Two-stroke engine geometry
Compression Ratio Calculator
Calculate geometric and trapped compression ratios from bore, stroke, measured chamber volume, gasket, deck position, piston crown volume, and exhaust timing—then solve the chamber volume needed for a target ratio.
Calculated output
CalculatedCompression summary
Volume geometry
DynamicCylinder section
Ratio comparison
LiveCompression bars
Audit the inputs
Per cylinderVolume breakdown
| Component | Calculation reference | Volume |
|---|---|---|
| Swept displacement | π × bore² ÷ 4 × stroke | — |
| Head chamber | Measured input | — |
| Head gasket | Gasket bore × compressed thickness | — |
| Deck volume | Piston position relative to deck | — |
| Piston crown correction | Dish positive; dome negative | — |
| Net clearance volume | Sum of clearance components | — |
| Exhaust closes | — | — |
Calculation method
Compression-ratio formulas
Geometric ratio uses the full stroke. Trapped ratio begins when the rising piston closes the exhaust roof, so it uses the volume swept from that closing position to TDC.
1. Swept volume
Vₛ = (π ÷ 4) × bore² × stroke
Millimetre cubed results are divided by 1000 to obtain cubic centimetres per cylinder.
2. Net clearance
V꜀ = chamber + gasket + deck + crown correction
A dish adds volume. A dome or piston protrusion subtracts volume. The final net value must remain positive.
3. Geometric ratio
CRgeo = (Vₛ + V꜀) ÷ V꜀
This is the full-stroke or geometric compression ratio and does not account for the open exhaust port.
4. Trapped ratio
CRtrapped = (Vtrapped + V꜀) ÷ V꜀
The tool uses rod length and exact slider-crank position at exhaust closing to calculate trapped swept volume.
5. Target clearance
V꜀ target = swept basis ÷ (target CR − 1)
Fixed gasket, deck, and crown volumes are removed to report the theoretical head-chamber volume needed for the selected target.
Measurement guidance
Compression ratio FAQ
What is the difference between geometric and trapped ratio?
Geometric ratio uses the complete swept displacement. Trapped ratio uses only the charge retained after the exhaust port closes, making it especially useful for piston-ported two-stroke comparison.
How should chamber volume be measured?
Bring the piston to true TDC, seal the rings with a light grease, install the intended head and gasket, and burette the assembled clearance whenever possible. If measuring only the detached head chamber, use the separate gasket, deck, and crown fields in this calculator.
What sign should I use for a piston dome or dish?
Enter a dish as positive because it creates extra clearance volume. Enter a dome as negative because it occupies clearance volume.
What does negative deck clearance mean?
A negative value means the piston crown protrudes above the cylinder deck at TDC, reducing clearance. Check squish clearance and mechanical interference directly.
Why is rod length needed?
Rod length changes piston position at a given crank angle. It is required to find the exact trapped cylinder volume at exhaust closing.
Does exhaust duration assume symmetrical timing?
Yes. Opening and closing are assumed symmetrical around BDC. Power valves, asymmetric ports, or an independently measured closing angle require a more specialized model.
Does this predict fuel octane or safe ignition timing?
No. Detonation margin also depends on trapped pressure, squish, chamber shape, fuel, air temperature, ignition timing, pipe behavior, cooling, load, deposits, and mixture.
Is the target chamber value a machining instruction?
No. It is a mathematical result. Confirm minimum squish, piston-to-head clearance, plug reach, sealing width, head strength, chamber shape, and safe material removal with a qualified engine builder.
Why can required chamber volume be negative?
The selected target is impossible with the entered fixed gasket, deck, and piston volumes. Those fixed components must change, or a different target must be selected.
Should I use total engine displacement in the ratio?
No. Compression ratio is calculated per cylinder. Cylinder count is used only to report total engine displacement.