In the last decade internal combustion engines have experienced a phase of deep and intense investigation finalized mainly to the optimization of performance and fuel consumption. In the field of high performance engines, both for automotive and motorbike applications, the attempt to reach higher and higher levels of power output has made the design process one of the most critical step in the engine production process. A high performance engine requires a rather carefully tuned intake and exhaust system in order to reach the maximum power output that the engine can potentially offer. In this scenario fluid dynamic simulation tools have encountered an increased demand to assist the design phase, mainly because of their flexibility and accuracy. 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 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 form 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 issue to address the highest performance at the desired operating condition. In particular, the evaporation of the fuel injection operates an increase of charge density due to the cooling effect produced by the fuel droplets, resulting in a sort of supercharging. Also the precise targeting 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 optimization of the fuel injection strategy both on the point of view of targeting and timing.