Abstract: A variable cam mechanism for an engine includes a pivot shaft and a plurality of rocker arms pivotally mounted on the pivot shaft, at least some of which operatively engage a valve. A cam shaft extends generally parallel to the pivot shaft and transverse to the rocker arms. The cam shaft has cams secured thereon which are registered with the respective rocker arms so as to selectively contact them when the cam shaft rotates. A spring biased boost cam engages one of the rocker arms. An actuator connected to the boost cam changes the spring bias and thereby changes the timing and duration of contact between the boosted rocker arm and its cam. By moving the actuator, the operator can select between regular, low-speed mode and a boosted, high-speed mode.