Ian says: Anything that stops and starts each revolution cannot, by definition, affect "revability". Heavier pistons, for example, do take up kinetic energy, but do not contribute in any way to flywheel type inertia.

Paul says: Rotating mass doesn't have any effect but reciprocating mass does, you can't start & stop pistons, piston pins & the little end of the conrod for free.

You're both right, but you're talking about two different things. Rotating mass has no affect on horsepower, which is a measure of the work done at a steady speed. Reciprocating mass does affect top speed, while rotating mass does not.

Rotating mass has a large impact on how quickly a motor can rev, obviously, while reciprocating mass is much less important. Rotating mass affects acceleration more than reciprocating mass (oversimplifying).

Interestingly, there are two principle types of chassis dynos, the inertia type (Dynojet, for instance) and the brake type (the more expensive models of SuperFlow). Inertia types measure how quickly a roller of known mass can be accelerated -- and rotating mass will influence the HP numbers obtained. A brake dyno measures power at steady speeds (again oversimplifying), and only the reciprocating mass will affect the readings.

--Phil