The present paper describes an experimental and numerical study on the effect of the nozzle flow number (FN) on the full load performance of a modern Euro5 diesel automotive engine, in terms of torque, efficiency and exhaust emissions. The improvement of the diesel engine performance requires a continuous development of the engine components, first of all the injection system and in particular the nozzle design. One of the most crucial factors affecting performance and emissions is the nozzle flow number and its influence becomes more and more important as high performance and low emissions are continuous requirements. Indeed, reducing the nozzle flow number, due to an increase of spray-air mixing, an improvement in PM-NOx trade-off is generally expectable. On the other hand, at full load, where peak firing pressure and exhaust valve temperature become the limiting factors, critical operating conditions can be easily reached reducing the nozzle hole diameter. Correspondingly, spray, combustion process and pollutants formation have been numerically analyzed in details by means of 3D CFD simulations. Results indicate that full load performance are attainable for all tested nozzle configurations, but reducing the nozzle flow number, an unexpected trade-off of smoke emissions versus FN has been detected. In this respect, the numerical investigations have put into evidence the crucial role of the spray dynamics and the flame evolution during the expansion stroke.