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Ethyl has conducted extensive fleet testing to investigate the effect of a manganese-based antiknock additive (MMT) on exhaust emissions from production cars. The fleet consisted of 48 cars - six cars each of eight models representing more than 50% of 1988 U.S. sales. Three of each model were tested for 75,000 miles using the base fuel. The other three used the base fuel plus 0.03125 gram manganese per gallon as MMT. Results of this testing show that use of the additive will not cause or contribute to the failure of emission control systems. Exhaust back pressure data for the 48-car fleet as well as a test of a close-coupled catalyst at 80 mph showed no indication of catalyst distress or plugging. Catalyst conversion efficiency was generally higher for units aged on the MMT fuel. Particulate emission data showed that less than 0.5% of the input manganese was exhausted as airborne for the FTP cycle.

Extensive vehicle fleet testing has demonstrated that use of MMT can reduce net tailpipe out emissions. The use of fuel containing the octane-enhancing, emission-reducing fuel additive leads to manganese oxide deposits in the vehicle exhaust system. Studies of the physical and chemical effects of manganese oxide deposits on the performance of catalytic converters conclusively demonstrated that MMT does not adversely affect catalytic converters and, in fact, protected the converters from phosphorus and zinc. Despite the overwhelming evidence that MMT is compatible with catalytic converters and vehicle emission control systems, concerns have recently been raised about the effect of manganese oxides on OBD-II catalytic converter monitoring.