With the introduction to Cobb Tuning making a comeback to producing hard parts, their anticipated release of a bypass valve is welcomed. Many of us have been waiting for a valve that is easy to adjust, sounds great and works 100% without any surging on the turbo compressor.

Question: So what exactly is a BPV and what is the difference between a BOV ( Blow off valve ) and a BPV (Bypass Valve)?

Answer: Its simple.. It all boils down to whether or not your car runs on speed density or mass air. If you have a car that is speed density, you will not have a mass air meter. So, effectively, you can run a Blow Off Valve which give off a very loud WHOOSH sound as you shift gears when in boost. A Bypass Valve takes that boosted air its venting, and recirculates it back into the intake. The importance of this on a mass air car is, that air was already metered and counted, therefore, venting that air to the atmosphere will cause your can to go rich. You are effectively removing air once counted and letting it blow in the air, so the car is going to add more fuel thinking that air is still incoming. Many enthusiast prefer the sound of a BOV, but generally they end up doing damage to their catalytic converters because of the rich conditions incorporated into running a VTA ( Vent To Atmosphere ) system on a MAF equipped car.

Question: Whats the purpose of these valves anyways?


1) Boosted cars produce tremendous amounts of pressurized air that is pumped very quickly throughout the intake system. When the throttle body is open, this air can move very quickly into the engine. Once the throttle body closes, that fast moving air has no place to go. You have an abundance of air trapped in a small place ( charge pipes ) which can burst, causing significant damage to your intake system and even your cars hood. On turbocharged cars especially, its main function as a Bypass Valve (BPV)/Blow Off Valve (BOV), is to release this boost pressure to prevent this pressurized air from being diverted back toward the turbo impellers. If this pressurized air is not released, the diverted air can cause the turbine wheels to slow and possibly even stall, causing wear and damage to the turbocharger. A turbochargers impeller wheels can spin in excess of over 60,000 RPM, so if the impeller was to fail, you would see a lot of damage.

2) Its second function is being able to hold peak boost during gear change without a significant loss of boost during this event. A good BPV/BOV can hold a high amount of boost, for the needed time of shifting gears, without degrading performance.

3) One important thing to remember on a Mazda, the stock BPV leaks, even at as low as 8 psi of boost. That is almost half of the rated 15 psi a speed can produce stock!

Cobb Tuning has an answer for those looking for that loud blow off sound but with a safety margin, the Cobb Tuning XLE BPV. It can be setup to run in 3 modes, one of them being a hybrid mode that allows a minute amount of VTA without placing the car in a lean condition. You get that loud WHOOSH but you also get that important counted air, recirculated back into the system.

Even though this valve comes in at a steep price of $299, its not any more expensive then the most over-rated BPV on the market, the HKS SSBOV… The Cobb XLE doesn't require any type of flange adapter. The HKS requires an adapter to be fitted to it before it can be mounted on a Mazdaspeed 3 or 6. Installing the adapter is a pain since it uses a snap ring… The most troubling part of the HKS is how much plastic is found inside the valve, but more daunting, its inability to be adjustable. $300? Really? There are numerous reports on our forum about the HKS falling apart… The plastic inside is breaking… This is definitely not the type of valve I want inside my car. Step aside, we have a new BPV on the market thats going out be worth the $299.

The Cobb Tuning XLE BPV is one unit that can be adapted to several cars. First off, if you plan to replace your Mazda with a scoobie, this can be done with a simple part change without having to buy a new valve. Below are the specs on the Cobb XLE:



* Body, mounting flange and secondary outlet cover are constructed of 6061 T6 aluminum.
* The body is hard anodized for additional strength.
* The primary outlet is rectangular in shape to provide maximum cross sectional area for venting pressurized air as quickly as possible.
* The secondary outlet cover can be removed to enable partial vent to atmosphere or full vent to atmosphere modes.
* Unit height is only 3.5 inches from base of flange to top preload adjuster,allowing it to fit into the tightest of locations.
* Body to mounting flange interface is o-ring sealed, allowing flanges of different shapes to work on multiple applications.


* Lightweight aluminum piston allows faster reaction time than heavier pistons.
* The aluminum piston has been hard anodized and impregnated with Teflon to provide smooth movement, reduce surface wear and minimize maintenance.
* Rounded piston profile smooths air flow as it passes through they cylinder.
* The rounded piston profile mates with a rounded piston seat for superior sealing when the BPV is closed.
* Tight tolerance o-rings are used to seal piston to cylinder bore properly at normal engine operating temperatures without sacrificing piston speed.

Preload Adjuster and Spring

* Adjuster allows adjustment of spring preload for fine tuning release pressure.
* Spring and spring seat are constructed of 303 stainless steel.
* Spring preload adjustment mechanism and fasteners are stainless steel.


* Valve vents pressurized air very quickly due to lightweight components, port shape and low sliding friction.
* Unique exhaust sound due to speed of air release.
* Construction materials make the XLE BPV compatible with alcohol/water injection due to inherent corrosion resistance. * Lighter than most aftermarket BPVs and BOVs.


Full Recirculation – Outlet is plumbed back into the intake tract pre-turbo. This configuration provides the best driveability with stock based MAF systems. This configuration generates the least amount of noise.

Hybrid Mode – Outlet is plumbed back into the intake tract pre-turbo and the auxiliary port cover is removed. When the piston is more than 30% open, the auxiliary port is opens, audibly venting to atmosphere. Less than 30% open results in full recirculation to improve driveability.

Full Atmosphere – Outlet and auxiliary port are open to atmosphere. Recommended only for MAP based air metering. This configuration generates the highest sound levels.


Spring Preload – Spring preload determines the pressure differential across the BPV piston required before the BPV opens and releases the pressurized intake charge. The COBB XLE BPV can be adjusted by loosening the locknut and turning the allen-head shaft clockwise for more preload or counter-clockwise for less preload. Once preload is set, the locknut is retightened and ensures the adjustment is secure.


Parts List:

* Bypass Valve
* O-ring (Mazdaspeed 3 Application Only)
* Paper Gasket (Mazdaspeed 3 or LGT/08+ WRX Applications Only)

Tools Required:

* 3/8″ Ratchet
* 10mm & 12mm Socket
* 10mm & 12mm Open End Wrench
* Pliers

Tools Required for Maintenance:

* 3mm Allen Key
* 4mm Allen Key
* 13mm Open End Wrench
* Moly-Based Grease

Installing this piece couldn't be easier. The stock bpv is black in color, made of plastic can can be found on a stock Mazda right at the cold pipe on the stock TMIC

1) Take a pair of pliers and remove the recirc hose on the back of the valve (large 23mm hose). Guide the pliers to the butterfly clamp, compress pliers to release tension and slide back clamp 3 inches from the lip. tug on the hose to release it from the valve.

2) Take the pliers and remove the small vac line in the same manner as you did the above recirc hose.

3) Remove the (2) 10mm bolts holding on the BPV onto the pipe and put them off to the side, we will reuse these.

Now, you have to decide if you want to run full recirc or hybrid mode. If you choose to run the louder hybrid mode, you will need to remove the trap door on the valve with a 3mm allen key/bit. Theres only 4 of these holding it on, so its really simple.

Once you have decided which route you will take, remove the paper gasket from the box and place it on the valve, using both bolts to hold it in place. Simple place the valve back onto the BPV flange and retighten it. Now word of caution here.. If you ARE using an alumn TMIC, I strongly suggest using a ratchet extension with a 10mm deep well socket to hand tighten both bolts evenly. Once they are hand tight, give one good 1/4 inch crank on both bolts and that's it. They hardly require much torque and we keep hearing about Hercules breaking off his bolts in the cold pipe or stripping the threads.

Once you got it torqued back down, simply connect both hoses and take it out for a drive. I seriously doubt you will need to make any adjustments to the valve, but if you do, keep reading.


Adjusting this valve is a no brainer. Simple place a 13mm open end wrench onto the jam nut located on top where the vac line attaches, then take a 4mm allen/bit and go clockwise to increase spring tension ( increase the chance of compressor surge / vent slower ) or counter-clockwide to decrease spring pressure ( decrease the chance of compressor surge / vent faster ).

Now a word to the wise for you noobs.. Increasing the spring tension here does NOT increase the amount of boost this valve can hold. If you want to hold more boost, a different spring will need to be installed in this valve.


It is recommended that every time you change your oil you inspect your COBB Tuning Bypass Valve to make sure the piston is still moving freely. You can do so by unbolting the BPV from the vehicle mounting flange and pushing on the piston. If you can feel some resistance, it is advised to remove the piston from the body and clean and re-grease the piston, we recommend ARP Ultra-Torque Fastener Assembly Lubricant, P/N 100-9911, but any moly-based grease will work fine. I prefer the redline red moly that I used during both the Cobb STS and TWM Performance STS installation.

Disassemble the COBB Tuning BPV

In order to remove the piston, unbolt the base flange from the body using a 3mm allen key. Depending on the BPV model you have, this may be 5-6 socket head cap screws. When removing the piston, be careful to not cut the o-ring. If your o-ring is damaged, you can contact COBB Tuning for a replacement.

Reassemble the COBB Tuning BPV

To reassemble the BPV, push the piston back into the body, so that the piston does not protrude from the body. You can hold the piston at that point by placing your finger over the vacuum connection.

Place the base flange onto the body and re-clock the flange. Secure the flange with the 5-6 socket head cap screws, depending on the model, using a 3mm allen key.


I am so happy to see such an easy valve hit the market that not only looks and sounds cool, but is fully functional, adjustable and isn't finicky right out of the box. For years I was running a Forge Valve that had a blue spring and 2 shims. It gave me issues with no shims, it also gave me issues with a red shim. It was 75% on mark with 2 shims.

With the Cobb XLE, I hold all of my boost during shift and its loud, meaning people know you are on the boost. It also doesn't sound like a couple of ninjas having a sword fight under the hood like the Forge does.

Keep your eye on the valve needing grease, esp is you are running the hybrid mode or your car is in a lot of moisture like rain or snow. This will def breakdown the grease in the piston.

Also keep in mind that the paper gasket IS needed on both the scoobies and Mazda's. I have seen some genius's attempt to go straight o-ring and unless your flange is 100% level, you are going to leak some boost without the gasket. So, use the gasket guys… it is re-usable and needed for our application to keep this valve from leaking.

I would like to thank Gary Sheehan for accidentally placing this in the box while getting the Cobb FMIC V2.0 out to us!


Source by Kevin Pugh