The HP formula I got out of a very early engine book was. (In inches) Bore x bore x stroke x RPM Divided by 18000 for a 4 cycle and 21000 for 2 cycle. It works out very close every time. Reg & Marg Ingold. Newcastle, NSW, Australia. randmingold at hotkey.net.au http://www.oldengine.org/members/randmingold