroke, more stroke better mechanical advantage at low speeds than bigger bore, bigger bore will produce more power over all however a smaller cylinder will burn more efficiently) volumetric efficiency, and burn efficiency (port proximity, swirl, tumble, spark plug placement.) So if we commit to these facts then how can putting pressure on the exhuast valve(back pressure is what I've heard it called) against the release of combustion to increase any of the above properties? What most ppl feel is the motor pickin up better than before once it starts to reach peak torque then on to peak horsepower, so by the seat of there pants they determine that the motor is doggy at low speeds, because the motor pulls harder later does not mean that it wasnt pulling as hard as it before earlier in the powerband. If this were the case by any means, then you would see professional race cars running 2-stroke style pipes and mufflers, 4-strokes are 4-strokes and 2' are 2's, they arent they same. Think about it guys, backpressure doesn't increase power anywhere in a 4-cycle engine.

 

does it possible have to do with the scavanging effect keeping velocity high. in the old days head porter would just make it big thinking big is better, know they have learned that shape and size does matter. the same has to apply to exhaust, every engine combination has an optimal size of exhaust area. every time i see header size comparison the smaller dia always makes more torque and the larger size usually makes more h.p. at a higher rpm.
the pipe size affects the engine power curve