Blowoff valve: What is the purpose of this device?

Blowoff valve
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The Blowoff valve also called the Dump valve, relieves pressure in turbocharged engines. The blowoff valve releases compressed air directly into the atmosphere.

This valve is intended to prevent the formation of high pressure between the turbocharger and the throttle valve and thus reduce wear and tear on the turbocharger and the engine itself.


How does the Blowoff valve work?

During the release of compressed air, a characteristic hissing or whistling sound occurs, depending on the design of the valve. Some valves are equipped with a tube-shaped outlet, due to which this sound is deliberately amplified. That is why some owners of cars with turbocharged engines buy this valve exclusively for its sound effect even when the engine does not need it.

The blowoff valve is connected via a vacuum hose to the intake manifold behind the throttle valve. When the throttle is open, the air pressure is equal on both sides of the blowoff valve piston, and the spring keeps the piston down.

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However, if the throttle is closed, the pressure of the compressed air from the turbocharger pushes the valve piston up and thus opens the air pressure outlet to the atmosphere or back to the turbocharger.

What is the role of Blow off valve?

Without a blowoff valve, the pressure between the throttle and the turbo is forced back through the free-spinning turbine, slowing it down. This has a very negative effect on the turbocharger's bearings and thus shortens its service life.

The blowoff valve also reduces turbo lag. Without the blowoff valve, the accumulated pressure reduces the speed of the turbocharger's turbine. When the throttle is subsequently stepped on, the turbo takes longer to spin up to the speed it left off when the accelerator pedal is stepped on.

There are three types of Blow off valves:


Atmospheric valves release the accumulated pressure directly from the pipe to the environment. These valves are characterized by a loud hissing or hissing sound.


It forces the accumulated pressure back into the intake tract before entering the turbocharger. This type of blowoff valve is quieter than the atmospheric valve, so it is used by cars that meet different noise or emission standards.

Dual port

A combination of the atmospheric and recirculating (above-mentioned) valves.

Engine sound with and without Blowoff valve

Hear how the engine sounds with and without the blowoff valve. The comparison starts at 2:33 in the video below.