A fuel-injected, turbocharged, six-cylinder air cooled Teledyne Continental Motors TSIO-360-C aircraft engine was operated over a range of test conditions to investigate factors influencing the effectiveness of air injection in reducing exhaust emissions. The test program included the standard EPA five-mode baseline cycle revised to a seven-mode cycle and engine fuel-air ratio leanout tests. All tests were carried out using induction and cooling air at a temperature of 59° F and a relative humidity of 60%. Such factors as composition and temperature of the reacted mixtures were evaluated by varying the engine fuel-air ratio and exhaust tube air injection flow rates for various engine loads. No attempt was made to optimize the location of the point of air injection. The standard exhaust system was not modified other than to weld air injection couplings below the cylinder heads in the exhaust tubes. Air was injected into the exhaust tube of each cylinder for all modes and an exhaust gas analysis was made for hydrocarbons, oxides of nitrogen, and carbon monoxide content. For the revised seven-mode EPA baseline cycle it was found that as the amount of injection air was increased both carbon monoxide and hydrocarbons showed a marked decrease while the oxides of nitrogen changed only slightly. When sufficient air was injected to lower the pollutant levels to meet the Environmental Protection Agency (EPA) standards it was found that the temperature of the exhaust gas mixture as it entered the turbine exceeded the maximum recommended temperature of 1650° F by approximately 225° F while operating in the high power modes. Leanout data on a mode basis showed that, in the lower power modes of idle and taxi, air injection was more effective in reducing hydrocarbons and carbon monoxide at the richer fuel-air ratios. At the higher power modes of takeoff, climb, and approach air injection was more effective at the leaner fuel-air ratios. In combining air injection at various rates of flow and leaning the engine mixutre through fuel management it was found that the EPA standard could be met while not exceeding a turbine inlet temperature of 1650° F.