Functional description

The blow-off valve prevents "pumping" against the closed throttle valve, thus improving engine acoustics. In addition, the blowoff valve protects the exhaust turbocharger against damage. Another effect: the exhaust turbocharger reacts quickly when the throttle valve is opened again. Without the blow-off valve, the exhaust turbocharger would work against the ram pressure of the closed throttle valve and become slower. On opening the throttle valve, the exhaust turbocharger would react with a delay. The engine control activates the blow-off valve. The blow-off valve has the positions: open and closed (no intermediate positions).

Fig 1: Exhaust Turbocharger Components

Courtesy of BMW OF NORTH AMERICA, INC.