Turbo or super charged intake tract diverter valve system, upstream of a throttle valve, includes a closure means (10) for a diversion aperture (4.1) in the intake tract (3) to vent pressurised gases within to a bypass path or atmosphere; the closure means having a transfer aperture (12) facilitating a net three due to a pressure differential on its opposite sides of the closure means (10) so as to close or keep closed the diversion aperture (4.1). When gas pressure on opposite sides of the closure means is equal, and when an upstream side (10.1) of the closure means (10) has a pressure greater than a downstream side (5.1), then it will open the diversion aperture (4.1). An actuation means opens a control aperture (6) to create the necessary pressure differential on the closure means (10) to thereby cause same to open the diversion aperture (4.1).