Originally posted by: Eli
Originally posted by: Vic
It's not a true statement. Power is a function of torque and rpm (work over time). In other words, all things being equal, power is the ability to produce a similar amount of work (torque) but at faster speeds (shorter durations of time). An increase in backpressure (to a reasonable extent of course) will actually increase torque and thus power at lower rpms but will hamper the ability of the engine to rev efficiently at higher rpm, thus reducing torque and consequently power at higher speeds. Reducing backpressure reduces torque (and thus power) at lower rpm, but allows the engine to rev more efficiently at higher rpms and thus retain more torque at those higher speeds, hence more power.
This is the correct answer.
It's something entirely different for two strokes.
You
do need some backpressure, even in a two stroke. On engines with a tuned exhaust, the energy pulses travel better through a more dense medium - in this case, exhaust gasses.
On two-stroke engines with no tuned exhaust, removing the muffler or putting a straight pipe will reduce low end torque considerably.