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This causes a surge which can raise the pressure of the air to a level which can be destructive to the engine e.g. damage may occur to the throttle plate, induction pipes may burst. The surge will also decompress back across the turbo as this is the only path that the air can take. This sudden flow of air will often cause turbulence and a subsequent whistling noise as the air passes past the compressor wheel.Turbocharger

The reverse flow back across the turbo acts on the compressor wheel and causes the turbine shaft to reduce in speed quicker than it would naturally. When the throttle is opened again, the turbo will have to make up for lost momentum and will take longer to achieve the required speed, as turbo speed is proportional to boost/volume flow. (This is known as Turbo Lag) In order to prevent this from happening, a valve is fitted between the turbo and inlet which vents off the excess air pressure. These are known as an anti-surge, bypass, blow-off (BOV) or dump valve. They are normally operated by engine vacuum.Turbocharger

The primary use of this valve is to maintain the turbo spinning at a high speed.Turbocharger The air is usually recycled back into the turbo inlet but can also be vented to the atmosphere. Recycling back into the turbo causes the venting sound to be reduced and is required on an engine that uses a mass-airflow fuel injection system (as opposed to a speed-density system). The reason for this is that the airflow sensor is normally located before the turbo and the ECU will inject enough fuel for the amount of air that flows through it. If some of the air that has gone through the sensor is dumped into the atmosphere, the engine will be over fueled until the BOV closes again. The benefits of venting to the atmosphere are simply the ease of installation (because there is no need to run an extra hose to plumb the charge back into the system) and that it makes a sound considered desirable by some. A dump valve will shorten the time needed to respool the turbo after sudden engine deceleration.wedding dress