Modern motorcycle pipes run a much more sophisticated exhaust system. It all starts at the exhaust port. A radius on the upper edge of the port and maybe the piston helps flow at small openings. The first picture shows a feeler to measure the timing. If you have huge exhaust area (bridged or triple port) the casting and header should reduce in area then expand like in the second picture. The "header" then starts its taper into a pipe that might look like the third picture. This is all designed by a combination of computer simulation and cut and try.
The modern low cost simulations are pretty good at least down to 50 cc engines. My experience is that they overstate the high rpm power for 26 cc gasoline engines. However, simulations have been used to model glow ignition engines. Their main contribution is modeling pressure pulses and simple flows in an engine. They use empirical models for scavenging, friction, and combustion.
My only practical experience with tapered headers is with Quickdraw headers and pipes. They come pretty close to the above description with an elbow that tapers up, but have an adjustable parallel section in the header. We get better power on a dyno with them than with conventional straight headers.
Lohring Miller
