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Among the many development trends of internal combustion engines, there is a controlled change in displacement, which aims to reduce fuel consumption, leading to a reduction of carbon dioxide emissions to the environment, i.e. fulfilling the economic and ecological assumptions of sustainable development of transport means. This trend operated for several years under the name downsizing, and now, due to the broader view on the issue - rightsizing. In rightsizing, it is possible to both reduce or increase the displacement of the engine in order to decrease the amount of consumed fuel, to obtain performance characteristics that meet the legislative and user satisfaction requirements, which translates into an adaptation of the engine to a specific model or version of the vehicle. The basic phenomena shaping the volumetric efficiency characteristics were defined and then a series of bench tests were carried out using a real engine as well as model tests using computational systems. Among others, the boost pressure and flow resistance were considered. On the basis of actual research and theoretical calculations, the relationships defined as the research objective were demonstrated and included in the form of general, detailed, utilitarian and prospective conclusions. The most important are: - the existence of dependence of the changes in the temperature of the charged air along with the degree of rightsizing, which reduces the useful efficiency of the engine, - rightsized engines, due to the fact that they have short intake manifold hoses do not fully use the possibility of dynamic charging, - the pressure drop of the flowing air due to the flow resistance in rightsized engines is significantly greater than in naturally aspirated engines, however the relative reduction of the filling ratio due to the flow resistance in the intake system in a wide range of rotational engine speed is smaller for rightsized engines. It has been proved that there is a relationship between the individual phenomena and factors such as: airflow resistance, increase in air temperature, inertial phenomena, wave phenomena, boost pressure from a turbocharger and mechanical compressor affecting the filling ratio of an internal combustion engine and its degree of rightsizing. As a result, a method of shaping the relationship between volumetric efficiency and engine speed under conditions of changing the displacement was developed.