The present invention relates to a variable valve timing system for an internal combustion engine which cyclically opens and closes a port providing fluid communication between a combustion chamber and a conduit leading thereto and which delays the closure of the port in response to a signal indicative of the power output required of the engine in order to vary the ratio of the mass of fluid inducted into the chamber to the mass of fluid expelled from said chamber during the period the port is open so that the mass of fluid retained in the chamber after closure of the port can be controlled without the use of a throttle valve.