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General stuff, then ending with why to buy a PERRIN BOV or ALTA BOV

Why do you need a BOV?  Every factory turbo charged car has a BOV.  Turbo charged cars need BOV€™s to release the built up pressure from the turbo when the throttle is closed between shifts.  With out a BOV, the pressurized air getting pushed backwards through the turbo stops the turbo from spinning.  So a BOV is needed to keep the turbo spinning, which minimizes turbo lag between shifts.

So if a stock car has one, why do I need one?  Factory BOV€™s are designed to hold factory boost levels.  Some can€™t even handle them, that well.  When the boost is turned up or a larger turbo is installed, factory BOV€™s will leak pressure causing loss in power.  An upgraded Perrin BOV will solve the leaking issue and you will never have to worry about your BOV holding pressure ever again.

The more boost, the more pressure it holds regardless of spring rate!

No need to have a stiffer spring for high boost applications

Adjustable spring tension!

Adjustable flow rate!

BOV cap can be screwed in to increase tension on spring, to adjust how quickly the BOV vents.

Internal adjustment to adjust how much the BOV will flow while venting.  It does this by changing the piston travel which allows the user to vent less air for lower boost applications, and more air for higher boost applications.

Will hold more boost, and flow more air than any other BOV!

Universal applications for tube to tube recirculation type BOV€™s (EVO, Eclipse, 1.8T€¦..)

Direct mount applications for WRX,

Vent to atmosphere applications for SRT-4, and other non MAF based systems.

With our Extensive testing on the WRX and EVO, we have found that 100% recirculating BOV€™s, always outperform vent to atmosphere BOV€™s.  Since the VTA type BOV€™s don€™t vent the air back into the intake after the MAF sensor, the ECU will add fuel based on that amount of air it ingested, not the amount of air vented.

Since the intake/boost system on a MAF based car needs to be kept in a closed loop, venting air to the atmosphere causes the car to run very rich.  This can cause drivability issues.

Get the sound of a VTA with a 100% recirculation BOV.

For cars that use a MAP sensor to determine fuel, don€™t matter.


Remove the stock BOV

Install BOV at loosest settings to start with, please note the grooves on body

Leave the flow adjustment at factory setting.

Install the Black tube to the pressure source

Install the silver tube to the intake, or recirculate tube.

Install manifold vacuum source to the back nipple on the BOV.

BOV directions for the WRX

Remove the stock BOV

Remove the recirculation elbow from the stock BOV and using same bolts install onto the Perrin BOV and attaching the vacuum hose

Attach the flange to the BOV, and orientate the flange similar to stock. (Make sure to screw the set screws into the flange and tighten)

Install the BOV same as old BOV comes off, using same bolts

Rotating the flange may be necessary to get the recirculation hose in the right location.

Make sure the BOV clears the hood before closing.

BOV directions for VTA

Remove old BOV

Attach the black tube to the boost tube where factory BOV came from.

Make sure to orientate BOV so there is no interference with other parts.

Hook the manifold vacuum source to the top of the BOV

If tuning needs to be done to the BOV we have provided instructions how to do so.  The cap should never be unscrewed past the o-ring, this will cause a boost leak and the BOV will not function correctly.  The piston in the BOV should travel a minimum of 1/16€, please use this as a frame of reference when adjusting the BOV€™s spring tension.  More spring tension is not needed to hold more boost.  Adjusting spring tension is necessary to adjust how quickly the BOV closes and vents.  The higher the tension, the slower the venting response.

1. Remove, or thread in completely the socket head screw to allow piston full travel.

2. Adjust the spring tension to desired venting response, or closing response. (Making sure to note what groove on the body, the cap is screwed down to)

3. With the car turned off, pull vacuum hose off the BOV and unscrew the socket head screw until it stops on the BOV cap.

4. Screw in 2.25 turns =1/16€ of piston travel

4.5 turns = 1/8€ of piston travel

6.75 turns = 3/16€ of piston travel

9 turns = 1/4€ of piston travel

11.25 turns = 5/16€ of piston travel

13.75turns = 3/8€ of piston travel

Stock setting is 3/8 from end of screw to nut, and cap screwed 13/16 from the top of the body.

The more the screw is turned in, the more air will vent from the BOV.

This part of the adjusting procedure can be done while the car is running for faster setup.

5. Once desired venting level is achieved, remove the BOV cap and spring

6. Carefully remove the piston and screw

7. Making sure to hold the threaded length of the screw tighten the nut down to the piston.  This will hold the desired amount of piston travel.

8. Install the piston back into the body making sure to use a small amount of grease around the piston.

9. Install the spring and cap, and tighten cap down to mark as noted before on side of BOV.

[quote=nmwg;2473766][SIZE=2]This is a question about blow off valves more in general. I know what their basic functionality is: they release compressed air produced by the turbo when the engine no longer needs it. I know that they usually vent the air into the atmosphere. I have read that that can screw up the engine performance because it causes it to run rich. I am under the understanding that if you route the blow off valve output back to the intake before the turbo but after the mass airflow sensor, that fixes the problem, but the blow off valve doesn't make the sound that it is known for. If that is the case, does that mean that everyone who is running a loud blow off valve has poorer engine performance, or are there ways to achieve both?

The other question is: can someone explain how directing the blow off valve output back into the intake before the turbo and after the sensor fixes the engine running rich problem? My understanding is that the mass airflow sensor takes a measurement of the air coming in so that the computer can calculate the proper fuel air mixture for the given throttle position. If the throttle is in the closed position, that should mean that a minimal amount of air would be needed for the engine combustion, so the computer should only allow that minimal amount of air in for combustion. In the case of a system with a blow off valve, this scenario would cause the blow off valve to activate and release the extra air, thus creating a situation where there isn't a proper amount of air left for the computer calculate fuel amount. I can see how this makes the engine run rich for a moment. What I don't understand is how putting that air back into the system before the turbo helps the running rich problem. Reason being.... The mass airflow sensor is still calculating air coming in from the outside and is unaware of the extra air being put back into the system via the blow off valve. I would think this would create the situation where there is more air in the system than the mass airflow sensor calculated. Maybe that doesn't matter because the computer only takes what it needs for combustion, so that long as there is not a shortage, things run smoothly. This still leaves me with the question of: as long as the blow off valve is open, how does it matter to engine combustion whether or not the extra compressed air is vented into the atmosphere or back into the system since the blow off valve is redirecting all the air away from the engine. I would think that by the time that the recirculated air from the blow off valve got back to the point to be used by the engine at least one of the pistons would have fired rich, or that any needed air for a no throttle combustion could be sufficiently supplied from the standard air intake.

Thanks for your answers.


Blow off valves, recirc valves, dump valves, bypass valves pop-off valves and whatever you want to call them all do the same thing. They relieve boost pressure!  In different car markets BOV€™s have different names which makes it a little confusing as to what they do.  This makes all the BOV€™s confusing as to what they do. The easiest way to think of a BOV is a big valve.  It just opens and closes relieving pressure at certain times.

First lets explain the basic turbo system.  Under normal operating conditions (light load) the engine is sucking in air through the throttle body.  The turbo sits upstream of the throttle body and is not doing much work at this point.  As the throttle is opened more and the engine is under more load, the turbo starts to build boost.  As boost goes up so do the power and the fun!  When the throttle closes all that boost has to go somewhere, this is where the blow off valve comes into play!  If the BOV doesn€™t exist, when the throttle closes the boost gets back up through the turbo. This back up of pressure puts great stresses on the turbo€™s compressor and turbine shaft and the air causes the turbo to slow down very quickly and abruptly. This phenomenon is called compressor surge. Compressor surge is not good for the turbo€™s longevity at all.

In the above situation, a BOV would be installed after the turbo, but before the throttle body.  Under high load and light load the BOV would be closed (remember its valve).  The BOV comes into play when the engine transitions from high load to light load.  This is when the turbo is creating boost, then suddenly the throttle body is closed and the pressure needs to be relieved.

Seems simple enough, it gets rid of the pressure that builds up when the throttle closes abruptly like when shifting. How the BOV actually open and relieves the pressure varies slightly, but for the most part the BOV is actuated using 2 things, manifold vacuum and the boost pressure it is trying to relieve.

Under high load the boost is holding the valve close and as the throttle closes the vacuum now in the intake manifold starts to pull the valve open.  On the opposite end of the valve the boost pressure still in the system also helps the valve open.

So now that you know how it works, lets add a few variables to what the BOV does with the relieved pressure.  On all OEM applications the BOV vents the air back into the intake system. This is done to keep the noise down, and to keep unwanted emissions from getting into the atmosphere.  The other option is to let the BOV vent the unwanted boost to the atmosphere.

There are 2 types of ECU€™s and  how they make the engine run. One uses a MAF (Mass Air Flow) sensor which reads air flow, and the other uses a MAP (Manifold Absolute Pressure) sensor to calculate air flow.  The MAF base systems have a sensor placed right after the air filter, where the MAP based systems have the sensor in the intake manifold.

Traditionally, a BOV, or blow off valve is one that blows the unwanted boost, out to the atmosphere

A recirc or bypass valve blows off the unwanted boost back into the intake system of the car.

In my

How different BOV€™s effect how the engine works.

On a car that d

On the Mini the BOV is electronically actuated by the ECU.

Blow Off Valve Explained Cover
Model: Christine Huang
First off, if you don’t own a turbocharged car, enjoy these photos of Christine, then turn the page. But if you’re honestly driving around an STI, Evo or SR20-swapped 240, or perhaps you’ve thought about turbocharging your Civic or Z, keep reading because you’re going to want to invest in a good blow-off valve.
The underlying objective of a blow-off valve (BOV) is to protect the turbo against damage while ensuring smooth and reliable drivability. It just so happens it makes a loud sound that gets everyone’s attention! But the noise it makes shouldn’t be what you’re most concerned about; you have to make sure you have the right valve for your ride that’ll keep your turbo happy and healthy.

Compressor Surge

Every component of a turbo system needs to work together. But when there’s compressor surge, it can create headaches, mess with performance and even cause damage to your precious car. Compressor surge occurs when you suddenly lift off the gas pedal and the throttle plate closes. A rush of boost heads into the engine and when it hits the closed throttle plate, it has nowhere to go but back into the turbo.
The effect of this boost backtracking into the compressor outlet and interfering with the compressor wheel is called compressor surge. Now why is it bad? As the boost returns to the compressor wheel, it can slow or stall the wheel putting stress on the wheel shaft and bearings. Not only damaging, this surge can also mess with turbo response and kill drivability. The wheel loses its momentum and also creates lag. A good blow-off valve can come to the rescue and keep the surge out of the turbo by venting it to the atmosphere. Pssshhhh!
Blow Off Valve Explained Diagram

Blow-off Basics

Here’s how it works… A blow-off valve is connected to the intake tract. Inside the valve’s main housing is a vacuum chamber with a spring, a diaphragm and valve. The diaphragm reacts to pressure changes and at a predetermined vacuum it’s pulled toward the vacuum source compressing the spring inside the housing. The spring is connected to a valve that pulls away from its seat and releases the unwanted boost pressure.
On some BOVs, an adjustment screw lets you control at what pressure in the intake system the valve is activated. You can also swap the spring to change the activation point.
Blow-off valves are often referred to by diameter, 40mm being a common size. Picking the best valve comes down to the how much boost you’ll be running and the physical space available for installation.
Blow Off Valve Explained Broken Down

When to Bypass

Not all turbocharged engines are made for blow-off valves, they might need a bypass (aka diverter or recirculating) valve instead. They both accomplish the same task but there’s a big difference in how they do it.
The type of engine management your car runs will come into play when choosing a valve. If you have a MAF-type system, it meters the airflow after it enters the intake system. In this case, when a blow-off discharges to the atmosphere, the ECU isn’t able to properly fuel the engine resulting in rich AFRs, hesitation, bad idle and even stalling. In these systems the excess pressure must be plumbed back into the turbo system before the compressor inlet, which is what a bypass valve does.
Remember that the key to picking a valve is to know your car and what kind of plumbing it needs. But if you know what you’re doing, here are our top picks for BOVs you should check out. They all come from reputable companies and they also offer a wide range of applications. Hit them up if you have questions but just don’t be “that guy” and ask about what noise it makes…

M7 Japan Hyper Sound BOV Type GT-7

M7’s BOV utilizes a dual drive system that helps relieve compressor surge without reducing compressor speed. Its design creates a tight and sharp blow-off noise. The valve will fit most applications but there are also specific kits for the R35 GT-R, Impreza and Evo. Price: From $292 Contact:

Forge Motorsport 50mm BOV

This is Forge’s newest valve. It’s a universal BOV that features the largest bore and flow rate they’ve ever manufactured. The diameter of the inlet port is 50mm. Its top port will accommodate any position for the vacuum hose and the valve can be adjusted with any of its valve springs. Price: $225 Contact:
Blow Off Valve Explained Forge Bov

Forge Motorsport nissan GT-R Recirculation Valves

Got a R35 GT-R? Forge has you covered with either its BOV or recirculating valves. These recirc valves pictured here offer a higher flow rate capability compared to stock and feature a 50mm bore capable of handling up to 34psi of boost. Price: $520 Contact:
  • Blow Off Valve Explained Silver Vs Black
  • Blow Off Valve Explained Gtr Bov
  • Blow Off Valve Explained Synapse Bov

Synapse Synchronic BOV

This valve is completely modular and can be set to either blow-off or diverter mode. It uses an interchangeable mounting flange system and should fit most turbo cars. By design, it is a pull-type valve only and features no diaphrams. It’s also designed to remain open under vacuum conditions. While universal, Synapse offers vehicle-specific kits for the Mazdaspeed 3 and 6, Genesis, Evo and more. Price: $243.95 Contact:


One of the most popular BOVs around, HKS is on its fourth design of its Super Sequential valve. It’s a dual stage pull-type relief valve that won’t leak under boost and will work on most stock and heavily-tuned vehicles. Price: $249.99 Contact:
Blow Off Valve Explained Hks Bov
Win Me! Thanks to Forge Motorsport, we’ll be giving away one of their 50mm BOVs to one lucky reader. Just e-mail us at and tell us why your turbo project needs it.

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