Octane number requirement mapping of an 8:1 CR Ford 2.3 liter engine was carried out under dynamic conditions at 1″, 3″ and 6″ vacuum accelerations. Equations were developed relating octane number requirements to engine rpm, manifold vacuum, spark advance, air-fuel ratio, and exhaust gas recirculation rate. These equations, together with Ford's extensive emissions and fuel consumption data, were used to determine whether octane number requirements constitute a limiting constraint in the optimization of engine-vehicle systems. In addition, steady-state and dynamic octane number requirements were compared at 9:1 CR. The results indicate that dynamic octane number requirements cannot be predicted from limited steady-state requirements data.