While a large number of dynamic simulation models have been presented for various four-cycle spark ignition engine subsystems in the literature, very few have been presented for the entire engine which can claim an acceptable level of accuracy for engineering purposes. This paper presents a nonlinear three state (three differential equation) dynamic model of an SI engine which has the same steady state accuracy as a typical dynamometer measurement of the engine over its entire speed/load operating range (±2.0%). The model's accuracy for fast transients is of the same order in the same operating region.Because the model is so mathematically compact, it has few adjustable parameters and is thus simple to fit to a given engine either on the basis of measurements or given the steady state results of a larger cycle simulation package. The model can easily be run on a Personal Computer (PC) using a ordinary differential equation (ODE) integrating routine or package.The paper shows how the model can be fitted using either experimental data or simulation results. Moreover it includes the results of simulations run on experimental data with large and fast throttle (tip-in/out) transients which show its excellent steady state and dynamic accuracy. The model is also useful for control system design and evaluation.