How to Figure Compression

An engine's compression ratio describes the combined volume that exits in an engine's cylinder when the piston within the cylinder is at its lowest position and at its highest position. The higher the compression ratio, the greater the power the engine produces. However, higher compression requires the use of a higher octane fuel to prevent premature ignition of the air/fuel mixture within the cylinder. Compression ratio can be calculated by adding the cylinder's volume by the cylinder head's chamber volume, then by dividing the result by the cylinder head's chamber volume. The challenge lies in determining the cylinder's volume.

Things You'll Need

• Engine specification's manual

Instructions

1. Consult the engine specification's manual and record the following specifications: bore, stroke, cylinder head's chamber volume.

2. Convert the bore and the stroke from cubic inches to cubic centimeters by multiplying each value by 2.54. For example, if the engine's bore is 4.0 cubic inches, the bore value would convert to 10.16 cubic centimeters. If the engine's stroke value is 3.5 inches, the stroke would convert to 8.89 cubic centimeters.

3. Calculate the engine's cylinder volume. The formula is as follows: cylinder volume = pi/4 x bore² x stroke. A value of 3.14 may be used for pi. For example, if the engine's bore is 10.16 cubic centimeters and the stroke is 8.89 cubic centimeters, the volume of each cylinder is 720.37533344 cubic centimeters.

4. Add the cylinder volume and the cylinder head's chamber volume. For example, if the cylinder volume is 720.37533344, and the chamber volume of the cylinder head is 60 cubic centimeters, the result would be 780.37533344.

5. Divide the result from step 4 by the chamber volume of the cylinder head to determine the compression ratio. For example, 780.37533344 divided by 60 equals approximately 13.006. Rounded up, the engine's compression ratio is 13:1.

References

• Auto Math Handbook, John Lawlor, 1992