In this work an integration between a 1D code (Gasdyn) with a CFD code (OpenFOAM®) has been applied to improve the performance of a Moto3 engine. The four-stroke, single cylinder S.I. engine was modeled, in order to predict the wave motion in the intake and exhaust systems and to study how it affects the cylinder gas exchange process. The engine considered was characterized by having an air induction system with integrated filter cartridge, air-box and intake runner, including two fuel injectors, resulting in a complex air-path from the intake mouth to the intake valves, which presents critical aspects when a 1D modeling is addressed. The exhaust and intake systems have been optimized form the point of view of the wave action. However, due to the high revolution speed reached by this type of engine, the interaction between the gas stream and the fuel spray becomes a key aspect to be addressed in order to achieve the best performance at the desired operating condition. In particular, the evaporation of the fuel injection causes an increase of charge density due to the cooling of the fuel droplets, resulting in a supercharging effect. Moreover, a precise targeting and injection timing of the spray is important in order to avoid liquid film formation in undesired zone, which usually lead to irregular power output. This work proposes an analysis, by means of coupled 1D-3D simulation, of the fuel spray propagation inside the air-box. Spray-wall interaction is considered as well as liquid film formation and evaporation. The final target of the study is the analysis of the fuel injection strategy both on the point of view of targeting and timing, finalized to the optimization of the injection system for the engine which will be used in the next championship.