Bob, I'm not an expert by any means, but I seem to recall from reading a couple of tomes that it has to do with back pressure. We know that for the intake, equal length pipes allow for better equalization of fuel. It may have been David Vizard, the Mini Wizard, or a book on tuning two strokes, but I recall the idea was to extract exhaust gases equally, so there is equal room in the combustion chamber for new fuel mixture. Manx Nortons often ran with a megaphone which acts to pull more exhaust out of the chamber by creating a depression. Unfortunately, that also pulls some unburned mixture out. And it tends to be pretty peaky. The fix, to create better midrange, was to fit a cone to the back of the megaphone to stuff some unspent fuel back in the chamber. Sorry for the long explanation, but I relate this to the idea of a tuned exhaust. Here is something I found online which explains it far better than I have.
https://www.wemakeyoufly.com/tuned-vs-eq...t-systems/
With piston port two strokes, a megaphone was found to pull fresh mixture out of the exhaust ports, spa reverse cone was added that essentially send a shock wave back up the exhaust to stuff the fresh mixture back into the combustion chamber before the piston blocks it.Having the right diameter of the expansion chamber is important, and sort of mimics a cam on a four stroke.
Best,
Erich
https://www.wemakeyoufly.com/tuned-vs-eq...t-systems/
With piston port two strokes, a megaphone was found to pull fresh mixture out of the exhaust ports, spa reverse cone was added that essentially send a shock wave back up the exhaust to stuff the fresh mixture back into the combustion chamber before the piston blocks it.Having the right diameter of the expansion chamber is important, and sort of mimics a cam on a four stroke.
Best,
Erich