Abstract: A "hang-on" after-cooler accessory unit adapted to be mounted on an existing supercharged internal combustion engine having a carburetor supplied with natural gas fuel. The unit is mounted on an existing engine by removing the engine carburetor from the intake manifold, placing the unit over the carburetor mounting studs on the intake manifold, replacing the carburetor over the mounting studs and inside the unit, creating an air-tight seal between the unit and the carburetor, and connecting the inlet of the unit to the discharge port of the engine supercharger. The unit includes a water-cooled radiator or heat exchanger through which the supercharged air passes on the way to the carburetor air intake. The accessory unit is designed to be easily mountable on any internal combustion engine of the carburetor type and greatly increases the efficiency of engine operation because of the resultant cooling of the supercharged air.