Reason for this post:

  • I want people to know how great this product is.
  • I wanted to clear up some details for customers that were incorrect from the manufacturer.
  • I was hoping Cobb would explain what makes this product so great but honestly I don’t think they know since everyone involved with the design (me) is gone.
  • I want to explain some of the thought that went into a product that from most manufacturers has little to no thought put into it.
  • BPV ? BOV

    Bottom of piston


Cobb has released new photos with the production units.  The pictures I have are of pre-production units (casting is the only difference).  Cobb is referring to it as the XLE (I am guessing Xtra-Limited-Edition or something along those lines) BOV.  It is my understanding that they will only sell what they had in our original production run then they are done.  But then I was also told they were just selling out of everything else they had in stock then they were done, but oh look, it’s all back.

This is not just a BOV.  Of course like any bypass valve, you can just fail to hook up the recirculation hose thereby making it a BOV.  But really it was designed to be a BPV, not a BOV.  All the cars it was designed to work on use a MAF sensor and therefore a BOV is undesirable due to it’s affect on operation, mainly idle.  It has a cover over an oval shaped hole on the rear of it that can be removed to vent a bit of air to the atmosphere and alter the sound a bit.

The original design intent was to build in at minimum, all features of the APS twin vent BOV that seems to be the most popular choice for the Subaru customers.  Of course a full on copy was never an option.  You never want to copy something unless you can make it better in some way.  So I created something that while it had all the features, it was immensely different in the execution.  The APS unit was so large it actually hit the hood on some of our shop cars.  So obviously the size needed to be smaller.  But how do you vent as much air with a smaller package? Don’t worry I found a way.  The piston on the APS unit had a lot of damage on it from regular use.  If you grease it daily, I am sure this could be avoided, but who is going to do that? Also on our time attack car which was running much more boost than stock the valve leaked essentially requiring us to make less power to keep our tune reliable.  Also, while the shape makes sense, it didn’t seem very sexy to me.

The features I built in to my design:

  1. The piston is rounded on the bottom to smooth air flow exiting the unit.
  2. The rounded bottom of the piston sits in a rounded seat for superior sealing of the bottom side of the piston
  3. The air exit is rectangular rather than round to give a larger exit path for the air making it vent as quickly as possible
  4. The piston is hard anodized and Teflon impregnated aluminum to make action smooth and minimize wear. Cobb claims the piston is dry film lubed, but in testing that did not hold up as well as Teflon.
  5. The piston weighs (IIRC) about 1/15 as much as the APS one to make it move faster.  You know, basic physics.
  6. The piston is o-ringed to crazy tight tolerance as to provide sealing at operating temp without sacrificing much piston speed. (I hear the current assembler may have changed something here I just hope he did it right)
  7. The spring and spring seat are stainless steel
  8. The spring seat has many holes in it for an internal air path for the vacuum lifting the piston. This is a feature that may not work on other designs due to their inferior sealing.
  9. The body is hard anodized aluminum.
  10. The fasteners used for the spring pre-load are all stainless.
  11. The bottom mount plate is o-ring sealed and interchangeable for different applications.
  12. The secondary vent hole can be covered to make it a full bypass valve rather than a partial BOV.
  13. The total height of the entire unit is only around 5 mm taller than the stock Subaru diaphragm part.

The result:

  • In most cases, the valve vents the air so quickly that the secondary vent to atmosphere isn’t even used.
  • The sound is very unique and subtle due to how quickly the air is evacuated.
  • On our time attack car we made an additional 40 HP without retuning just due to the fact we didn’t leak air like we did with the APS.
  • I think it looks better. Although it would have been nice to have a more precision cast piece, like die cast. But that would have been expensive which would have had to be passed onto the customer.
  • This piece survives the use of alcohol and water injection where others corrode very quickly in those environments.
  • It is lighter weight and smaller than most aftermarket BPV’s & BOV’s unless they are plastic diaphragm parts.
  • Price came in on target.

I was personally in charge of testing for these on our MazdaSpeed3 (MS3) time attack car, the Nissan R35 GTR, the 2008 Subaru WRX, our 2006 Subaru STi time attack car, a 2002 WRX drivetrain in an older Subaru Impreza, and a 2008 Subaru STi with upgraded turbo and intercooler. Once testing began, we had 0 issues with the function.  The only issue we had at all was when a prototype JB welded recirculation hose connection came apart, which was quickly fixed with a couple spots of real weld to go with the JB weld.


One response to “COBB Tuning BOV

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