Emissions optimization, particularly for complex engines, relies on developing mathematical correlations or 'Response Surface Models' of engine outputs. There is a clear tradeoff between the complexity of the model and its usefulness for a range of optimization tasks. Several current Sl engine management systems use a 'Spark efficiency curve' to elegantly describe the response of engine torque to spark advance using a prescribed characteristic line that is fitted to a spark sweep with only two degrees of freedom. This provides a powerful method of data reduction. A similar technique has been developed to characterize engine emissions data. A single efficiency curve with only two degrees of freedom can be used to accurately describe the response of, say, hydrocarbon emissions to spark over a wide range of engines and operating conditions. The technique offers similar data reduction advantages for engine management system developers and those optimizing engines using high DoF models. The form and application of the efficiency curves is discussed and examples of application to tasks such as engine emission optimization are discussed.