"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)."

I have always thought there was a very noticable difference between early & late B20's, so I think reciprocating mass is important. Late B20/B21 cranks tend to be more bent then early B20 cranks & B18 cranks hardly bend at all. That bending force has to equate to more drag don't you think?

Cheers,
Paul.