A performance control system is provided that is adapted to regulate the output of an engine while maintaining stable combustion whereby the perceived operational feel and exhaust efficiency is improved. The performance control system includes an accelerator position sensor for detecting the position of an accelerator by a driver, an electronically controlled throttle adjuster for adjusting the opening of a throttle valve mounted in the intake pipe of an engine with a motor, and a control unit for controlling the throttle adjuster based on the output of the accelerator position sensor. When the engine output, characterized by engine speed or vehicle speed, reaches a predetermined value, the opening of the throttle valve is reduced from the standard opening of the throttle valve, based on the position of the accelerator.