Abstract: A vapor injection system for introducing water vapor into an internal combustion engine includes a housing having first, second, and third chambers within the housing. An inlet conducts water into the first chamber from an unpressurized water container. A filter separates the first chamber from the second chamber. A small orifice extends from the second chamber into a third chamber. An airtight tube connects the third chamber to an opening in a carburetor of the internal combustion engine, the opening being located above a throttle valve of the carburetor. When the engine is running, reduced air pressure exists in the carburetor above the throttle valve and therefore in the third chamber. The orifice is sufficiently small that water passing from the second chamber into the lower pressure third chamber immediately vaporized as it enters the third chamber. The water vapor passes through the tube into the carburetor.