An experimental “Synchrothermal” system with very low exhaust emission levels was developed based on duPont thermal exhaust manifold reactors. Rich fuel-air mixtures were used to decrease nitrogen oxides. Recirculation of a controlled amount of exhaust gas under selected operating conditions further decreased nitrogen oxides. Injections of air synchronized with individual exhaust valve opening promoted more complete combustion of hydrocarbons and carbon monoxide. Acceptable reactor temperatures under high-speed operation were maintained by diverting some of the injected air to the intake manifold. Muffler back pressure and a fixed recycle line proportioned the exhaust recycle which was injected between the carburetor venturi and the throttle plate. Recycle flow was blocked off during idle, wide open throttle and choked operation.Composite exhaust emission levels below 0.3% carbon monoxide, 10 ppm unburned hydrocarbons and 150 ppm nitrogen oxides were obtained by the federal test procedure. Fuel consumption data indicating a fuel penalty of 16% were obtained from extended runs on a Mileage Accumulation Dynamometer. No attempts at obtaining system or component life data were made in these initial studies.