THE BEECHMONT CREST AUTOMOTIVE LEARNING ZONE
VARIABLE COMPRESSION PISTON
This is a piston design that prevents engine knocking (pinging). A variable compression piston consists of two pieces: a piston head and a main body. The piston head fits over the main body piece.
Engine oil pressure between the head and body piece forms a hydraulic cushion. During normal driving, oil pressure pushes the head of the piston upward to maximize the compression ratio. At low engine speeds, combustion pressure forces the head of the piston downward. This lowers the compression ratio, and also prevents engine knocking.