Compression Ratio (CR) tells you the relationship between minimum and maximum cylinder volume. The cylinder is largest when the piston is at BDC. It is smallest when the piston is at TDC. The ratio is expressed as:
Max. Cylinder Vol. : Min. Cylinder Vol.
Actual measurements are used to calculate the ratio. Then, the numbers are reduced to simplify the expression. For example, 10 : 1 is a common CR.
How is it calculated?
Before you calculate CR, you need to know the following measurements. (Click the links to learn more about each measurement.)
- Cylinder Bore Diameter
- Stroke Length
- Combustion Chamber Volume
- Piston Head Volume
- Deck Clearance
- Compressed Head Gasket Thickness
After you have the measurements, just plug them into our Compression Ratio Calculator.
How does it affect performance?
An engine's CR is a clue to the power it can make. A high CR means more air and fuel are compressed into a smaller space. This usually means more power.
When the air and fuel ignites, it creates extreme pressure inside the cylinder. Higher pressure pushes down harder on the piston. More force on the piston creates more power and torque.
High CR indicates more pressure. However, the amount of pressure in the cylinder depends on other factors. Valve and ignition timing also play a role.
High CR has pros and cons. You will need to consider the following:
- Stock parts may fail under increased compression. You may need to upgrade your connecting rods, pistons, and other parts.
- High compression engines require fuel with a higher Octane Rating to prevent Detonation.
- Engines with power adders will require a lower CR.
- Camshaft selection and Ignition Timing may need to be adjusted.