Piston Speed Calculator (Mean and Peak)

Mean piston speed = 2 x stroke x rpm. The piston covers the stroke twice for every crankshaft revolution, once travelling up and once travelling down, which is where the factor of two comes from.

Peak piston speed is the highest instantaneous speed the piston reaches, which occurs around mid-stroke. It is always a little higher than the mean, and the gap depends on the connecting rod length.

Rod ratio is the connecting rod length divided by the stroke. A longer rod reduces the maximum rod angle, which lowers peak piston speed and the side loading on the bore for the same mean speed. A shorter rod raises the peak and makes the piston accelerate harder near the top and bottom of the stroke.

Because mean speed rises in a straight line with rpm, the speed to rpm option simply solves the same relationship for engine speed, which is handy when you want to know the rpm that produces a chosen mean speed.

A 3.48 in stroke turning at 6,000 rpm. The mean speed follows directly from the formula.

Piston speed is a design and comparison tool, not a safety limit. The speed an engine can sustain depends on its materials, its intended use and how it is built and cooled. Two engines with the same mean speed can have very different service lives depending on the rest of the combination.

Use the figures here to compare changes such as a longer stroke or a higher rev limit, and to see the trade-offs before any parts are ordered. Any decision about a real build should be checked with a qualified engine builder.