mm
mm
mm
°From the cam card. Use the figure at the same lift the card quotes, often 0.053 in (1.35 mm) or seat timing; state which.
Result
Dynamic compression ratio (to 1)9.22
How it is calculated, with your numbers
- Piston distance below TDC when the valve closes
x = stroke / 2 × (1 − cos(180 − ivc)) + rod − √(rod² − (stroke / 2 × sin(180 − ivc))²)100 / 2 × (1 − cos(180 − 40)) + 170 − √(170² − (100 / 2 × sin(180 − 40))²) = 91.37 mm - Swept volume per cylinder
sw = π / 4 × bore² × stroke / 1000π / 4 × 100² × 100 / 1000 = 785.4 cc - Clearance volume per cylinder
clear = sw / (cr − 1)785.398 / (10 − 1) = 87.27 cc - Volume swept after the valve closes
dsw = π / 4 × bore² × x / 1000π / 4 × 100² × 91.3679 / 1000 = 717.6 cc - Dynamic compression ratio (to 1)
dcr = (dsw + clear) / clear(717.602 + 87.2665) / 87.2665 = 9.22
Working uses the metric units the formula is written in.
- bore Bore (mm)
- stroke Stroke (mm)
- rod Connecting rod length (centre to centre) (mm)
- cr Static compression ratio (to 1)
- ivc Intake valve closes (degrees after BDC) (°)
- x Piston distance below TDC when the valve closes
- sw Swept volume per cylinder
- clear Clearance volume per cylinder
- dsw Volume swept after the valve closes
- dcr Dynamic compression ratio (to 1)
What it means
The piston only starts compressing once the intake valve closes, some way after bottom dead centre. Until then, part of the charge is pushed back into the intake. The dynamic ratio uses the stroke that remains after the valve closes, found from the crank and rod geometry, so it is always lower than the static ratio printed in the spec sheet.
That is why cam and piston choices go together: a longer-duration cam closes the intake later and bleeds off compression, so it is often paired with higher static compression. Too high a dynamic ratio for your fuel risks knock; too low and the engine feels soft at low rpm.
Assumptions and limits
- Ignores intake ram effect, which raises the effective ratio at high rpm.
- Rod length and timing figures from the engine and cam makers; timing at the lift quoted on the cam card.
Sources
- Heywood, J. B., Internal Combustion Engine Fundamentals, McGraw-Hill, 1988.
- Bell, A. G., Four-Stroke Performance Tuning, Haynes (cam timing, duration, overlap and lobe centres).
Related calculators
Compression ratio calculatorStatic compression ratio from bore, stroke, chamber, gasket, deck height and piston volume.Cam timing calculator (duration, overlap, lobe centres)Duration, overlap, lobe centrelines and lobe separation from the four valve events on a cam card.Big bore and stroker kit calculatorNew displacement and compression ratio after a bigger bore, a longer stroke or both.