Originally Posted by NitroVein

You've got the old 9886, 9853. Onroad seem to often use the 2015, and both on & offroad uses the 2069, and that's just on top of my head.
Even Novarossi has gone for outside source for a pipe that worked well for other brands, that should tell you something.
New 1/8th off-road tuned pipe - NOVAROSSI WORLD

Different engines do use different size squish area, but it's usually difficult to see exactly how much there is in difference.