Compression Ratio Calculator
Calculate static engine compression ratio, swept cylinder volume, and total displacement in cubic inches (CID) and cubic centimeters (cc).
Recommended Tools & Equipment
Tested hardware and components for high reliability
How Engine Compression Ratio is Calculated
The static compression ratio (CR) is the ratio of maximum cylinder volume when the piston is at Bottom Dead Center (BDC) to minimum cylinder volume when the piston is at Top Dead Center (TDC).
Compression Ratio Formula
CR = (Swept Volume + Clearance Volume) / Clearance Volume
Fuel Octane Guidelines by Compression
- 8.0:1 to 9.0:1: Regular 87 Octane / Ideal for turbo or supercharged boost
- 9.1:1 to 10.5:1: Premium 91-93 Pump Gas (Modern naturally aspirated)
- 10.6:1 to 12.0:1: 93 Octane / E85 Ethanol recommended
- 12.5:1+: Dedicated race fuel (100+ Octane) or pure E85
Frequently Asked Questions
What is the difference between static and dynamic compression ratio?
Static CR is purely geometric. Dynamic CR accounts for when the intake valve closes (IVC) during the compression stroke. Camshafts with longer duration close later, lowering dynamic compression.
Why do dished pistons lower compression while domes raise it?
Dished pistons create extra chamber volume at TDC, expanding the clearance space and lowering the compression ratio. Domed pistons take up space in the combustion chamber, shrinking clearance volume and raising compression.
How does head gasket thickness affect quench distance?
Optimal quench distance (piston-to-cylinder-head clearance) is typically 0.035" to 0.045". It combines deck clearance and crushed gasket thickness to create turbulence and suppress detonation.