The effects of air-fuel mixture quality and cylinder-to-cylinder air-fuel distribution on exhaust emissions have been determined on two engine-vehicle combinations. California Motor Vehicle Pollution Control Board (CMVPCB) test cycle emissions were measured on vehicles using a pre-mixed and pre-heated air-fuel charge supplied by a steam jacketed, nine cubic foot vaporization tank. The vaporization tank provided a near constant air-fuel mixture ratio for all operating modes of the 7-mode CMVPCB test cycle. The two vehicles were evaluated at nominal air-fuel ratios of 14:1, 16:1 and 18:1.Cylinder-to-cylinder air-fuel distribution during the transient operation of the 7-mode CMVPCB test cycle was measured on a 200-CID six cylinder and a 289-CID eight cylinder engine. The procedure employed was to record the total carbon emissions (CO + CO2 + CH4 equivalent) for each cylinder during successive test cycles. Distribution measurements thus established were found to be repeatable within ± 0.40 air-fuel ratios. Comparing data with that of a conventional carbureted induction system showed that the vaporization tank significantly improved cylinder-to-cylinder air-fuel distribution for all modes of the cycle. This improvement permitted lean operation and resulted in low carbon monoxide concentrations. CMVPCB cycle hydrocarbon emission levels were similar for both the vaporization tank and the carbureted induction system when compared at equivalent air-fuel ratios. At the leaner air-fuel ratios obtainable with the vaporization tank, an actual increase in cycle hydrocarbon levels was noted.Mixture quality was visually observed through a transparent section of the intake manifold installed on a six cylinder engine. Carbureted mixtures were found to be extremely heterogeneous while wet and globular in form. Manifold wetness with the vaporization tank induction system was nonexistent throughout the 7-mode test cycle.The installation of a 4-inch high intake manifold riser extension, which provided an additional intake charge mixing volume, resulted in an improved cylinder-to-cylinder air-fuel distribution with the conventional lean carburetion. Application of exhaust heat to this mixing chamber accomplished little in improving the cylinder-to-cylinder air-fuel distribution.