Dynamic Compression Ratio Comparison Calculator
Compare two cam timing choices against one engine’s static compression ratio. The result shows the intake-closing angle used for each cam, the piston stroke left to compress, and the resulting geometric ratio.
Dynamic compression ratio difference (Cam B − Cam A)-0.35
- Cam A dynamic compression ratio
- 9.15
- Cam B dynamic compression ratio
- 8.8
- Cam A effective stroke (in)
- 3.15
- Cam B effective stroke (in)
- 3.02
- Cam A trapped stroke
- 90.61%
- Cam B trapped stroke
- 86.68%
- Calculated intake closing angles
- Cam A: 42.00° ABDC; Cam B: 50.00° ABDC
- Comparison summary
- Cam B closes later, Cam B has the shorter effective stroke, and Cam B has the lower geometric dynamic compression ratio. Effective-stroke difference: 0.14 in.
Ratios are geometric estimates based on the selected intake-closing convention; use the same timing checking height for both cams.
How to use this calculator
- Choose inches or millimeters, then enter stroke and rod length in that same unit.
- Enter the engine’s static compression ratio from a measurement or a separate static-ratio calculation.
- Choose direct closing angles or enter each cam’s intake duration and installed intake centerline.
- Use timing figures measured at the same checking height for both cams before comparing the results.
How dynamic compression is compared
Static compression ratio describes the full swept cylinder volume and the clearance volume when the piston is at top dead center. A running engine does not begin trapping the fresh charge at bottom dead center if the intake valve remains open as the piston starts upward. This calculator compares the geometric compression that remains after each cam’s intake valve closes.
For a direct entry, the intake-closing angle is used as entered in degrees after bottom dead center (ABDC). For a cam card entry, the calculator derives it as IVC = ICL + D ÷ 2 − 180, where ICL is installed intake centerline in degrees after top dead center and D is intake duration in degrees. That relationship is only meaningful when the duration and centerline describe the same installed timing convention.
Exact piston position and effective stroke
The piston does not move linearly with crank angle because the connecting rod swings. With full stroke S, rod length L, crank radius r = S ÷ 2, and closing angle α, the calculator uses the crank-slider relation below. The crank angle is θ = π + α × π ÷ 180.
x = r + L − [r × cos(θ) + √(L² − r² × sin²(θ))]
Here x is the piston distance remaining from intake closing to top dead center: the effective stroke. The trapped-stroke percentage is 100 × x ÷ S. A later closing angle puts the piston farther up the bore, so it leaves less effective stroke. Rod length has a smaller but real effect because it changes piston position at a given crank angle.
Turning effective stroke into a ratio
Static swept volume divided by clearance volume is C − 1, where C is static compression ratio. Scaling that swept portion by effective stroke gives DCR = 1 + (C − 1) × x ÷ S. The comparison result is Cam B DCR minus Cam A DCR; a negative value means Cam B has the lower geometric ratio. At an intake closing of bottom dead center, effective stroke equals full stroke and the dynamic value equals the static ratio.
What this calculation does not predict
This is geometric cam-comparison math, not a cylinder-pressure or detonation model. Actual trapped charge also depends on valve motion, port flow, engine speed, volumetric efficiency, boost, temperature, residual exhaust gas, and pressure waves. Cam makers can publish timing at different checking heights, so advertised duration from one card should not be compared with duration at 0.050-inch lift from another. Installed timing measured with a degree wheel can be more representative than a catalog value.
Worked example
Consider an engine with a 3.480-inch stroke, a 5.700-inch rod, and 10.00:1 static compression. Cam A has 224° intake duration installed on a 110° intake centerline, so its calculated intake closing is 110 + 224 ÷ 2 − 180 = 42° ABDC. Cam B has 240° duration on the same 110° centerline, so it closes at 50° ABDC.
The crank-slider calculation gives Cam A an effective stroke of 3.153 inches, or 90.61% of full stroke, and a geometric dynamic compression ratio of 9.155:1. Cam B leaves 3.016 inches, or 86.68% of full stroke, and gives 8.801:1. Cam B therefore has a 0.354 lower geometric dynamic compression ratio than Cam A.
Common questions
What is dynamic compression ratio?
Dynamic compression ratio is a geometric estimate that starts compression at intake-valve closing rather than bottom dead center. It scales the engine’s static swept volume by the piston travel still remaining after that event. It is useful for comparing cam timing on the same engine, but it is not a direct measurement of cylinder pressure.
How do I calculate intake closing from duration and centerline?
For a symmetrical timing approximation, add half the intake duration to the installed intake centerline and subtract 180 degrees: IVC ABDC = ICL ATDC + duration ÷ 2 − 180. Use the installed intake centerline, not lobe-separation angle. The duration and centerline must describe the same timing convention.
Should I use advertised duration or duration at 0.050-inch lift?
Either can support a comparison when both cams use exactly the same checking height and convention. Do not mix advertised duration from one cam card with duration at 0.050-inch lift from another, because the calculated closing angles would no longer be comparable. Direct intake-closing figures at a shared checking height are preferable when available.
Why does a later intake closing lower dynamic compression?
After bottom dead center, the piston begins moving upward while the intake valve can still be open. Closing later means the piston has already traveled farther toward top dead center, leaving less cylinder travel to compress the charge geometrically. Less effective swept volume produces a lower calculated ratio.
Is intake centerline the same as lobe-separation angle?
No. Intake centerline is the crank angle of the intake lobe centerline relative to top dead center after installation. Lobe-separation angle describes the angular separation of the intake and exhaust lobe centerlines. Advancing or retarding a cam changes installed intake centerline without changing its lobe-separation angle.