What Is A Blow-Off Valve And What Does It Do?

Once all the rage among wannabe tuners, the blow-off valve has been a controversial modification for years, but what does it actually do?

Noise is a key avenue down which we can explore our love of cars. The variety and character of sounds you hear at a good car meet, from a classic Subaru flat-four burble to the furious razz of a well-sorted rotary, there are so many ways to get your fix. Blow-off valves, or dump valves, are divisive.

The premise is simple. In a turbocharged car, the system builds boost pressure to force extra air into the cylinders. The usual combustion business follows, with a greater amount of fuel/air mixture for bigger explosions and more power. But if the driver lifts off while the turbo is boosting, that air pressure goes from essential to potentially damaging in a matter of milliseconds.

What Is A Blow-Off Valve And What Does It Do?

When you ask the turbo to change states of operation so quickly it places a huge amount of strain on the moving parts within it. Lift right off while boosting hard, and although air flow through the pipe reduces to almost nothing the turbo is still spinning and could enter ‘surge’ conditions. We all know turbo flutter, right? That’s the sound made by a turbo under surge strain, and although it sounds fantastic it’s actually no good for the mechanicals.

Here’s where bypass or diverter valves come in. Any modern turbocharged engine has to have one, usually activated by vacuum and designed to recirculate that excess air pressure to a pre-turbo point in the system, thus recycling it, balancing the pressure and continuing the air flow into the turbo.

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But they don’t add anything to the audible charisma of the car. If we’re being honest, an efficient bypass valve only makes a car more boring. They are technically better than than blow-off valves, but more complicated to engineer in. Blow-off valves were developed as a solution to compressor surge, just like bypass valves, but they were easier to fit because they were only ‘attached’ at one end.

Fitted between the turbo and the throttle body, as close to the throttle body as possible to ensure the fastest response to a surge situation, the humble dump valve simply exhausts the excess air pressure into the atmosphere, making an addictive hissing noise you’ll have no problem hearing from the driver’s seat, although the volume of the burst should be proportionate to how hard the turbo is boosting.

Some higher-quality blow-off valves will even let you choose how much air is vented to atmosphere and how much is recirculated back into the turbo.

These clever little slices of naughtiness aren’t all fun and games, mind you. If they’re fitted downwind of a mass air-flow (MAF) sensor in the system then the MAF sensor won’t account for the lost air and will keep sending incorrect readings to the fuel injection system. After every hiss of the blow-off valve, the engine will over-fuel slightly – at least, if you’re still on the throttle at all. That can lead to stuttering throttle response, coked-up spark plugs, backfiring or stalling.

Set up badly it’s like any other mod – it’s better not to bother. Set up properly, a blow-off valve is a neat way to add some aural character to your car without giving your insurers a heart attack.



This article literally blew off my mind.

07/24/2018 - 16:29 |
112 | 2

In reply to by Anonymous (not verified)

I didn’t understand any of it.. it wooshed straight over my head 😂

07/24/2018 - 16:46 |
78 | 2
Anton 3

In reply to by Anonymous (not verified)

Could you please let off these STU-TU-TU-pid jokes, thanks.

07/24/2018 - 16:48 |
40 | 0

In reply to by Anonymous (not verified)

I’m not so sure about it, I might bypass this article

07/25/2018 - 06:41 |
2 | 0


07/24/2018 - 16:33 |
4 | 14
TurboToddler (Straight-five)

Am I the only one who thinks those high-pitched BOVs sound bad? Seems like everybody online loves them.

07/24/2018 - 16:46 |
12 | 2

I like the sound that the AMG engine in the Huayra makes, but I think it’s a diverter valve.

07/24/2018 - 17:24 |
2 | 0

I showed this article to a non-car guy. He said: “You see why I don’t wanna be a car guy… A whole freaking book page of text and 2 videos just explaining how a mini hydrant looking thingy works? Pff…” 😂

07/24/2018 - 17:20 |
42 | 0

In reply to by CS55

“Mini hydrant looking thingy”

This is what I shall call blow off valves from now on.

07/25/2018 - 18:22 |
6 | 0

It’s just for good noises, ok

07/24/2018 - 18:50 |
0 | 0
Rand0m_c1tizen (Jon)

A blow off valve goes pssh after the braaps

07/24/2018 - 19:29 |
10 | 0

pssshhh, i already knew how these wroked.

07/24/2018 - 23:53 |
0 | 2
Alec Winkelmann

Blow off valves are awesome on cars that need them but i hate when i pull up to a car meet and there’s that one stock genny or sentra etc, that only has a bov and they milk the hell out of it. Blow off valves are an awesome thing but they are turning into a ricers favorite toy. That’s the way it is in Texas so it might not be like that in other places. I’m curios to get ya’lls opinion on this too

07/25/2018 - 02:24 |
0 | 0
Keiyu Mamiya

69 up votes!!!!😂😱😂

07/25/2018 - 03:34 |
2 | 6
What Is A Blow-Off Valve And What Does It Do?

Everytime a German speaking Person talks about Blow-Off Valves they use the Term “Pop-off” and everytime i could punch them squarely in the head for that… Get your facts right!

07/25/2018 - 09:10 |
0 | 0


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