How to use this calculator
- Enter cylinder geometry. Enter bore, stroke, rod length and cylinder count.
- Enter clearance volumes. Add chamber volume, piston dish or dome volume, deck clearance and gasket dimensions.
- Enter intake closing. Use the intake valve closing angle after bottom dead center from the cam card.
- Compare ratios. Read dynamic compression ratio beside the static ratio and effective stroke.
How it works
Dynamic compression ratio starts with the same clearance-volume stack as the static compression ratio calculator, then replaces full stroke with the effective stroke left after the intake valve closes. Swept volume is still V_s = (pi/4) x bore^2 x stroke, but dynamic swept volume uses V_dyn = (pi/4) x bore^2 x x_ivc.
The piston position at intake closing comes from slider-crank geometry: x_ivc = r(1 - cos theta) + L - sqrt(L^2 - (r sin theta)^2), where theta = 180 deg + IVC, r = stroke/2 and L is rod length.
Use this page when the cam card matters. For pure deck, gasket, chamber and piston geometry, use the static compression ratio calculator. Then pair the result with piston speed, BMEP, engine airflow and fuel injector flow when the build question moves from geometry to power and fueling.
Worked example
Verified against the live calculator
An 86 mm bore, 86 mm stroke, 143 mm rod and 60 degree ABDC intake closing has about 69.4 mm effective stroke. With 50 cc chamber volume, 5 cc piston dish, a 1 mm gasket and 0.5 mm deck clearance, static compression is about 8.82:1 and dynamic compression is about 7.32:1.
Frequently asked questions
What is dynamic compression ratio?
Dynamic compression ratio is a geometric compression ratio that replaces full stroke with the effective stroke remaining after the intake valve closes.
Which intake closing angle should I enter?
Use the cam card intake closing angle after bottom dead center. Most dynamic-compression references expect advertised or seat timing, not the later 0.050 inch checking point.
Why is dynamic compression ratio lower than static compression ratio?
The piston starts moving upward before the intake valve fully closes, so part of the stroke does not trap and compress the final charge.
Does this predict octane requirement?
No. It is a geometry screen. Octane requirement and knock margin also depend on cam ramps, fuel, mixture, chamber shape, quench, boost, temperature, altitude and ignition timing.
Is piston dish volume positive or negative?
Enter dish, valve relief and bowl volume as positive because they add clearance volume. Enter dome volume as negative because it removes clearance volume.
Method & assumptions
- Use cam-card intake closing after bottom dead center. Advertised or seat timing and 0.050 inch timing are not interchangeable.
- Positive piston volume adds clearance; negative piston volume removes clearance.
- The result is geometry only. It does not model leakage, volumetric efficiency, boost, cam ramps, cranking pressure or knock limit.