Formula
The compression ratio dictates how tightly the air/fuel mixture is squeezed. Higher ratios yield more efficiency and power but require higher-octane fuel to prevent pre-ignition (detonation).
Worked Example
This calculator provides estimates based on standard mathematical formulas. Real-world results will vary based on mechanical condition, environmental factors, and other variables.
Frequently Asked Questions
Street engines on pump gas typically run between 9.0:1 and 10.5:1. Race engines on high-octane or E85 can run 12.0:1 to 14.0:1 or higher.
Most naturally aspirated engines run safely at 9:1 to 11:1 on premium pump fuel. Higher ratios need higher octane or risk knock.
Higher compression extracts more work per combustion cycle, increasing power and efficiency — up to the limit where the fuel's octane can no longer prevent detonation.
Static compression is the geometric ratio from the volumes. Dynamic compression accounts for when the intake valve actually closes, giving a more realistic running figure — especially with aggressive cams.
Yes. A thicker gasket increases clearance volume, which lowers the compression ratio slightly — a common way to drop compression for forced induction.