What each input means
- Bore, stroke, chamber, piston, gasket and deck: the same inputs as the static compression ratio calculator. Together they set the clearance volume left above the piston at top dead center.
- Rod length: center to center. It decides where the piston sits at a given crank angle.
- Intake valve closing: degrees after bottom dead center (ABDC), from the cam card. If the card only gives duration and intake centerline, closing ABDC = centerline + duration ÷ 2 − 180.
Which closing point to use
Cam cards list the closing point at 0.050 in of tappet lift, and usually at seat (advertised) timing too, which the cam maker measures at a much smaller lift of its choosing. Seat timing closes later, so it always gives a lower dynamic ratio for the same cam. Neither is wrong, but two results only compare if they use the same convention. Pick one, write it next to the number, and check which one any target figure you read was based on.
The formula
r = stroke ÷ 2, l = rod length, θ = 180 + closing angle ABDC
piston below TDC = r + l − (r × cos θ + √(l² − r² × sin² θ))
effective stroke = piston below TDC at θ
dynamic swept = π ÷ 4 × bore² × effective stroke
dynamic ratio = (dynamic swept + clearance) ÷ clearanceThe first line is exact crank and rod geometry: r × cos θ + √(l² − r² × sin² θ) is the distance from the crank center to the wrist pin, which equals r + l at top dead center. No short-rod approximation is used.
Worked example
Bore 4.000 in, stroke 3.480 in, rod 5.700 in, 64 cc chamber, 5 cc dish, head gasket 4.100 in bore and 0.041 in thick, piston 0.025 in below the deck. The example cam card shows 224° duration at 0.050 in on a 110° intake centerline, so the valve closes at 110 + 112 − 180 = 42° ABDC.
- Clearance volume 83.02 cc, static ratio 9.63:1.
- Crank angle 222° after top dead center: the piston is 3.153 in (80.09 mm) below top dead center, 90.6% of the stroke.
- Dynamic swept volume: 649.3 cc.
- Dynamic ratio: (649.3 + 83.02) ÷ 83.02 = 8.82:1.
If the same example card gave 280° seat duration on the same centerline, the valve would close at 70° ABDC: effective stroke 2.575 in and a dynamic ratio of 7.39:1. Same cam, two conventions, 1.44 points apart (8.822 against 7.386 before rounding).
Related calculators
Work out the static ratio first with the compression ratio calculator, and the pressure ratio of a boosted engine with the boost calculator. The engine set also covers rod ratio, volumetric efficiency, BMEP and turbo airflow and compressor map points.
Questions
How do I find the intake valve closing point?
Read it from the cam card, which lists it in degrees after bottom dead center. If the card only gives duration and intake centerline, add half the duration to the centerline and subtract 180: 224° on a 110° centerline closes at 42° ABDC.
Should I use 0.050 in or seat timing for dynamic compression?
Either, as long as you use the same one every time. Seat timing closes later, so it gives a lower number. In the example on this page, closing at 42° ABDC gives 8.82:1 and closing at 70° ABDC gives 7.39:1.
Does rod length change dynamic compression?
Only a little. With the example engine closing at 42° ABDC, a 5.700 in rod gives 8.82:1 and a 6.000 in rod gives 8.81:1. The longer rod has lifted the piston slightly further when the valve closes.
Why is dynamic compression lower than static compression?
Static compression counts the full stroke. The piston cannot compress anything until the intake valve shuts, which is after bottom dead center, so only part of the stroke counts. In the example 90.6% of the stroke is left, and 9.63:1 static becomes 8.82:1 dynamic.