The aim of this diploma thesis is supercharging of SI engines and design influence on performance. The main objective of this thesis is to propose appropriate modifications on AR67203 engine of Alfa Romeo 155Q4 personal vehicle in order to achieve significantly better performance parameters and a constant torque in the widest possible speed range. That is why I analyze design and modifications that affect the overall engine performance and their appropriate application to the selected engine. An important point of this thesis is the right choice of turbocharger, in order to have an effective cooperation with a modified engine. The calculation study and the simulation in Lotus Engine Simulation software serve this purpose. I also give information about ECU programming. The results, as well as a practical output in the form of measured performance parameters of modified engine, are evaluated at the end.