In this paper an integrated experimental and numerical approach is applied to optimize a new 2.5l, four valve, turbocharged DI Diesel engine, developed by VM Motori. The study is focused on the EGR system.For this engine, the traditional dynamometer bench tests provided 3-D maps for brake specific fuel consumption and emissions as a function of engine speed and brake mean effective pressure. Particularly, a set of operating conditions has been considered which, according to the present European legislation, are fundamental for emissions. For these conditions, the influence of the amount of EGR has been experimentally evaluated.A computational model for the engine cycle simulation at full load has been built by using the WAVE code. The model has been set up against experiments, since an excellent agreement has been reached for all the relevant thermo-fluid-dynamic parameters.The simulation model has been used to gain a better insight on the EGR system operations. Furthermore, the influence of the most important geometric parameters (EGR valve seat diameter, intake manifold throttle diameter) on the amount of recycled gas for a few critical operating conditions has been investigated.