Mean Piston Speed and RPM-Limit Reading
Whether an rpm target is safe for the stroke, the single best predictor of reciprocating stress.
Example
You enter
- Crankshaft stroke (in) 3.48
- Engine speed (rpm) 6000
You get
- Mean piston speed 3480
- Mps ms 17.68
Checked against published worked examples from: Engine Builder magazine (Wiseco).
Details, formula, and sources
ScopeMPS = stroke_in x RPM / 6 (ft/min), independent of bore. A 3.48 in stroke at 6,000 rpm runs 3,480 ft/min (street/endurance); rev to 7,000 and it hits 4,060 (performance) - and a longer 4.00 in stroke would already sit at 4,000 at 6,000 rpm, the trade a stroker accepts. Street builds stay under ~4,000, performance 4,000-4,500, race over 4,500. Average, not peak; guidance bands. A shop aid; the component ratings govern.
Formula or tablemps_fpm = stroke_in x RPM / 6 (= 2 x stroke x RPM); mps_ms = mps_fpm x 0.00508; regime by ft/min band.
Source and editionThe mean-piston-speed relation MPS = 2 x stroke x RPM and the practical rpm-limit bands, as compiled in the engine-building references, by name.
Free accessThe mean-piston-speed relation is a public engine-building result; the regime bands are standard builder guidance.
What governsEstimate. AHJ and licensed professional govern.
API field namesstroke_in, rpm, mps_fpm, mps_ms
- Mean speed MPS = stroke_in x RPM / 6, the average piston speed over a strokeengine-building references
- Regime bands street/endurance under ~4,000, performance 4,000-4,500, race over 4,500 ft/minbuilder guidance
- Average not peak peak is roughly pi/2 higher and rod-ratio-dependent; not computed herekinematics