Determining Compression Ratio

The calculation of compression ratios (C.R.) is based on the following formula:

Compression ratio= (Swept Vol. + TDC Vol.) divided by TDC Vol. Swept Vol.= (3.1416 x Bore Dia. x Bore Dia. x Stroke) divided by four.

T.D.C. Vol.= Head vol. + gasket vol. + deck vol. + dish/dome vol.

Head Vol. is published or measured in cubic centimeters (cc).

Gasket Vol.= (3.1416 x Gasket Bore x Gasket Bore x Compressed Gasket Thickness) divided by four.

Deck Vol.= (3.1416 x Bore Dia. x Bore Dia. x Deck Clearance) divided by four.

Dish/Dome Vol. is published or measured in cc's.

NOTE: Any volume below the piston compression height is a positive value and any volume above the piston compression height is a minus value.

DIMENSIONS: When calculating use cubic centimeters or cubic inches but do not mix them. The following conversions are listed: 1 inch = 2.54 centimeters .061 cubic inch = 1 cubic centimeter (cc) 1 cubic inch = 16.387 cubic centimeters.

EXAMPLE: Find C.R. for a Chevy 350 using KB120 .030 pistons:

Head Vol.= 76cc = 76 x .061 = 4.636 cubic inches.
Dish/Dome Vol. = 7cc = 7 x .061 = .427 cubic inches.
Gasket Thickness = .035".
Gasket Bore Dia = 4.166".
Deck Clearance = .005".
Deck Vol. = (3.1416 x 4.03" x 4.03" x .005") divided by four =.064 cubic inches.
Gasket Vol. = (3.1416 x 4.166" x 4.166" x .035") divided by four =.477 cubic inches.
Swept Vol. = (3.1416 x 4.03" x 4.03" x 3.48") divided by four =44.389 cubic inches.
TDC Vol. = 4.636 + .427 + .064 + .477 = 5.604 cubic inches.

COMPRESSION RATIO = (44.389 + 5.604) divided by 5.604 = 8.92.

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