Compression Ratio Calculator
Enter swept volume and all clearance volumes (chamber, gasket, piston) to get the true static compression ratio — the number that decides fuel tolerance.
Last updated: 2026-09-15
How the calculation works
- CR is total cylinder volume at BDC divided by volume at TDC — the clearance volume collects chamber, gasket and piston features.
- Dome pistons subtract volume (they intrude into the chamber); dish pistons add it.
- The result is static CR — dynamic CR (after intake closes) is always lower.
Formula
CR = (V_swept + V_clearance) / V_clearance V_clearance = chamber + gasket + piston
| Symbol | Meaning | Unit |
|---|---|---|
CR | Static compression ratio | :1 |
V_clearance | All volume above the piston at TDC | cc |
Worked example
Interpreting the result
Static CR drives knock tendency: pump gas tops out around 10.5–11:1 static in iron engines, a bit higher with aluminum chambers and good quench. Every 1 point of CR is worth roughly 3–4% thermal efficiency — the reason premium fuel exists and why squish/quench design matters as much as the raw number.
Assumptions
- Volumes measured or manufacturer-specified at TDC.
- Static CR — ignores intake valve closing point (dynamic CR).
Limitations
- Dynamic/effective CR depends on cam timing and rod length — build a different number.
- Gasket bore vs cylinder bore mismatch changes gasket volume.
Frequently asked questions
What compression ratio for pump gas?
About 10.5:1 static max for iron-head engines on 91 octane; aluminum heads and good chamber design stretch that to 11:1. Beyond that needs race fuel or boost control.
Why is my dynamic CR lower?
The intake valve closes well after BDC; below a certain RPM the rising piston pushes mixture back out. Dynamic CR uses the volume at intake closing — typically 1.5–2 points lower than static.