Blow Off Valves Explained

A blow off valve is a pressure release system present in turbocharged engines. Its purpose is to prevent compressor surge and reduce wear on the engine.

Definitions
A compressor bypass valve (CBV), also known as a compressor relief valve or diverter valve, is a vacuum-actuated valve designed to release pressure in the intake system of a turbocharged car when the throttle is lifted or closed. This air pressure is re-circulated back into the non-pressurized end of the intake (before the turbo) but after the mass airflow sensor.
A blow off valve, (BOV, sometimes hooter valve, not to be confused with a dump valve) performs the same task but releases the air into the atmosphere instead of recirculating it. The blow off action produces a range of distinctive hissing sounds, depending on the exit design. Some blow off valves are sold with a trumpet shaped exit that intentionally amplifies the sound. The TurboXS model RFL blow off valve is well known among tuners for this kind of design and some turbocharged vehicle owners may purchase a blow off valve solely for the auditory effect even when the function is not required by normal engine operation. Motor sports governed by the FIA have made it illegal to vent unmuffled blow off valves to the atmosphere. In the United States, Australia and parts of Europe cars featuring unmuffled blow off valves are illegal for street use.
Blow off valves are used to prevent compressor surge, a phenomenon that readily occurs when lifting off the throttle of an unvented, turbocharged engine. When the throttle plate on a turbocharged engine closes, the high pressure air in the intake system is trapped by the throttle and a pressure wave is forced back into the compressor. The compressor wheel slows rapidly and may even stall, and the driver will notice a fluttering air sound. The rapid slowing or stalling stresses the turbo and imparts severe turbo lag if the driver accelerates immediately after the surge event.

Disadvantages
In the case where a mass airflow sensor is used and must be located prior to the blow off valve, the engine control unit (ECU) will meter out excess fuel because the atmospherically vented air is not subtracted from the intake charge measurements. The engine then briefly operates with a fuel-rich mixture after each valve actuation.
The rich mixing can lead to hesitation or even stalling of the engine when the throttle is closed, a situation that worsens with higher boost pressures. Occasional events of this type may be only a nuisance, but frequent events can eventually foul the spark plugs and destroy the catalytic converter, as the inefficiently combusted fuel produces soot (excess carbon) and unburned fuel in the exhaust flow can produce soot in the converter and drive the converter beyond its normal operating temperature range.
One way to mitigate the problem is to reduce the boost pressure, which reduces the required venting volume and yields less charge over calculation by the ECU. The air can also be recirculated back into the intake, a typical stock setup for cars with an upstream MAF sensor. The situation can also be corrected by switching the fuel metering system over to a manifold absolute pressure sensor, a conversion that usually requires a compatible aftermarket ECU or piggy-back fuel controller. The MAP sensor monitors the absolute pressure in the manifold at all times and will correctly detect the change that occurs when the valve vents, allowing the ECU to reduce fuel metering accordingly.

Operation
A blow-off-valve is connected by a vacuum hose to the intake manifold after the throttle plate. When the throttle is closed, manifold vacuum without pressure develops in the intake manifold after the throttle plate and "sucks" the blow off valve open. The excess pressure from the turbocharger is vented into the atmosphere or recirculated into the intake upstream of the compressor inlet.