Engine calculator
Displacement from bore, stroke and cylinders; static and dynamic compression from the chamber, gasket, deck, piston and the cam's intake closing; rod ratio and piston speed; airflow and horsepower–torque; and gearing — speed at rpm in every gear with a shift chart, rpm at cruise, and which final drive to pick.
Added
Works with
- Small engines Cars and EVstwo-stroke mix, rejetting for altitude, compression tests and power for small engines
- Turbocharger matching Cars and EVsboost it — pressure ratio, air flow on a compressor map, intake temperatures, injectors and fuel pump for the power
How to use it
The sums I do when planning an engine or a gearing change, in one place. Three tabs share the same engine, so the displacement from the first is used for airflow on the second.
How to use it
Displacement & compression. Give bore, stroke, the number of cylinders and the rod length (in mm or inches — switching converts what you’ve typed), then the combustion chamber volume, head gasket bore and compressed thickness, how far the piston sits below the deck at top dead centre (negative if it comes up above), and the piston’s volume: dish and valve reliefs positive, a dome negative. The page gives displacement in cc, cubic inches and litres, the static compression ratio, the bore/stroke and rod ratios, and mean and peak piston speed at the rpm you give.
For dynamic compression, give when the intake valve closes in degrees after bottom dead centre — or the intake centreline and duration from the cam card, and it works the closing point out as centreline + duration ÷ 2 − 180. Use seat-to-seat (advertised) figures; with only the 0.050″ figures, add about 15° to the closing point (30° to the duration).
Airflow & power. Airflow in cfm at a given rpm and volumetric efficiency; horsepower and kW from torque at an rpm, either unit.
Gears & speed. Pick a gearbox or type the ratios, the final drive, a tyre size (225/45R17, 33x12.50R15 or a diameter in mm) and the redline. The chart shows rpm against road speed in every gear, with the drop on each upshift at redline; the table gives speed per 1000 rpm, top speed in each gear, rpm at your cruising speed, and the rpm each upshift drops to. Choosing a final drive takes a target rpm at cruise and lists the common ring-and-pinion ratios with what each gives.
The sums
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Displacement = π/4 × bore² × stroke × cylinders.
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Static compression = (swept + clearance) ÷ clearance, where clearance is chamber + gasket (π/4 × gasket bore² × thickness) + deck (π/4 × bore² × deck clearance) + piston volume.
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Piston position at crank angle θ after TDC, crank radius r = stroke ÷ 2 and rod length L: r(1 − cos θ) + L − √(L² − r² sin² θ). Dynamic compression uses the distance still to travel when the intake closes (θ = 180° + closing ABDC) in place of the full stroke. It ignores the charge the cam’s overlap and the intake’s ram effect add at speed, which is why it is a guide to detonation at low rpm, not a boost gauge.
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Mean piston speed = 2 × stroke × rpm ÷ 60. Peak speed comes from the formula above, differentiated numerically; it happens a little before 90° on a short rod.
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Airflow (four-stroke) = cubic inches × rpm × VE ÷ 3456.
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Horsepower = lb·ft × rpm ÷ 5252; kW = N·m × rpm ÷ 9549.
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Road speed = rpm ÷ (gear × final drive) × π × tyre diameter. Tyre diameters are nominal; a loaded tyre rolls 2–3 % short of it.
The gearbox presets are typical published ratios; check your own box, since makers changed them between years and options.
Related: tyre sizes and speedometer error, OBD-II diagnostics for reading rpm and speed from the car, gear designer for tooth geometry, and RC car gearing.
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