Engine / Compression
Compression Ratio Calculator: Static and Dynamic Estimates
Calculate static compression from bore, stroke, chamber, piston, gasket and deck measurements. Optionally estimate dynamic compression using intake-valve closing and rod length.
Compression Ratio Calculator: Static and Dynamic Estimates: Sums the clearance volumes above one piston and divides total bottom-dead-centre volume by top-dead-centre volume. The optional dynamic result replaces full stroke with the ideal slider-crank distance remaining at intake closure. It is an estimate, not a pressure or knock prediction. Runs 100% locally in your browser with zero server file uploads.
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| Part | Volume (cc) |
|---|---|
| Swept volume | 499.557 |
| Chamber | 50 |
| Piston dish / dome | 5 |
| Gasket | 5.945 |
| Deck | 0.000 |
Formulas and sign conventions
A = π × bore² ÷ 4; Vs = A × stroke ÷ 1000; Vc = chamber + piston + π × gasket bore² × gasket thickness ÷ 4000 + A × deck ÷ 1000. Static CR = 1 + Vs/Vc. Lengths are mm and volumes cc. A dish adds volume; a dome subtracts volume. A piston above the deck has negative deck clearance.
Dynamic estimate: r = stroke/2, L = rod length, θ = 180° + intake closing ABDC; x = r(1 − cos θ) + L − √(L² − r² sin² θ). Dynamic CR = 1 + (A × x/1000)/Vc. This ideal slider-crank model assumes zero pin offset and compression starting at valve closure.
Dynamic CR is a geometry estimate, not a cylinder-pressure or knock prediction. Use advertised seat closing timing, not a 0.050-inch timing figure. Cam ramps, leakage, boost, temperature and gas flow are not modelled. This does not verify mechanical clearance.
NASA: compression ratioStatic compression
NASA Glenn, Compression and Expansion, defines the geometric compression ratio. CR = (Vs + Vc)/Vc. Clearance Vc is chamber + signed piston volume + gasket volume + signed deck volume. https://www1.grc.nasa.gov/beginners-guide-to-aeronautics/compression-and-expansion/
Dynamic geometry estimate
Let r = stroke/2, L = connecting-rod centre distance, θ = 180° + intake closing ABDC, and A = π × bore²/4. Distance from TDC at closure is x = r(1 − cos θ) + L − √(L² − r² sin² θ). Dynamic CR = 1 + (Ax/1000)/Vc with lengths in mm and volumes in cc. This follows the right-triangle geometry of a zero-offset slider crank.
Universidad Carlos III de Madrid, Mathematical Expressions for Piston Motion, defines rod length, crank radius and angle from TDC: https://e-archivo.uc3m.es/rest/api/core/bitstreams/97ad7c8d-95b7-47be-bdaf-9cb51d5768d7/content . Valve closure is treated as instantaneous; the result omits flow, thermal and pressure effects.
How to use it
- Enter bore, stroke and chamber volume per cylinder.
- Enter signed piston volume, compressed gasket dimensions and signed deck clearance.
- Optionally enable dynamic compression and enter seat-timing intake closure and rod length.
Privacy & limitations
The calculations run in your browser without uploading dimensions.
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Frequently asked questions
Which sign should a piston dish or dome use?
A dish or valve relief adds clearance volume, so enter a positive cc value. A dome displaces chamber volume, so enter a negative value.
What is deck clearance?
It is the piston-to-block-deck distance at top dead centre. A piston below the deck has positive clearance; above the deck is negative. The calculator does not verify mechanical clearance.
Which intake closing angle should I use?
Use the advertised seat closing angle in degrees after bottom dead centre. A 0.050-inch timing figure is not interchangeable. Cam ramps, gas flow, leakage and boost make actual compression behaviour more complex.
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