The convergent cone on an expansion chamber would help, but having a divergent cone like they do on 2- strokes wouldnt really do anything for our cars, its there on 2 strokes to reflect a positive wave back to the port to help keep the incomming intake in the cylinder, if timed exactly rite, it can be used as sorta a mini super charger, buy alowing a little intake out the exaust, then rite before the exaust port closes, cramming it all back in with the positive wave. this wouldnt really work on a rotarey though because our combustion faces are seperated, and a lot of other reasons. The small exit on 2 stroke exaust is so that as rpm's rise the pressure in the pipe also rises, rising pressure equals faster travling sound, which make the pipe tuned for a higher rpm, as rpm rises, effectivly matching the pipe to the car at all rpm, this is very hard to get rite, but would work on a rotary, however there is a downside, which is increasingly large amounts of backpressure. on 2 strokes the scavenging, and supercharging effects of the pressure waves make up for this enough to make large gains with this set up, on a rotary how ever, it wouldnt because the only wave yuo can really use is the scavenging wave. the increase in back pressure probably wouldnt be worth it.