Blow-Off Valve

What is a Blow-Off Valve? | Tuning 101

One of the most discussed components in the tuning scene is undoubtedly turbochargers. And, the reason for that is very straightforward – the more boost you have, the faster you go.

The problem with boost is that it can also create various issues when calibrated incorrectly. Increasing the air pressure entering the engine is significant to produce more power. However, it also puts a lot of strain on internal components.

When you lift off the gas pedal, the throttle plate closes, which causes the pressurized air to be trapped between the intake manifold and the turbocharger. To prevent internal components from being damaged by excessive air pressure, modern turbocharged engines are equipped with blow-off valves (BOV).

Blow-off valves are simple, yet essential parts of any turbo engine producing a significant amount of boost. The primary purpose of BOVs is to release the pressure when the throttle plate closes and takes the load of the turbocharger. Moreover, some of these valves also keep the compressor spinning when releasing engine load, reducing the “turbo lag” effect when you hit the gas pedal again.

Aside from the safety consideration, BOVs are also appreciated for the hissing sound they create. If you have ever heard a tuned turbocharged engine, you’re probably familiar with the “”PSSHHHHTUTUTU”” sound they make when the clutch pedal is depressed. That sound comes from the blow-off valve.

With that being said, most OEM turbo engines are already come with pressure-releasing systems from the factory. Sure, they don’t sound as nice, but will do the job just fine, and even better in some circumstances. So what is a blow-off valve, what are the real benefits and do you really need one? Let’s tackle those questions and see if installing a fancy aftermarket blow-off valve is really worth the hassle.

Blow-Off Valve
GReddy Type FV Horn Blow Off Valve

What Does a Blow-Off Valve Do?

The primary role of a blow-off valve is to release the pressure buildup when the throttle plate closes. To better understand what happens and why blow-off vales are so crucial, we’ll take a peek at what happens inside the intake manifold when you lift off the gas pedal after an acceleration.

When you step on the gas with a turbocharged engine, the turbochargers sends pressurized air into the intake manifold and the combustion chamber. When you lift off your foot from the gas pedal, the throttle plate closes, but the turbocharger continues to spin. Because the throttle plate is now closed, the pressurized air builds up inside the intake.

Subsequently, the pressure inside the intake continues to rise. This is called pressure surge, and it can damage your turbocharger but also the intake gasket. Moreover, back pressure also causes turbulence, reducing the speed of the turbocharger, which, in turn, reduces the turbo’s efficiency and creates lag when hitting the accelerator again.

The blow-off valve’s job is simply to release the pressurized air out of the intake manifold. Different types of BOVs are available, but the most common ones are internal and external models. Internal release models are the most common ones and are mostly used by OEM manufacturers since they are more discrete and more eco-friendly. This model releases the pressurized air into the intake or exhaust manifold. External models, on the other hand, sound a lot louder since they release the pressure directly into the atmosphere. It’s important to mention that external BOVs are often illegal in many countries.

Blow-Off Valve
TurboSmart USA Blow Off Valve Kompact Dual Port

How Does a Blow-Off Valve Work?

Blow-off valves are quite simple. In fact, they are nothing more than mechanical vacuum valves. Most models have a diaphragm connected to a spring, opening and closing the valve. When a vacuum is applied, the valve closes. When the vacuum drops, the spring pushes the diaphragm, and the valve automatically opens.

Practically, when the gas pedal is depressed, the pistons suck air into the engine, which creates a vacuum in the intake manifold. In that case, the blow-off valve stays closed as long as the car accelerates. When the gas pedal is released, and the throttle plate closes, the vacuum drops and the BOV releases the pressure.

When you hit the accelerator again, the throttle plate opens, which lowers the vacuum, closing the diaphragm and the valve, increasing the boost to help the car accelerate. Simple yet effective.

Blow-off valves are often attached near the intercooler with a weld-on flange. The location might vary from one car to another, but the blow-off valve is always on the intake system after the turbocharger.

Blow-Off Valve
Blitz DD Blow Off Valve for Mitsubishi EVO VII-X 01-11

Different Types of Blow-Off Valve

Vented BOVs

Vented models create the famous blow-off valve sound. These valves simply release all the boosted pressure in the atmosphere. On most models, there is a trumpet on the exit, which amplifies the sound even further.

Vented blow-off valves are great for mitigating pressure surge. However, because the pressure falls rapidly, you might experience turbo lag when you hit the accelerator again. In simple terms, it will take more time for the turbocharger to build up boost again. For that reason alone, these models aren’t the best choice if you want the best performance from your engine.

However, in most cases, you can just remove your OEM BOV and install and aftermarket vented model, or at least not without some adjustment to the ECU mapping. Since vented BOVs expel air pressure a lot faster and more efficiently than OEM blow-off valves, the Mass Air Flow sensor might send incorrect data to the ECU, which may result in a wrong air/fuel ratio.

Diverter Valves

Instead of releasing the pressurized air into the atmosphere, diverter valves recirculate it either into the intake manifold before the turbocharger or directly into the exhaust system, before the catalytic converter. That way, the pressure before and after the turbo compressor evens out.

Thanks to the even pressure on both sides, the turbo lag will be significantly reduced. For that reason, diverter valves often come as standard equipment on most modern OEM turbo engines. Besides, diverter valves don’t have a negative effect on MAF sensors since the air is recirculated back into the system.

However, diverter valves don’t produce the typical sound tuner love so much.

Hybrid Valves

Thankfully, there is a way to experience that captivating sound and no turbo lag at the same time. To do that, you’ll need a hybrid blow-off valve.

As their name suggests, hybrid BOVs release some of the air into the atmosphere and some into the intake manifold. That way, you will still hear the hissing sound, while experiencing none of the drawbacks of a vented BOV.

Now, of course, hybrid BOVs won’t be as loud as vented valves, or limit turbo lag as diverter valves. Nonetheless, some higher-end models work great on both fronts. Hybrid BOVs are a great compromise.

Blow-Off Valve
TurboSmart Blow Off Valve Supersonic for Mazda/Subaru

Do You Need an Aftermarket BOV?

Blow-off valves, no matter the type, are mandatory on modern turbo engines. This means that if your engine is turbocharged from the factory, it will probably be equipped with one already have one. So, why install an aftermarket one then?

Well, aftermarket blow-off valves are especially useful when running on a very high boost. For example, you might need an aftermarket blow-off valve if you plan to install a larger snail. The factory one might not be enough for that kind of upgrade. In many cases, tuners often want the visual and audible appeal of aftermarket solutions.

With that being said, stock BOVs can often work at mildly higher boost pressures. So, if you only tuned your engine via software changes or adjusting the wastegate a little higher than normal, the stock BOV might be enough. However, if you plan to crank up the boost significantly, you’ll probably need a bigger model.

In all cases, if you decide to go down the aftermarket path, make sure that you purchase a quality BOV. Cheap or counterfeit blow-off valves sold on eBay, for example, will definitely give you the looks and sounds, but won’t deliver on the performance front. That’s often due to improper sealing — cheap valves often leak after a little while, reducing that precious boost.

Final Words

If you just got a new turbocharged car, you might be itching to purchase a louder blow-off valve, and that’s totally understandable. However, for proper engine functioning, make sure that the new part won’t hinder the performance of your car.

And, I’m not talking only about losing a few horsepower. Cheap aftermarket BOVs can make your engine behave erratically, especially under acceleration.

The best practice is always to search for aftermarket parts that will work correctly in your car. For that, you can visit a specialized speed shop or shop for one online.

No matter where you decide to buy yours, make sure to buy one from reputable manufacturers. Cheap tuning parts are never worth it. They might look appealing on the outside, but most of the time, they’re simply not worth the money.

As a general rule, make sure to buy your BOV from renowned tuning part manufacturers like Greddy, HKS, TurboSmart, Blitz, and so on…

Find the perfect blow-off valve for your car here!

About Ivo G

Hey guys and girls, I’m Ivo, a passionate car enthusiast with a particular weakness for rear-wheel-drive and stick-shift gearbox cars. Meanwhile, I don’t mind getting my hands dirty – I have experience as a Certified Head of Service at a Toyota Dealership, and knowledge that I wish to share with you.

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