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Rod ratio
The rod ratio affects side load, piston motion and how the engine behaves high up in the revs. Enter the rod length and the stroke, and you get the ratio directly.
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Rod ratio
1,90:1
Rod ratio = rod length divided by the stroke. A higher value gives lower side load against the cylinder wall.
How the calculation works
The rod ratio is simply the rod length centre to centre divided by the stroke. It is a dimensionless number, so it does not matter whether you measure in mm or inches as long as both measurements are in the same unit.
A high ratio, that is a long rod relative to the stroke, gives lower side load against the cylinder wall and less piston rock, which favours high rpm. A low ratio gives more dwell near top dead centre and can favour filling and torque lower down.
Example
A 152 mm rod with an 80 mm stroke gives a rod ratio of 1,90:1, which is high and favours high rpm.
Optimal rod ratios
| Application | Rod ratio |
|---|---|
| Street performance | 1,55-1,65 |
| Circuit racing | 1,65-1,75 |
| Drag racing | 1,75-1,85 |
| Pro Stock | 1,80-1,90 |
| Sport bike (MC) | 1,85-2,00 |
| Formula 1 | 1,90-2,10 |
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