What are Blow Off Valves?

Blow-off valves are responsible for the “psshhh” sound you hear in turbo-charged cars.  What the sound really is, is the sound of charged air being released into the atmosphere.  The purpose of blow-off valves is to prevent compressor surge when the throttle is released or closed.  When the driver lets go of the gas pedal, the throttle closes but the compressor of the turbo-charger continues to turn.  In the end of the discharge line (at the intake manifold), air pressure would start to build up because of the closed plate.  This increased pressure has nowhere to go, but back to the turbo-charger causing the compressor wheel to slow down, stop or even turn the opposite way.  During such occurrences, the compressor wheel may be damaged as well as the bearings and other parts of the turbo charger.  In addition, even after the throttle is opened after, the trapped gas has to be consumed first before the wheel can start to build up momentum again.  This is what is called turbo lag.

But with a blow-off valve installed, this elevated pressure doesn’t stay within the system, and is vented out instead.  What makes up the blow-off valve is a piston held in place by a preloaded spring.  When the throttle is closed, air pressure builds up in the section behind the throttle plate.  A vacuum also develops on the other side of the plate (in the intake manifold area).  From here, a vacuum hose is connected to the area enclosed behind the piston.  When the load of the spring is overcome by the suction of the vacuum hose and the pressure that builds up from the trapped air, the valve opens and compressed air is released.

Releasing the air pressure into the atmosphere, however, does not solve anything.  The release of air, itself, brings about other complications.  For one, the air flow sensor of the car still reads the amount of air that passes through the compressor, thus confusing the ECU (Electronic Control Unit) of the engine and injects the combustion chamber with a corresponding amount of fuel regardless whether the air does go in or is released into the atmosphere.  As a result, successive explosions will occur within the combustion chamber and in even in sections of the exhaust system causing unnecessary damage.  Unburnt fuel can cause the fouling of the spark plugs and possible damage to the catalytic converter as well.  Because of the occurrence of compressor surges and turbo lags, it is also very common for cars to stall unexpectedly, which is not only at most times inconvenient but also dangerous.
Yet, despite these things, there are still people who choose to install blow-off valves for the sound it makes and the pops and cracks it makes with the exhaust.

There are other devices which function similarly as blow-off valves like bypass or divert valves.  In the same way, bypass valves also relieve the intake manifold of overpressure, but instead of releasing the air into the atmosphere, it is redirected back to the low pressure side of the compressor, maintaining the net air volume and setting the turbo-charger system in equilibrium.

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