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