A computer simulation of a three lobe, roots blower for use as a positive displacement supercharger has been developed. The simulation considers three to five control volumes existing at various times in the supercharger cycle, and numerically integrates continuity and energy equations for the control volumes throughout the cycle. Instantaneous properties for the control volumes and instantaneous flows throughout the supercharger are predicted. Comparisons between predicted and experimental performance show that the simulation can adequately model performance trends of the supercharger. Use of the simulation in providing a supercharger map and supercharger performance on an engine is shown.