At the rpm a 250R turns the pumping action of the piston and crankshaft is not very effective. With the modern high energy exhaust systems we use today the pipe design is much more effective at pulling in fuel than changing crankcase volume. I have tested some pretty significant volume changes with little or no hp change. I have made small exhaust pipe diffuser profile changes that made a very noticeable hp difference. By diffuser I am referring to the diverging cone part of the pipe starting at the head pipe. This part of the pipe effects scavenging efficiency or the pumping effect of the pipe. Changing the angles of the diffuser cone has an effect of how hard the pipe pulls on the intake charge and for how much duration. Steep cones pull harder but over a narrower rpm range. The trick is to find the correct combination of angles to give the widest power band and still have as much energy as possible. I don't care to much whether the tin is on or off. When we balance the cranks we have it welded on because when the heavy metal is welded into the flywheel it will burn the o-rings.
I think I will continue to spend my energy on exhaust system, rather than case volume! I still don't know every thing I want to know about pipes, and I have spent a large amount of my time thinking and working on this sense about 1977. You think maybe I should get a life? lol!
