Southwest Research Institute (SwRI) is exploring the feasibility of extending the performance and fuel efficiency of the spark ignition (SI) engine to match that of the emission constrained compression (CI) engine, whilst retaining the cost effective 3-way stoichiometric aftertreatment systems associated with traditional SI light duty engines. The engine concept, which has a relatively high compression ratio and uses heavy EGR, is called “HEDGE”, i.e. High Efficiency Durable Gasoline Engine. Whereas previous SwRI papers have been medium and heavy duty development focused, this paper uses results from simulations, with some test bed correlations, to predict multicylinder torque curves, brake thermal efficiency and NOx emissions as well as knock limit for light and medium duty applications. The parametric effects of high EGR dilution, variable valve timing, manifold and coolant temperatures are explored more readily than could be conducted with prototype test engines that are limited by EGR distribution, diesel swirl ports distribution and probably poor fuel preparation and wall wetting. Results are compared to state of art light duty CI engine performance. Conclusions are drawn as to the performance and fuel efficiency potential of the HEDGE concept.