Abstract: A continuously variable cam mechanism designed to change the opening and closing characteristics of the cam operated intake and exhaust valves of an internal combustion engine as a function of engine speed. A cam on the camshaft is split into two adjacent cam pieces forming a single effective cam. The two cam pieces rotate around the camshaft in opposite directions to enlarge or reduce the profile of the combined cam lobe. Each cam piece carries a track follower which engages one of two oppositely directed spiral tracks carried by the camshaft, so that as the camshaft is made to move longitudinally with respect to the longitudinal position of the cam pieces, as a function of engine speed, the cam pieces are rotated about the camshaft in opposite directions.