mm
mm
mm
Result
Bore / stroke ratio1.166
Rod / stroke ratio1.749
How it is calculated, with your numbers
- Bore / stroke ratio
bs = bore / stroke80 / 68.6 = 1.166 - Rod / stroke ratio
rs = rod / stroke120 / 68.6 = 1.749
Working uses the metric units the formula is written in.
- bore Bore (mm)
- stroke Stroke (mm)
- rod Connecting rod length (centre to centre) (mm)
- bs Bore / stroke ratio
- rs Rod / stroke ratio
What it means
A bore/stroke ratio above 1 is "oversquare": a wide piston with a short stroke, which leaves room for bigger valves and allows higher rpm. Below 1 is "undersquare", typical of engines built for low-speed torque. Superbikes are strongly oversquare; many adventure twins sit just above 1.
The rod/stroke ratio affects how quickly the piston accelerates away from top dead centre and how hard it pushes sideways on the cylinder wall. A longer rod (higher ratio) is smoother and kinder to the bore.
Assumptions and limits
- Geometry only.
Sources
- Heywood, J. B., Internal Combustion Engine Fundamentals, McGraw-Hill, 1988.