Q: What is rod ratio?
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A: Rod ratio is the relationship between the Connecting Rod Length and the Stroke Length of the crankshaft.
How is it calculated?
Use the following formula to calculate rod ratio:
Rod Length ÷ Stroke Length = Rod Ratio
For example, a stock small block Chevy 350 has 5.7 in. connecting rods and a 3.48 in. stroke:
5.7 in. ÷ 3.48 in. = 1.64 Rod Ratio
How does it affect performance?
Rod ratio can result in a little more power or a slightly longer piston ring life.
Typically, only high-performance engine builders make adjustments to rod ratios. There are many other factors that have much larger effects on engine performance.
| Lower Ratio | Higher Ratio |
|---|---|
| More wear, vibration, and heat | Less friction, wear, and heat |
| Shorter engine block deck height | Holds compression, improves combustion efficiency |
| More vacuum at low rpm | Less vacuum at low rpm |
| Better throttle response | Poor throttle response |
| More low-end torque | Less low-end torque |
| Common in daily drivers and street performance | Common in race engines |
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This is another in a series of weekly Q&A Mailbag sessions with Summit Racing‘s tech department, in which there are hundreds more. Click here to see them all.
[…] Q: What is rod ratio? … A: Rod ratio is the relationship between the Connecting Rod Length and the Stroke Length of the crankshaft. How is it calculated? Use the following formula to calculate rod […] Read full article at http://www.onallcylinders.com […]
Don’t start playing with this unless you have lots of money and nothing else to do.
Exactly rod ratio is virtually irrelevant on a pushrod motor.
The 1955 Studebaker truck V8 had a stroke of 2.8125″.
What was the rod length and piston speed ? It had a bore
of 3.5625″, 224.3 cubic inches, a compression ratio of
7.5 to 1. Horsepower was 160 @ 4500 rpm. Does a long
connecting rod moving through a short stroke generate more
torque than a short connecting rod with the same stroke ?
I heard that increasing the bore delivers a proportional increase in horsepower, but increasing the stroke only gives
A 50% increase in horsepower. I don’t know. I’m asking.