Calculators · Engine
Compression ratio calculator
Compression ratio compares the cylinder volume with the piston at the bottom of its stroke to the volume left when it reaches the top. It decides how hard an engine squeezes its charge, which affects efficiency, power and the fuel octane it needs. These calculators work out the static ratio from real engine measurements, the combustion chamber size needed for a target ratio, and the theoretical pressure a compression test would show.
Static compression ratio
Adds up every volume above the piston at top dead centre — combustion chamber, head gasket, deck clearance and piston dish — and compares it with the swept volume. This is the ratio manufacturers quote.
V_c = chamber + gasket + deck + dish CR = (V_swept + V_c) / V_c
Chamber volume for a target compression ratio
Reverses the calculation: given a target ratio and the rest of the engine, it tells you how large the combustion chamber must be. Useful when choosing cylinder heads or planning skimming.
V_c = V_swept / (CR − 1) chamber = V_c − gasket − deck − dish
Theoretical compression pressure
Estimates the peak cylinder pressure a compression test would read, from the compression ratio and atmospheric pressure. Real readings are lower because valves close after bottom dead centre, rings leak and cranking speed is low.
P_abs = P_atm × CR^n P_gauge = P_abs − P_atm
Worked example: checking a rebuilt 2.0-litre engine
- Swept volume of one cylinder (86 × 86 mm): 499.6 cc.
- Head gasket: π/4 × 8.7² × 0.07 = 4.16 cc. Deck clearance: π/4 × 8.6² × 0.02 = 1.16 cc.
- Clearance volume: 45 (chamber) + 4.16 + 1.16 + 5 (dish) = 55.32 cc.
- Compression ratio: (499.6 + 55.32) / 55.32 = 10.03 : 1.
- Theoretical compression at sea level with n = 1.3: about 19 bar gauge. A healthy engine on a real compression test typically reads less.
Typical compression ratios and compression test readings
| Engine type | Compression ratio | Typical compression test |
|---|---|---|
| Naturally aspirated petrol | 9.5 – 11 : 1 | about 10 – 14 bar (145 – 200 psi) |
| Direct-injection petrol | 10.5 – 13 : 1 | about 12 – 16 bar |
| Turbocharged petrol | 8.5 – 10.5 : 1 | about 9 – 13 bar |
| Diesel | 15 – 18 : 1 | about 25 – 35 bar (360 – 500 psi) |
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About these results
Results are theoretical estimates from standard engineering formulas. Real-world figures depend on conditions these formulas do not capture. Always follow the manufacturer's specifications for maintenance, loading and safety decisions.
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