Compression Ratio Calculator

This online calculator is designed to estimate the change in compression ratio due to a change in bore and/or stroke.

Enter the Stock Cylinder Bore Diameter from the table below, the amount that the bore is increased (overbore), and the stroke length.  Click the Calculate Compression button below for results.
Stock Cylinder Bore Diameter:   Inches

Stock component engines: cast iron crankshaft, stock connecting rods, stock cast pistons

Heavy duty engines: forged or nodular iron crankshaft, shot peened connecting rods, forged pistons

Racing/Performance engines: forged crankshaft, high strength alloy rods, high strength alloy pistons

Cylinder Over Bore:  Inches
Stroke Length:   Inches
Head Chamber:   CCs
Gasket Thickness:   Inches
Deck Height:
Note: Neg. number above deck, Pos. number below deck
  Inches

Piston Dish: 

  CCs
Gasket Volume:
  cubic in.

Number of Cylinders: 

 

Maximum Engine RPM (stock):

  RPM

Mean Piston Speed (stock):

  FPM

Maximum Engine RPM (heavy duty):

  RPM

Mean Piston Speed (heavy duty):

  FPM

Maximum Engine RPM (race):

  RPM

Mean Piston Speed (race):

  FPM
 
Stock and New Engine Displacement
and estimated Compression Ratio 
 

Stock

New Bore

Cylinder Bore Diameter:
  Inches

  Inches

Displacement:
  C.I.D

 C.I.D.

Compression Ratio:

  to 1

  to 1

Head Chamber Vol.:   cubic in.   cubic in.
Piston Dish Vol.:   cubic in.    cubic in.
Swept Volume:    cubic in.   cubic in.
T.D.C. Volume:   cubic in.   cubic in.
Deck Volume:   cubic in.   cubic in.

Common Pontiac Engine Specifications

Pontiac Bore Stroke
230 3.875 3.25
250 3.875 3.52
326 3.718 3.75
350 3.875 3.75
389 4.0625 3.75
400 4.12 3.75
421 4.0937 4.00
428 4.12 4.00
455 4.15 4.21