What does the blow off valve do?

If you are a fan of speed racer films like Fast and Furious, you may often hear a hissing sound like whistling in cars used for speeding. This hissing sound that sounds "pssshhhh.... pssshhhh..." will feel louder when a gear shifts while speeding. So, are you curious where the hissing sound comes from?

Yes, we will usually find this hissing sound in Turbo-engined cars equipped with a component called "Blow Off Valve." The different Blow Off Valve models used makes the "pssshhhh" sound produces will also be different.

What is a blow-off valve?

Blow Off Valve (BOV) is one component of air pressure safety on turbo engines in addition to the wastegate valve. If the wastegate valve is placed in the exhaust manifold channel, the Blow Off Valve locate in the air intake section, precisely between the Turbo and Throttle Valve.

Blow Off Valve serves to remove excess air pressure generated by the Turbo due to the closing of the Throttle Valve, either when shifting gears or when the gas pedal is released suddenly at high rpm.

The image below is the process of removing excess air pressure that will produce the hissing sound.

How does a blow-off valve work?

Inside the Blow Off Valve, there is a piston behind which there is a retaining spring. This retaining spring will push the piston to close the hole connected to the outside air.

When the throttle valve position is open, the air pressure from the Turbo directly enters the engine combustion chamber. In this position, no air pressure collects in front of the Throttle valve so that the air pressure does not push the Blow Off Valve piston up.

When the Throttle Valve is closed due to releasing the gas pedal (usually when shifting gears ), pressurized air from the Turbo will gather in front of the Blow Off Valve piston and press the piston up so that it opens a hole to the outside air.

It's when the compressed air comes out that a "pssshhhh..." hissing sound occurs. When the air pressure in the front chamber of the throttle valve decreases, the spring will push the piston to close the air outlet again.