What mandates connecting rod length? Say you have a crank with a 3.875" stroke, what size connecting rod would you use and why... a 5.7", 5.85", 6"...

 

What kind of displacement are you trying to obtain?

 

If you're going for a 396 with that crank it would be 5.85".

 

The short answer...... compression height.

Comp height is the distance from the wrist pin center to the top of the piston (flat portion). Once you get down to less than 1.1", you need to start looking at shorter rods or more deck height. The later is not always a feasible option especially with GenII blocks.

-Mindgame

 

This issue gets discussed ad nauseum, maybe you should consider a search? Keeping in mind that rod length is well nigh irrelevant except for max effort motors, here's my take on the question.

Forced induction and NA have slightly different requirements. NA motors need to spin fast to make hp. At high rpm, the long rod has some advantages. Primarily that the piston/rod combo is lighter, especially the piston. This decreases forces on the bottom end. The long rod also accelerates the piston slower, though this is a pretty small effect. The short rod has the advantage of lower peak cylinder pressure, as it dwells less near TDC. That's why the peak piston acceleration is higher. Also, a short rod means a taller compression height, which leaves more room for a dished piston crown that is used in many blower cars and also room for giving the rings generous support with beefy ring lands.

Basically though, it is meaningless at the level most of us are shooting for.

Rich Krause