This paper presents the results of Phase II of the Hydrogen Enrichment for Aircraft Piston Engines Program. The overall program, and specifically the Phase I results, have been discussed by Menard et. al.4 previously. In Phase II, a Lycoming TIO-541-E engine was equipped with a gasoline operated hydrogen generator and installed on the dynamometer stand of the experimental facility at AVCO-Lycoming Division. A matrix of lean-out curves at sea level for different spark advances, hydrogen enrichment rates and power levels were obtained. An analysis of these data shows that while the lean-out to very low equivalence ratios with hydrogen enrichment does not present any difficulty, it seems that for certain types of engines one may achieve even better results with gasoline only, if the engine is properly timed. It has been found also that in this ultralean region, power recovery can be achieved by boosting the manifold pressure without exceeding the engine red line temperatures even at relatively high powers. Test cell altitude checks confirm the possibility of utilizing certain ultralean techniques at useful altitudes, without overheating the engine or inducing detonation.The hydrogen generator flight model and the engine were installed in the flight configuration of the flight test stand. Fuel economy measurements and limited emission measurements were obtained on the flight test stand, in order to spot check the dynamometer results and the potential for reduced emissions. It has become evident that with proper timing, one can run the engine ultralean (fuel/air ratios as low as 0.05) with gasoline only during a steady-state operation without sacrificing performance (except for critical altitude). Under such conditions, fuel economy improvements of up to 20% have been obtained. The emission measurements conducted so far appear to indicate that, if these techniques can be implemented during taxi, idle, and approach, it may be possible to meet the Federal Emissions Standards.