What Is A Blow Off Valve – Functions and Benefits

Turbochargers are the commonly discussed components when it comes to tuning car engines to squeeze out a little bit of extra power. And the reason is this: the more boost you have, the more horsepower the vehicle will gain.

When you remove your feet from the accelerator, the throttle valve closes, and the turbocharger keeps spinning. This means the turbo keeps generating pressurized air. And this air gets trapped in the intake manifold since it has nowhere to go. But to prevent this pressurized air from accumulating in the intake tract, car manufacturers equip a blow-off valve in every modern turbocharged engine.

blow off valve purpose

Blow off valve Explained.

The blow-off valve is a simple yet important pressure-release component in turbocharged engines that is responsible for releasing the pressurized air that accumulates in the intake valve when you stop depressing the accelerator. Helping take off load from the turbocharger and prevent the turbo from failing sooner than it should.

Normally, when you remove your foot from the gas pedal, the throttle valve will close while the turbocharger keeps spinning. As the turbocharger is spinning, it is generating compressed air. This air is meant to go into the engine, but since the throttle valve is closed, the pressurized air will be trapped in the intake manifold. Since the air has nowhere to go, it will cause a pressure surge.

Here’s where the blow-off valve comes into play. To release this pressure surge and protect the turbocharger from damage, car manufacturers invent blow-off valves. Instead of allowing pressurized air to accumulate in the intake manifold and cause a pressure surge, the blow-off valve releases this air into the atmosphere until the throttle valve opens again.

What does the blow-off valve do?

As explained earlier, the main function of the BOV is to release the air pressure that builds up when you remove your foot from the accelerator. To better understand what the BOV does, let’s see what goes on in the intake manifold as the throttle valve closes.

The moment you depress the accelerator on a turbo-equipped engine, the throttle valve opens, and the turbo sends pressured air into the combustion chamber through the intake manifold. As you take off your leg from the accelerator, the throttle valve closes while the turbo keeps spinning. Since the turbocharger is still spinning and the throttle valve is closed, the pressured air gets trapped in the intake manifold.

As this continues, the pressure inside the intake manifold begins to rise. This is commonly known as a pressure surge, and it can damage the intake gasket and the turbo itself. This can also cause turbulence, minimizing the speed and efficiency of the turbocharger.

Simply put, the blow-off valve is responsible for releasing the pressurized air out of the intake manifold.

There are various types of BOVs, but the internal and external blow-off valves are common. The internal BOVs are usually factory-fitted. This type of blow-off valve releases the trapped air into the exhaust or intake manifolds.

In contrast, the external BOVs are usually aftermarket products that can be fitted on any turbocharged engine, whether it comes with a BOV or not. This type of blow-off valve releases the pressurized air into the atmosphere. As a result of this, the external blow-off valve sounds louder than the internal one.

What is a Turbosmart blow-off valve?

Turbosmart blow-off valves are Australian-made aftermarket BOVs engineered with precision and a focus on reliability and performance. The turbosmart BOVs are built and machined from durable, rugged billet aluminum.

They come in various types – dual port, divert, and internal blow-off valves. The dual port splits the pressurized air, releasing a small portion into the intake tract and a larger portion into the atmosphere. The divert type releases 100% of the pressurized air into the atmosphere, while the internal or plumb valve recirculates 100% of the compressed air into the intake tract.

Turbosmart blow-off valve replacement products suit all road cars for streets and performance applications, race, and, you guessed it, motorsport use.

Which is better, GFB or the Turbosmart blow-off valve?

GFB and Turbosmart are two family names when it comes to tuning your engine to gain more power. Both Turbosmart and GFB are super good and better than some OEM blow-off valves. However, turbo smart BOV is a bit better than GFB at a gradually bleeding boost, yielding more power when depressing the gas pedal.

Blow-off valve benefits.

You have seen the functions of a BOV and how it works; let’s take a closer look at the blow-off valve benefits to see if it is worth the extra cash you will spend for modification.

Pressure relief

As explained earlier, when you remove your foot from the gas pedal and the throttle valve closes, the turbocharger keeps spinning. Generating pressurized air that could get trapped in the intake manifold. But the blow-off valve kicks in and releases the pressurized air into the engine or atmosphere.

Turbocharger protection

The pressurized air trapped in the intake manifold has nowhere to go as far as the throttle valve is still closed. As the turbocharger keeps spinning, the pressure increases and causes a phenomenon known as pressure surge.

If it continues for long, the pressure surge can damage the turbocharger. Fortunately, the BOV releases the pressurized air—preventing pressure surge and turbocharger damage.

Sport car sound

Another benefit of the BOV is the unique whoosh sound — like that of a race car. While this is not a functional benefit, to some car enthusiasts, the unique sound is more than enough reason to install a blow-off valve.

Tuning and adjustability

Blow-off valves help the turbocharger maintain consistent and reliable boost — ensuring optimal engine performance. Plus, some BOVs are adjustable, meaning you can fine-tune how they release the air pressure. This also allows you to modify the engine tuning based on your driving style.

Increased sealing

Most OE blow-off valves have an acceptable level of pressure leaks. But to overcome this, the turbocharger has to work harder to gain the predefined amount of pressure boost. Most engines overwork the turbocharger.

But most quality aftermarket blow-off valves like the turbosmart and GFB are built and engineered to ensure there’s no pressure leak from the intake system. As a result, they release stress from the turbocharger compared to the standard BOVs. I understand this discussion is about the benefits of BOVs in general, but this is one of the key features that makes quality aftermarket blow-off valves right for the money.

blow off valve benefits

What happens when the blow-off valve goes bad?

The BOV plays a crucial role in releasing the trapped pressurized air out of the intake manifold. This prevents compressor surge and damage to the turbocharger. So, if the blow-off valve fails, it’ll cause a compressor surge—a situation where the pressurized air in the intake reverses direction and puts a lot of strain on the turbocharger.

That will lead to compromised airflow, bad gas mileage, surging or erratic engine power output, the illumination of the check engine warning light, and unusual hissing, rattling, or squealing noise from the engine. If you notice any of the bad blow-off valve symptoms and effects, do not conclude you have a bad BOV yet because other faulty engine problems can cause these issues. Instead, diagnose the BOV to know if it is bad or not.

How to install a blow-off valve

Installing a blow-off valve is pretty simple and can be changed by anyone, depending on the BOV design. Here’s a simplified step on how to install a new blow-off valve.

Step 1: Locate the BOV in your car

The first step is to locate the old blow-off valve in your car. I recommend you use your owner’s booklet for a visual aid on the BOV location. In most vehicles, there will be two vacuum hoses connected to the BOV. The upper vacuum line connects to the throttle body.

Step 2: Disconnect the BOV

Disconnect the hoses holding the BOV in place by using either screwdriver or a wrench to remove the hose clamps or screw bolts. Slide the vacuum lines out of the way by prying them off with a screwdriver. After that, set the old blow-off valve aside before proceeding to the next level.

Step 3: Install the new BOV

Grab the new blow-off valve kit you purchased and install it by following the reverse method you used in disconnecting the old one. Ensure all the hose clamps or screw bolts are tight and snug.

Does a blow-off valve add HP?

A blow-off valve is a device that helps pressurized air in the intake manifold escape when the throttle body is closed. It does not increase engine performance by adding horsepower; instead, it prevents pressure surge — giving you efficient turbo performance.

Are Turbosmart blow-off valves good?

Turbosmart BOVs are engineered with precision to offer excellent sealing of the intake systems. They reduce the stress on the turbocharger and compressor surge compared to standard OE blow-off valves. Plus, they don’t leak pressure regardless of how much boost the turbo produces.

Can a BOV damage my engine?

No. Blow-off valves are designed and built to enhance engine performance by ensuring the turbocharger works effectively. It doesn’t have any negative effect on the engine. However, you need to maintain it properly and replace it whenever it goes bad.

Do blow-off valves hurt performance?

They do not add horsepower to the engine; they keep the turbocharger running effectively and efficiently, which in turn adds more boost to the engine. So, no. Blow-off valves do not hurt engine performance.

Is it legal to have a blow-off valve?

It depends on two factors —the type of blow-off valve and your region. In most countries and cities such as Canada, NSW, and Australia, any modification on the intake system that makes the vehicle exceed the maximum noise level by the regulation is an offense. And BOVs have a unique sound that makes the car a bit louder. So, if the sporty sound is above the maximum noise level, the blow-off valve is illegal. And all blow-off valves that vent air into the atmosphere are illegal.

Blow-off valves that recirculate 100% of the pressurized air back into the engine are legal as long as it does not exceed the maximum noise level by your local regulation.

Be Wise While Selecting Blow Off Valve

If you want to get an aftermarket blow-off valve for a louder engine sound, that’s totally understandable. I love that sporty sound that comes from a street car. It gives a sporty feeling. However, for the love of boost, do not purchase a BOV that will hinder performance.

I’m not talking about losing a few horsepowers. Cheap blow-off valves are poorly designed, and they can make your car run erratically, especially when accelerating. Whether you are shopping online or offline, I recommend you get a BOV that is tailored for your car and from a reputable manufacturer.


Hi there, I am R. Hasan Tito, a mechanic, and owner of this website. My friend and I created this website to share our knowledge, expertise, and experience with our fellow mechanics' community and car users. I am a specialist and certified automotive mechanic (Both Heavy Commercial and Private Cars). I worked as a Mechanic and Mechanic Supervisor for over fifteen years at Global Rebound Automotive companies - Toyota, TATA, BMW, Nissan, TVs, and Others. Now, I enjoy my new role of leading a team of automotive experts (in their respective fields) and publish new content on a regular basis on my website and social media.

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