Abstract: Water injection is carried out in a pressure system, with water from a reservoir pressurized by an injection pump energized only above predetermined torque demand levels, under the control of a vacuum switch sensing intake manifold vacuum. Water injection is also precluded until the engine reaches operating temperature by a vacuum switch connected to a PVS valve. A water spray nozzle is mounted in the air cleaner and directs droplets into the carburetor intake. A purging pump causes purging of a short section of the feed line upstream of the injection nozzle after the engine is shut off to minimize water drippage into the engine carburetor.