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Compression Ratio Calculator

Calculate engine static compression ratio from bore, stroke, combustion chamber, head gasket, and piston deck volumes.

L ading . . .

The compression ratio calculator computes the static compression ratio of an internal combustion engine from bore, stroke, combustion chamber volume, head gasket volume, and piston deck volume. This is an essential calculation for engine builders selecting pistons, cylinder heads, and gaskets to match a target compression ratio.

The Compression Ratio Formula

Static compression ratio compares the cylinder volume at bottom dead center to the volume at top dead center:

CR = 1 + (0.7854 × B² × S) / (CCV + HGV + PDV)

Where B is bore, S is stroke, CCV is combustion chamber volume, HGV is head gasket volume, and PDV is piston deck volume. The numerator is the swept volume of one cylinder. The denominator is the total clearance volume.

How to Use

Select your unit system (inches/cubic inches or millimeters/cubic centimeters). Enter the bore, stroke, combustion chamber volume, head gasket volume, and piston deck volume. Positive PDV values represent dished pistons; negative values represent domed pistons. The calculator displays the compression ratio as X:1 format, along with the swept and clearance volumes.

Understanding Compression Ratios

Higher compression squeezes the charge harder, raising thermal efficiency and power. Typical ranges include:

  • Pump gas street engines: 9.5:1 to 11:1
  • Forced induction: 8:1 to 9.5:1
  • Race engines (high octane): 12:1 to 15:1
  • Diesel engines: 14:1 to 23:1

Frequently Asked Questions

How do you calculate compression ratio?

CR = 1 + (swept volume) / (clearance volume). Swept volume per cylinder is 0.7854 × B² × S. Clearance volume is the sum of chamber volume, head gasket volume, and piston deck volume.

What is a good compression ratio for pump gas?

On 91-93 octane pump gas, naturally aspirated engines typically run 9.5:1 to 11:1 with aluminum heads and 9:1 to 10:1 with iron heads. Forced induction builds drop to 8:1-9.5:1.

What is the difference between static and dynamic compression ratio?

Static CR is a geometric ratio measured with the engine stopped. Dynamic CR accounts for intake valve closing timing, which always reduces effective compression at running speed.

How does compression ratio affect horsepower?

Higher compression raises thermal efficiency. A rule of thumb: every full point of CR adds 3-4% to peak power. Returns diminish above 12:1.

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