Two cycle recips can use a divergent/convergent cone type expansion chamber. The timing of the pulses alternately draws gases out of the combustion chamber to assist scavenging to the point of overscavenging at the operating point, and then "stuffs" the overscavenged mixture back in at the last moment as the port closes to effectively supercharge the engine

Monty,

The rotary is similar to 2-cycles only in that it has a port rather than a valve in the exhaust. I have seen Mazda factory race exhaust systems from the unmuffled racing era. They use a "continous divergence system" The pipe is tapered slightly from the port all the way to the exit. (expanding) Several later tuned systems use extraction principles just like a common piston four-cycle. A solid pipe of a length tuned to your most common RPM is the most important part of the system. After the first 20" you can use different collection methods as long as the restriction is minimal.

Bill Jepson