The trade-off between engine operating efficiency and NOx emissions in the prechamber three-valve stratified-charge engine is examined in a series of parametric studies using an improved model developed at M.I.T. (1). Engine geometric, operating, and combustion parameters are varied independently and the effects on brake-specific-fuel-consumption, exhaust temperature and brake-specific-NO observed. Parameters chosen for study are: timing of the start of combustion, overall air-fuel ratio, prechamber air-fuel ratio at the start of combustion, main-chamber combustion duration, prechamber size (prechamber volume and orifice diameter), EGR (in main and prechamber intakes), and load. The results quantify trade-off opportunities amongst these design and operating variables which are available to the engine designer.