The Pluses and Minuses of Piston Head Volume and Figuring Compression Ratio

Last week a customer asked me "why some piston manufacturers give a positive cc volume on pistons with a dome, and KB does the oppsite with a minus cc volume."

KB has always treated piston head volumes by how they affect combustion chamber volumes. In the case of a piston that has a dome, the dome removes volume from the combustion chamber, and gives us a negative number. The reverse is true of a dish style piston. The dish increases the combustion chamber volume, giving a positive value.

The internet or catalog provides the piston head cc volume listed under the photo. KB's compression calculators, require cc numbers as shown in the catalog or internet with the printed positive or negative value. Some small dome pistons may have a positive value because the valve relief volume is larger than the dome volume.

Scott Sulprizio

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