In this paper, a generic methodology is proposed to simulate the static and dynamic responses of a SI gas engine. The predicted simulation of engine performances is based on a one zone thermodynamic model. The turbocharger is modeled by using polytropic coefficients, the intercooler by its efficiency. The Ventury effect carburetor model is based on physic properties and the butterfly valve model uses a classical approach. A comparison between the simulation and experimental results is realized in terms of static and dynamic performances in closed loop. Comparisons with actual data obtained on a 210 kW engine shows that the maximum error is less than 5 %.