Steady-state data obtained from a 400 CID V-8 conventional engine at one speed/load/compression ratio combination are reported to show the effect of engine emission constraints on fuel consumption and octane number for trace knock. The important trends shown by these results were: Various calibration combinations of air/fuel (A/F) ratio, exhaust gas recirculation (EGR), and spark retard can be used to control NOx emissions. A substantial reduction in NOx emissions was achieved by using EGR and optimized spark timing without suffering a fuel economy penalty; but hydrocarbon (HC) emissions tended to be high. When both HC and NOx emissions were reduced by optimal adjustments of EGR and spark timing, fuel consumption was increased, but octane number for trace knock was decreased. At controlled NOx emission levels, the lowest HC emissions were obtained at A/F ratios of 16 to 18:1. The minimum achievable HC emission level increased with decreasing NOx emissions. Injection of secondary air at the exhaust ports generally decreased HC emissions at A/F ratios richer than stoichiometric when the engine exhaust temperatures were greater than 1100°F. Use of secondary air at lean A/F ratios increased HC emissions. In general, through the use of the basic engine parameters investigated, the substantial reduction in engine emissions of HC and NOx was associated with a fuel consumption penalty and with a reduction in the octane number for trace knock measured under the emission test conditions.