Blow Off Valve: What it does, and how it works
|Blow off valves are air pressure release valves that can be mounted on the intake piping of a turbocharged car anywhere in between the turbo and the throttle body. They are also known as dump valves, or vent valves. The blow off valve pictured below is made by Greddy and is the Type-S model.|
A blow off valve has few purposes, but is a very important piece.
The functions of a blow off valve
Without a blow off valve, when you let off of the gas, the compressed air in your intake piping increases to great pressures as the turbines in the turbocharger are forced to a screeching hault. The extreme pressure forces the air back through the turbocharger, increasing the wear on the turbo.
With a blow off valve, when you let off of the gas the air pressure left in your intake piping is relieved as the blow off valve opens up. This allows your turbo to continue spinning in the proper direction, preventing damage to the turbo and allowing for a faster return to positive air pressure in the intake piping.
How it works
A small diameter hose runs from the intake manifold to the blow off valve. When the pressure in the intake manifold is positive (in boost), the pressure in the small hose keeps the wastegate closed.
Once you let off of the gas, the throttle plate closes, and all of the air in the intake manifold is sucked into the engine. With negative, or zero pressure in the intake manifold, the blow off valve opens up and releases all of the compressed air from the intake piping.