We spent considerable time tuning 2 strokes in the 1980's, with limited success.
Gordon Jennings, "Two Stroke Tuners Handbook is one of the few things I'd attempt to save from a total house fire. It is the holy grail of experimental tuning of more than just 2 cycle engines.
The steeper the angle of the converging cone the greater the reflection wave and it will chop the RPM at a pre determined point. OEM motorcycle pipes use this as a rev limiter.
A more gradual angle of the converging cone the more the engine will "string out" or continue to climb RPM with a sacrifice in peak power.
We experimented with the outlet pipe diameters extensively. You will feel .025" ID change in the seat of your pants.
The outlet pipe ID controls the pressure of the return wave pushing the over scavenged fresh mixture back into the exhaust port.
If you consider the after market expansion chambers of today are well designed quality units. I submit they are probably very close to optimum.
BUT, to send pipes all over the country to different weather conditions, altitudes, engines, length of races and amateur tuners. There must be a certain amount of safety built in or your product would get the reputation as an engine killer. So IMHO they make the outlet pipe ID a little too large. In the case of the brand and style of pipes we worked with in the 1980's,,,,,,, some were .050" too large.
Decrease the ID .075" was fast full range but the piston crown would suffer. You could watch the CHT rapidly rise no mater of the fuel mix.
At .100" smaller the clutch engagement changed, she felt strong pulling out of the pits but went flat and hot long before max RPM.
Larger outlet pipe ID from manufactured and the pipe was slow and cold.
We welded a large thick washer to the end of the outlet. Reamed it .100" larger. (we didn't know at the time were it needed to be) We drilled and tapped 3 mounting holes in this washer and had a series of washers with various hole ID's as much as .150" smaller we could fasten to it. In the end .050" smaller than manufactured was best power, range and length of race. Still hot but manageable,, air cooled.
Mr. Nunley manufactured the Maco pipes of the day,, if I'm not mistaken. it will be interesting when he decides to post to this thread.
Best, WP