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Exhaust length is actually important, although it is not as simple as longer being better. Your engine does not spit out air at a constant rate. Each time an exhaust valve opens, a discrete pressure pulse (sound wave) is sent down the pipe. Much of this reflects back to the valves. If the wavefront gets back just as the exhaust valve is about to close, then extra exhaust is pulled out of the cylinder, and some extra intake is sucked in (both exhaust and intake valves are open at this point). So it acts like a mild supercharger, but only works over a narrow rpm band. A very similar thing is done to optimize reflection timing in the intake. Some engine nowadays actually have variable-length intake runners (don't ask me how) to exploit the effect over a wider rpm band.
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