Piston Speed Calculator
Calculate mean and exact peak piston speed from stroke, connecting-rod length and RPM. You can also solve the RPM or stroke that produces a target mean piston speed.
Piston speed formulas
Mean piston speed is the distance traveled during the intake and compression or power and exhaust strokes divided by time: mean speed = 2 × stroke × RPM ÷ 60 when stroke is in meters and the result is in meters per second. In US units, mean speed (ft/min) = 2 × stroke (in) × RPM ÷ 12.
Peak speed is not simply the mean speed multiplied by a universal constant. This calculator uses the entered connecting-rod length and standard slider-crank geometry. With crank radius r, rod length L and crank angle θ measured after top dead center, piston travel is x = r(1 − cos θ) + L − √(L² − r² sin² θ). The tool differentiates that position with respect to time and solves the zero-acceleration point to locate the exact speed peak.
Checked example
A 3.48-inch stroke at 6,000 RPM produces a mean piston speed of 3,480 ft/min, or 17.68 m/s. With a 5.70-inch center-to-center rod, the calculator also determines the exact peak speed, its crank angle after TDC, rod/stroke ratio and theoretical TDC acceleration.
What each calculation mode solves
| Mode | Known inputs | Solved value |
|---|---|---|
| Piston speed | Stroke, rod length and RPM | Mean and exact peak speed |
| RPM | Stroke, rod length and target mean speed | Required engine RPM |
| Stroke | RPM, rod length and target mean speed | Required crankshaft stroke |
Mean speed versus peak speed
Mean speed is useful for comparing engines because it depends only on stroke and RPM. Instantaneous peak speed also depends on rod length. A finite connecting rod shifts the peak away from 90 degrees after TDC and changes the peak-to-mean ratio. Enter the actual center-to-center rod length rather than overall length.
Limits and input checks
- The model represents a conventional in-line slider-crank with zero wrist-pin offset. It does not model offset bores, articulated rods, opposed-piston layouts or non-sinusoidal mechanisms.
- Stroke is full TDC-to-BDC travel, not crank radius. Rod length is center to center.
- The rod length must be greater than half the stroke so the square-root term remains physically valid throughout the cycle.
- Mean piston speed is an average of distance traveled, not the arithmetic average of signed velocity over a full cycle; signed velocity averages to zero.
- The TDC acceleration is a rigid-body kinematic result. It is not piston force and does not include mass, gas pressure, friction or component flexibility.
- Use the target modes for comparison and design exploration only. Confirm RPM, stroke and durability limits with measured data, the component manufacturer and a qualified engine builder.