Connecting Rod to Stroke Ratios

I spent some time, in the last post, discussing rod-to-stroke ratios but never actually looked at them. So here is a quickie.

First, rod-to-stroke ratios are determined by dividing the rod length by the stroke length.

For example, the stock 350 connecting rod is 5.7” from center to center. Rod lengths are determined by measuring the distance between the center of the big end hole and the small end hole. The stroke for a stock 350 is 3.480”.

We then compute the ratio as follows:

5.7”
-------- = 1.64
3.480”

1.64 is the rod-to-stroke ratio.

I have yet to see a reason for not putting the longest rods you can in an engine. The longest length you “can” fit depends on the purpose of the engine. If you stretch the rod lengths too far then you get troubles with the piston. As you increase rod length you have to decrease compression height (compression height [or distance] is the distance from the center of the wrist pin to the top pf the piston*). As you decrease compression height you begin crowding the rings and that can lead to reliability issues. There are various tricks to reducing compression height, like using a smaller diameter wrist pin, but these tricks are both expensive and relatively short lived. So for me the longest rods I “can” fit are with pistons with compression heights that permit reliability and a long service life. For the purposes of this evaluation I’m choosing (somewhat arbitrarily) 1.2” for the minimum compression height. Further research will determine what my true minimum is.

Back to rod-stroke ratios. All of the previous to say that I am presently trying to put the longest rods possible in the motor and want to see how these ratios look compared to each other.

Stroke	3.480”	3.750”	4.000”
Max Rod Length	6.08”	5.95”	5.82”
Rod-Stoke Ratio	1.747	1.587	1.455

It is ironic that as the stroke increases, and you need a longer rod to maintain rod-stroke ratio, the maximum rod length decreases.

Well, I don’t know what to do with these numbers but I know I don’t like the ratio for the 4.000” stroke.

More study is needed

Builder

* This isn’t strictly true with domed pistons but if you cut the dome off so the piston was flat, then you would have the compression height.