Compression Ratio Calculator

Your engine’s static compression ratio is easily calculated if you know six measurements or volumes:

  • Bore— The diameter of the cylinder bore
  • Stroke— The distance the piston travels in the cylinder
  • Deck Height— The distance between the top of the cylinder bore and the top of the piston, when the piston is at TDC (Top Dead Center, or at its highest position
  • Compressed thickness of the head gasket— Usually 0.040″, but varies with gasket manufacturer and application
  • Piston top volume— If the piston is dished, domed, or dimpled, that has an affect on compression ratio.
  • Combustion chamber volume— How much open space is in the head above the cylinder

 If you want to take the formulas with you and put them in your tool box, here they are. 16.387 is a number than converts cubic inches to cc’s.

Calculate Compression Ratio

Calculating engine compression ratio

  • Find the owner’s manual; this will help you to take your measurements
  • Use an engine degreaser to make your engine as clean as you can before starting
  • Use a bore gauge to measure the cylinder’s bore
  • Find out the combustion chamber’s volume (from the manufacturer or owner’s manual)
  • Find out the piston’s compression height (from the manufacturer)
  • Find the piston’s dome/dish volume (from the manufacturer)
  • Calculate the clearance from piston to deck (bore × bore × 0.7854 × gap between piston and deck at top dead center (TDC))

– Measure the thickness of your head gasket and bore

– When you have all the figures, use this formula for the calculation of your engine’s compression ratio:

CR = (cylinder volume + clearance volume + piston volume + gasket volume + chamber volume) / (clearance volume + piston volume + gasket volume + chamber volume)

Displacement  =(Bore ÷ 2)2  x  3.14  x  Stroke  x  16.387
Head Gasket space  =(Bore ÷ 2)2  x  3.14  x  Gasket thickness  x  16.387064
Deck Height space  =(Bore ÷ 2)2  x  3.14  x  Deck Height  x  16.387064
Compressed Volume  =Head Gasket space  +  Deck Height space  +   
Piston Top volume  +  Combustion chamber volume
Uncompressed Volume  =Compressed Volume  +  Displacement
Compression Ratio  =Uncompressed volume  ÷  Compressed volume

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