laZy

Does anyone know what the optimal rod to stroke ratio is? The longer the rod the less stress it puts on the cylinder walls but at what point does it become too much? To keep CR up in a destroked motor you can use a longer rod but does that hurt its ability to rev higher? Basically to sum it up, what are the principles of rod/stroke ratio?

racer7088

Just to give an example most modern headed race engine don't seem to go for rod ratios over 1.75 anymore even when they can get them and will make the deck smaller to save weight and get better packaging if it's allowed. Some race engines go as low as 1.50 nowadays even for endurance engines. The C5R is a good example and it's WON several 24 hour races now for Chevrolet.

laZy

Does it make a difference as to how quickly the motor can accelerate, or what its limit is? Does a higher or a lower ratio accomplish these goals?

racer7088

A shorter rod will usually allow earlier power whereas the longer rod motor will rev a little longer before the heads stall out. These differences are fairly tiny to almost nonexistent though usually. Shorter rods have lower pumping losses but slightly more friction. All in all it isn't that important of an engine spec really.