The present invention relates to an electronically driven pressure boosting system that is used to boost the torque output of an internal combustion engine. Engine operating parameters (120-126) are fed to an engine control unit (ECU) (32). A supercharger (10) is controlled by the ECU and when powered by an electrical supply system (16,27) boosts engine torque (36). When the supercharger is in an idle mode of operation at low supercharger speeds it does not boost torque, and when it is in a boost mode of operation at high supercharger speeds, it does boost engine toque. The supercharger requires a lag time to accelerate from an idle speed to a boost speed. The electronic control system (32) monitors the engine operating parameter(s) (120-126) and calculates therefrom a likelihood that the engine torque (36) will need to be boosted (34) by the compressor (10). When the compressor (10) is operating at an idle speed, the electronic control system (32) controls said idle speed so that the lag time varies inversely with the calculated likelihood that the compressor (10) will be needed to boost engine torque (36).