There are a couple of schools of thoughts on this. One is a longer rod slows down the piston speed, allowing the piston to spend more time at top dead center, building combustion pressure. Another is short rods improve pumping work and blowdown during EVO, which increases power. My engine builder friend in NC is trending toward short rod motors. He has built record holding engines in Stock Eliminator and various tire classes. NASCAR engines still have long rods. I'm still a long rod guy; I'm putting 7.263" rods in my 292. I think forced induction engines benefit from shorter rods. Basically you can pick whichever and there's a theory backing it up.
