Most mufflers and tuned pipes provide exhaust back pressure that is needed to obtain the most H.P. from a 2 stroke engine. As for tuned pipes, the length of the header in relation to the expansion chamber affects the RPM as well as the amount of horsepower that an engine puts out. Commonly, tuners will refer to a properly tuned engine as "on the pipe" when it is running the strongest.

In the case of our aircraft engines, the load (prop diameter and pitch) is also a consideration in a tuned pipe application. The back pressure is needed so that the engine can breath most efficiently, exhausting as much of the burned fuel as possible without exhausting the next air/fuel charge which is moving up the transfer ports as the piston is moving towards the crankshaft. The distance between the closing of the transfer ports, (by means of the piston covering the ports) and the closing of the exhaust port is commonly called the port timing.

It is a mistake to believe that running the engine without a muffler or tuned pipe will increase the performance. It may increase the RPM's, that does not always translate into more HP. A quick look at a power curve chart or "power band" will confirm this statement.

On the other hand, if you are looking to lose your hearing in short order, or irritate your neighbors quickly, the straight pipe is the way to go.[:'(]