A method for estimating steady state values of engine operating characteristics is provided so that engine control can later be based on the estimated values. An amount of change in a specific engine characteristic (for which the steady state values are to be estimated) is set as a product of an unknown value and a dimensionless known value for predetermined times from when a specific engine control parameter is changed. A value of the specific engine characteristic at a given time is calculated as a sum of the products at the given time, with the products respectively corresponding to plural prior changes in the specific engine control parameter. Unknown values are calculated multiple times based on values of the specific engine characteristic measured in a conformance test, and the steady state values of the specific engine characteristic are estimated.