Compression Ratio to PSI Calculator
Convert compression ratio to expected cranking cylinder pressure and back. Adjust the polytropic index for heat loss and ring leakage, correct for altitude, and use the reverse solver to diagnose your compression test readings.
CR-to-PSI Reference Table — Sea Level
Every compression ratio from 7:1 to 15:1, with three common polytropic index values. Real engine cranking pressures cluster around n=1.25–1.35. The isentropic (n=1.4) column shows the theoretical maximum for an ideal engine with zero heat loss and zero leakage.
| Compression Ratio | Hot Engine (n=1.25) | Typical Crank (n=1.3) | Worn Rings (n=1.35) | Isentropic (n=1.4) |
|---|---|---|---|---|
| 7:1 | 167 PSI | 184 PSI | 224 PSI | 224 PSI |
| 8:1 | 198 PSI | 219 PSI | 270 PSI | 270 PSI |
| 9:1 | 229 PSI | 256 PSI | 319 PSI | 319 PSI |
| 10:1 | 261 PSI | 293 PSI | 369 PSI | 369 PSI |
| 11:1 | 294 PSI | 332 PSI | 422 PSI | 422 PSI |
| 12:1 | 328 PSI | 372 PSI | 476 PSI | 476 PSI |
| 13:1 | 363 PSI | 412 PSI | 533 PSI | 533 PSI |
| 14:1 | 398 PSI | 454 PSI | 591 PSI | 591 PSI |
| 15:1 | 434 PSI | 497 PSI | 651 PSI | 651 PSI |
Altitude correction: Every 1,000 ft of elevation reduces atmospheric pressure by ~0.5 PSI. At Denver (5,280 ft), the baseline is 12.2 PSI instead of 14.7 PSI — every PSI reading drops by about 17%. Use the altitude input in the calculator above for precise correction.
Polytropic vs Isentropic Compression — Why Your Gauge Reads Lower
Isentropic compression (n = 1.4) is the textbook ideal — no heat transfer between the compressed air and the cylinder walls, and zero air leakage past the piston rings. It's useful as an upper bound, but no real engine ever achieves it.
Polytropic compression (n = 1.25–1.35) models what actually happens during cranking and running. The compressed air transfers heat to the cooler head and bore, and a small volume of air leaks past the rings. Both effects reduce peak pressure — that's why your compression gauge reads 370 PSI on a 10:1 engine instead of the ideal 430 PSI.
What moves the n-value?
Compression Test Results — What Your PSI Actually Means
A compression gauge doesn't give you a number — it gives you a pattern. Learn to read the pattern and you can tell whether you're looking at normal wear, ring fatigue, valve leakage, or a blown head gasket before you tear the engine down.
Normal / Healthy
All cylinders within 10% of each other. No visible oil consumption or smoke. Rings seat properly, valves seal.
Worn Rings
Oily PCV, blue smoke on acceleration. May improve with engine rotation during test. Leak-down test confirms ring wear.
Leaking Valves
One or two cylinders consistently low. Often improves with a squirt of oil (oil seals rings temporarily but not valves).
Head Gasket Leak
Cross-cylinder pressure loss, bubbles in overflow tank, milky oil. Usually between two adjacent cylinders or into coolant jacket.
Severe Wear
End-of-life engine. Piston slap, heavy oil consumption, poor vacuum. Requires rebuild or replacement.
How to Run the Compression Test Correctly
- Warm engine to operating temperature, then turn off ignition and fuel pump.
- Remove all spark plugs so the engine can spin freely.
- Hold throttle fully open during cranking (eliminates intake restriction bias).
- Connect gauge, crank engine 3–5 seconds, record highest reading.
- Squirt 3–4 squirts of oil into the low cylinder and retest — ring wear improves, valve wear doesn't.
Frequently Asked Questions
Can I use this calculator for diesel engines?+
Yes — the polytropic formula works for any spark- or compression-ignition engine. Diesels run higher compression ratios (15:1–25:1) and typically have n ≈ 1.28–1.32 during cranking, similar to gasoline. Just note that diesel compression testers often report in bar instead of PSI (1 bar ≈ 14.5 PSI).
Why is my measured PSI lower than what the calculator predicts?+
Three common reasons: (1) You used n=1.4 (isentropic) instead of n=1.25–1.35. (2) The engine has worn rings or valves (the calculator assumes good condition). (3) Altitude was not corrected — every 1,000 ft drops atmospheric pressure by ~0.5 PSI. Check all three before tearing anything apart.
What is the 'rule of thumb' for PSI per compression ratio?+
At sea level with n=1.3: approximately PSI ≈ 35 × (CR − 1) + 15, but the exact formula is always P = 14.7 × CR^1.3. For quick mental math: 10:1 ≈ 370 PSI, 11:1 ≈ 440 PSI, 8:1 ≈ 250 PSI. The reference table above is more accurate.
How does dynamic compression ratio affect PSI?+
Always use dynamic compression ratio (DCR) for pressure estimates, never static. A 10.5:1 SCR engine with a mild cam (DCR ≈ 9.0) produces ~310 PSI, not ~400 PSI. That's why two engines with the same static CR can have very different cranking pressures.
More Engine Build Calculators
Every tool in the Engine Calculator family shares the same compression math — your numbers stay consistent.