What mean piston speed is
In every crankshaft revolution the piston travels down one stroke and back up one stroke. Mean piston speed is that distance divided by the time it takes, averaged over the whole revolution. The piston stops at the top and bottom and moves fastest in between, so its peak speed is higher than the mean. Builders use the mean figure to compare engines with different strokes and rev limits on the same scale.
What the inputs mean
- Stroke: how far the piston moves from top dead centre to bottom dead centre. It is set by the crankshaft throw, and engine data sheets list it in millimetres or inches.
- Engine speed: crankshaft revolutions per minute.
The formula
mean piston speed (ft/min) = 2 × stroke (in) × rpm ÷ 12 = stroke (in) × rpm ÷ 6mean piston speed (m/s) = 2 × stroke (mm) × rpm ÷ 60,000The 2 is one stroke down plus one stroke up. Dividing by 12 turns inches into feet; dividing by 1,000 and by 60 turns millimetres per minute into metres per second.
Worked example
A 3.48-inch (88.4 mm) stroke at 6,500 rpm:
3.48 × 6,500 ÷ 6 = 3,770 ft/min. In metric, 2 × 88.392 × 6,500 ÷ 60,000 = 19.15 m/s.
The same crank at 7,000 rpm gives 4,060 ft/min. Piston speed rises in a straight line with rpm, so 10% more rpm means 10% more piston speed. A longer stroke does the same at a fixed rpm.
Why it matters
The load on the connecting rods and the piston grows with speed and with how hard the piston changes direction at each end of the stroke. That is why short-stroke engines tend to rev higher than long-stroke engines of similar size. This page only gives the speed. Whether a given engine can run a given rpm depends on its parts, their weight and how it is built, so ask the engine builder or the parts maker.
Questions
How do you calculate piston speed?
Multiply the stroke in inches by the rpm and divide by 6 to get feet per minute. A 3.48-inch stroke at 6,500 rpm is 3,770 ft/min, or 19.15 m/s.
How do I convert piston speed from ft/min to m/s?
Multiply ft/min by 0.3048 and divide by 60, which is the same as multiplying by 0.00508. So 3,770 ft/min is 19.15 m/s.
What is the piston speed of an 86 mm stroke at 7,000 rpm?
2 × 86 × 7,000 ÷ 60,000 = 20.07 m/s, which is 3,950 ft/min.
Is mean piston speed the same as peak piston speed?
No. The piston stops at each end of the stroke and moves fastest near the middle, so its peak speed is well above the mean. This calculator gives the mean, the figure usually quoted.