This paper proposes a fast and simple model of a Compressed Natural Gas indirect injection system to predict the system dynamics with high accuracy for different operating conditions.In a retrofit system the CNG Engine Control Unit (ECU) is able to translate the petrol ECU control strategy in CNG operation. The adaptation of the engine to the natural gas type is handled by using the factory engine control strategy embodied in the factory ECU. The ECU monitors the engine through various sensors and controls its operation using a variety of actuators. Because of many input parameters, the control over the engine becomes quite complex. The use of advanced emission control systems makes such process even more complicated. The new Gas Control Unit is used to reprogram the ECU control parameters to correspond with the use of Natural Gas along with the characteristics of the Natural Gas injectors. Since gaseous fuels are not affected by evaporation effects, the intake fuel-air mixing transfer function has very little dynamic effects.The control system has been tested on a set of Air-Fuel ratio dynamic transients measured on a commercial engine, showing good accuracy in predicting the Air/Fuel ratio. Experimental results on an actual engine show high effectiveness of the presented method.