A coupler, such as a piston pin, is pivotally coupled to a piston such that the piston can pivot about a first axis relative to the coupler. A connecting rod is coupled to the coupler for pivoting about a second axis. The relative positions of the first and second axes can be shifted by pivoting an eccentric portion of the coupler to thereby vary the compression ratio of a piston cylinder within which the piston slides. The coupler comprises a pivot member engager portion that is selectively shifted from first to second positions to vary the compression rates in response to shifting of a pivot member. The pivot member engager is shifted from first to second positions as the piston approaches the bottom dead center position. The pivot member and pivot member engager disengage from one another as the piston travels away from the bottom dead center position.