Tech Tips

Instructions For Calculating Compression Ratio

Compression Ratio (CR) is defined as the ratio between the total volume above the piston at BDC, and the clearance volume above the piston at TDC.

To Determine Compression Ratio You Need To Know:
1. Cylinder volume (V) or cylinder displacement, determined by cylinder bore and stroke (indicated by movement of piston from BDC to TDC).

The formula for cylinder volume in cu. in. is:
Bore x Bore x Stroke x .7854

2. Clearance Volume (VCL) is the volume above the piston (actually above the top piston ring) at TDC. It consists of several small volumes which are measured in cubic centimeters or C.C.'s. as follows.

The formula for Deck Clearance Volume is:

Bore x Bore x Depth of Piston @ TDC x 12.87
The formula for Gasket Volume is: *refer to chart
Bore of Gasket x Bore of Gasket x Thickness x 12.87

Valve Notch Volume:

Available from your piston MFG, or must be C.C.'d using a burette. This figure must be added to clearance volume.

Piston Dome Volume:

Available from your piston MFG, or must be C.C.'d using a burette. This figure must be subtracted from clearance volume.

Combustion Chamber Volume:
Available from your Cyl, Head MFG, or must be C.C.'d
using a burette.

3. To Convert Total Clearance Volume in C.C.'s to cu. inches multiply by .06102.

4. Add the two volumes together, (V + VCL) then divide by VCL.
The formula is:

CR= V + VCL
       VCL

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