Increasing design emphasis on factors such as styling, fuel reduction and soundproofing raises a number of additional problems concerning under-bonnet aerodynamics and heat exchange. Because experimental work on successive prototypes entails heavy penalties in terms of development lead-time, it is becoming more and more important to integrate simulation from the pilot study stage, as a way to minimize the number of prototypes. Fortunately, early integration of under-bonnet air-flow modelling is becoming an increasingly viable proposition, thanks to the spectacular increase in computer processing power, which stimulates the development of more efficient meshing software and facilitates the generalized implementation of CAD techniques throughout the design processes.Modelling thus emerges as a new investigatory method that enhances the design office's capabilities by enabling it to adopt a sharper design focus right from the pilot project stage. However the computed results of airflow simulation still require experimental validation. In this paper we address this issue by comparing the results of experimental wind-tunnel tests with simulated airflow values for the engine compartment of a Renault Mégane fastback vehicle. Our analysis primarily concerns measured and computed air speed and turbulence values at the air inlet and in the engine compartment.