Static Compression Ratio Calculator

 Definitions:

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

Static Compression Ratio Calculator

Bore inches Diameter of cylinder
Stroke inches Distance piston moves
Head Gasket
thickness
inches Measure your head gasket tthickness. It's usually 0.040", but not always...for example, on K1200 BMWs it is 0.070". On 113/135" ORCA motors it is 0.00 because they use metal seals and no head gaskets.
Deck Height inches Move piston to TDC, and measure the distance between the flat top surface of the piston (not the top of a dome or the bottom of a valve relief) and the top of the cylinder. Enter a negative number if the piston's flat surface sticks up past the top of the cylinder. At TDC the piston will "rock" so be careful in your measurements. ORCAs are .035".
Piston top
volume
cc's You'll have to calculate this value the hard way i.e. do it yourself or call the piston manufacturer. Enter a negative number for a domed piston. Enter a positive number for a dished or dimpled piston. If you have a flat piston with no valve reliefs enter 0.00.
Combustion
Chamber
cc's Get out your 100ml burette, your light, non-nasty grease, your handy dandy plexiglass or lexan sheet, your Marvel Mystery Oil and a six pack and get ready to party. Don't forget to cc the combustion chamber.
Static Compression Ratio:
  Take Out Window (Hold the Mayo between your....)

 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.

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