Water-Cooled VW Performance Handbook

Compression ratio calculator

You need a couple of measurements that are not easy to get in order to calculate your compression ratio, but once you have your figures, here's the calculator. Note that for domed pistons, the volume will be a negative number.

Bore mm
Stroke mm
Head volume cc
Piston volume cc
Gasket thickness mm
Cylinder volume cc
Gasket volume cc
Compression ratio   :1

For example, a four-cylinder motor with a bore of 79.5 mm, a stroke of 86.4 mm, a head volume of 27 cc, a piston volume of 26 cc, and a 1.75 mm thick head gasket will have a compression ratio of 7.95:1.

The formula for compression ratio is:

Compression ratio =   CD + PV + HV + HGV
PV + HV + HGV

Where:

  • CD = cylinder displacement
  • PV = piston volume
  • HV = cylinder head volume
  • HGV = head gasket volume