An inlet system can be selected by experiments or numerical analysis. Physical and mathematical models of filling a four-stroke, spark ignition engine with air are discussed in this article. Based on these models software enabling geometrical parameters to be chosen was elaborated.The main hypothesis is that for initial numerical choice of an inlet system, theoretical analysis can be limited to dynamic phenomena in this inlet system and the process of load charge, compression and decompression.Model verification was carried out by engine experiments. Modelling and experiments were performed for a four-stroke two cylinder engine with a carburettor which was adopted to an MPI system. For the adaption an inlet system with separate pipes was made. The results obtained show the real advantages of the dynamic charge in terms of performance. Experiments and modelling were limited to the length of pipes.The physical model of the process was based on the following points: 1 The air in both inlet system and cylinder is treated as a semi-ideal gas. 2 The air motion is a one-dimensional wave flow. The mathematical model was formulated using: the law of mass conservation the law of momentum conservation the law of energy conservation 3 The first law of thermodynamics was used to describe the phenomena in the engine cylinder. As a result, the mathematical model consists of differential equations which describe phenomena in both inlet system and cylinder as well as several additional relations.The following conclusions were made: 1 Length of inlet system influences maximum torque and rev. speed. 2 The longer the inlet system the higher the torque and the smaller the rev. speed. 3 The main hypothesis was proved. Using the model and software the initial choice of the inlet system can be made both for newly designed engine as well as for a modified engine.