Abstract: An intake expansion chamber apparatus (10) for internal combustion engines is disclosed which preheats and expands the vaporized fuel/air mixture as the mixture flows between the carburetor and the combustion chamber. The apparatus includes an expansion chamber section (18) through which the fuel/air mixture flows between an inlet (20) connected in fluid communication with the intake manifold and an outlet (26) joined in fluid communication with the intake manifold at a location downstream from the outlet. The fuel/air mixture is heated during its passage through the expansion chamber by a heat transfer surface (30) to improve the vaporization and to more completely combust the hydrocarbons upon ignition. A safety valve (46) is provided to release pressure built up within the expansion chamber as during an engine backfire.