A comprehensive test program was conducted using a single cylinder CFR engine to evaluate lean supercharged operation (LSO) of the spark ignition engine for improved efficiency. The intake manifold pressure and temperature and the exhaust pressure were controlled to simulate the addition of a turbocharger to the engine.Results of the experimental work indicated that LSO has the potential of improved engine efficiency and NOx emissions comparable to, or lower than, the naturally aspirated engine. For equal power output from the engine, efficiency increases of 14% were accompanied by reductions in Brake Specific NOx (BSNOx) emissions of approximately 76%. For a case of equal BSNOx emissions, an efficiency improvement of 6.4 points (over 40%) was observed. For operation at realistic lean supercharged conditions, the hydrocarbon and carbon monoxide emissions were not significantly different than those for the naturally aspirated engine. Exhaust energy estimates for the lean supercharged conditions tested indicated that sufficient exhaust energy is available to power a typical automotive type turbocharger.