Q&A

Mailbag: What is Connecting Rod Ratio and Why Does it Matter?

(Image/Wiseco)

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 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 RatioHigher Ratio
More wear, vibration, and heatLess 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

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.

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4 Comments

  1. Pingback: Mailbag: What is Connecting Rod Ratio and Why Does it Matter?

  2. Don’t start playing with this unless you have lots of money and nothing else to do.

  3. Dan Braspennickx says:

    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.

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