*Blow Off Valves* - The Basics of Blow Off Valves, Bypass Valves & Hybrid Valves - Subaru WRX Forum
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#1 Old 12-28-2008, 02:11 AM
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*Blow Off Valves* - The Basics of Blow Off Valves, Bypass Valves & Hybrid Valves

There seems to be some misconceptions about the way these things work. Most people new to turbocharging are just looking for the additional sound. So I figure I'll do my best to clear things up in the simplest way possible without getting too technical. Feel free to let me know if I need to add/address anything!

What is the function of these valves?
A turbocharger compresses air with the help of exhaust gases... Just because you let your foot off the gas, that doesn't mean boost building has stopped. That excess boost has to go SOMEWHERE. The duty of the factory Bypass Valve is to recycle or re-route this air back to the intake tract (more specifically, the turbo inlet hose). Atmospheric, or Blow Off Valves vent to atmosphere, and Hybrids will do a bit of both. First, I will post diagrams I mocked up, and go from left to right through the illustrations, describing each condition with each type of valve. This is intended to illustrate the mechanical inner workings of the valves. Then I will write a little more about the function and why to stay away from Atmospheric valves. Hopefully I'll be able to demonstrate WHY a BOV can be potentially harmful, while BPVs and Hybrids may not be.

STOCK BPV:

* When the throttle is open - The piston is not under pressure, so the compressed air from the turbo flows right through.
* When the throttle is closed at low boost levels (Example: Shifting around 2200 RPM) - The piston is under light pressure, and the stock spring is not very strong, so that excess air is effectively routed back to the intake tract. (The MAF sensor expects this excess air to be plumbed back to the intake, so the ECU demands more fuel to balance the A/F ratio.)
* When the throttle is closed at high boost levels (Example: Shifting around 4500 RPM) - The piston is under heavy pressure, so that excess air is effectively routed back to the intake tract. (The MAF sensor expects this excess air to be plumbed back to the intake, so the ECU demands more fuel to balance the A/F ratio.)

ATMOSPHERIC BOV:


* When the throttle is open - The piston is not under pressure, so the compressed air from the turbo flows right through.
* When the throttle is closed at low boost levels (Example: Shifting around 2200 RPM) - The piston is under light pressure, and the since the spring is heavier in aftermarket applications, the excess air creates a back-pressure by having nowhere to go other than back where it came from... The turbo's compressor wheel receives resistance and can stall. This is referred to as Compressor Surge. This can seriously damage your turbo over time. The second ill effect is that your ECU has demanded fuel, but that air is not routed back to the intake, thus causing an extremely rich moment between the shift (Often resulting in a fire ball out the exhaust in cat-less application).
* When the throttle is closed at high boost levels (Example: Shifting around 4500 RPM) - The piston is under heavy pressure, and the piston moves far enough to expose the opening and vents that excess boost off to the atmosphere. This creates a very rich moment during the shift, because your ECU has demanded fuel, but that air is not routed back to the intake, again causing an extremely rich moment between the shift (Often resulting in a fire ball out the exhaust).
**Additional Notes** - If the valve is adjusted to a lighter spring tension, you will effectively prevent the stalling, but it will also allow the piston to push too easily creating boost leak, before you even reach your target boost pressures. Now you may think you have a "perfectly tuned" the valve.. But this is VERY unlikely, because boost pressures vary. Your valve's spring will ALWAYS be too strong or too tight. Causing boost leak, or compressor Surge, and always leading to that rich mixture between shifts.

HYBRID or 50/50 BOV:

* When the throttle is open - The piston is not under pressure, so the compressed air from the turbo flows right through.
* When the throttle is closed at low boost levels (Example: Shifting around 2200 RPM) - The piston is under light pressure, the piston is only pushed far enough to reveal the first opening which routes the excess boost to flow back to the intake. (Just like stock)
* When the throttle is closed at high boost levels (Example: Shifting around 4500 RPM) - The piston is under heavy pressure, and the piston moves far enough to expose both openings: theoretically this satisfies the plumb back to the intake, and then vents off any additional boost off to the atmosphere.
**Additional Notes** - The idea behind a Hybrid valve is simple and effective. It's got two openings. One to plumb back to the intake, and one to vent to atmosphere. The Vent opening is further back than the Intake opening, so that under light load and light boost levels, the valve functions EXACTLY like a by pass valve. But once you're shifting under full boost, there is so much pressure that the piston is pushed even further back revealing BOTH openings. Theres only so much boost that first opening can plumb back in that moment, so the rest is vented off to the atmosphere.

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The Mechanics of it all:
The movement of the piston isn't entirely dependent on Spring Tension alone. One of the largest contributing factors to the function of the BPV is pressure differential between the intake tract and intake manifold. This applies to all the valves mentioned above. Basically, when the vehicle is under boost with your foot to the floor, the pressure in the intake tract and the intake manifold are the same keeping the piston in place. Once you let off the throttle and the plate closes off, the turbo is still pulling air through the intake tract continuing to build pressure, but the intake manifold goes into vacuum. In this case, one side of the piston loses pressure and the piston moves, opening the BOV/BPV.

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"Rabble! Rabble! Don't put BOVs on MAF metered engines!!!" - Many people will also mention MAF readings being a reason why you shouldn't put a BOV on the car. And while some people may just regurgitate the information without knowing what's going on, I know there are those who are interested in knowing. Basically, if you refer to the diagram below, you will see air entering the ram scoop, through the air box and past the MAF sensor. The MAF sensor reads the amount of air passing through and sends the information to the ECU. Your ECU will demand the proper amount of fuel to counteract it in order to maintain proper Air/Fuel ratios. Look at where the BPV is placed. It's after the turbocharger and before the intake manifold. If you Blow Off the excess air, recall that the same air was initially metered and accounted for by your MAF sensor on it's way in through the intake. So while your ECU is counting on that air to be re-routed back to the intake tract(specifically into the turbo inlet hose), blowing that air off will result in a rich mixture of A/F and you will burn VERY rich during the shift. While some after market companies may refer to it as a 'cooling effect', it does have some negative effects over time and can seriously cause damage to your engine and/or turbocharger.



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Conclusion - The entire point of this post was to dissolve rumors and hopefully replace it with some technical insight to those who didn't quite know the difference, or just wanted to know more about the way these valves function. Obviously, the more you know about a product, the more of an educated decision you can make while being aware of any potential gain or harm. If there's any biased message, it's that Atmospheric BOVs are unsafe in applications like WRXs (and most other stock turbo, MAF metered cars). Hybrids are, theoretically, the best of both worlds. The stock BPVs can only hold so much boost because the springs are just strong enough to hold factory level boost (sometimes a tad less, and end up causing boost leaks). Another limitation is that they are generally made of plastic and just can't hold higher boost pressures without failing. Atmospheric BOVs are capable of holding higher boost, but are also adjustable to allow more leak or more resistance. (Again, an 'absolutely perfect' compromise is impossible). Hybrids or 50/50 valves try to give you that BOV sound at high boost levels while effectively functioning like BPVs under lower boost. Boost leak is safer than compressor surge. But neither is really a good thing. One loses power, one harms the turbocharger. My Advice? Upgrade as needed. If you're going to be running higher boost levels than stock, a replacement Bypass Valve may be the better choice as it will not hurt your performance and will in fact hold boost better than the factory unit. Just don't expect any head turning sounds from it

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Happy Motoring!
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#2 Old 12-28-2008, 11:23 AM
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Although your descriptions are accurate, I disagree with your analysis. The main reason you recommend hybrid and atmospheric BOVs is because they handle more boost and the FACTS are that the stock BPV handles way more than stock boost levels without any problems AND that there are lots of aftermarket fully recirculating valves (perrin) that handle more than we'd ever need.

In other words, it's a stupid mod that does #### with your MAF and gives no gain.

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#3 Old 12-28-2008, 12:54 PM
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Although your descriptions are accurate, I disagree with your analysis. The main reason you recommend hybrid and atmospheric BOVs is because they handle more boost and the FACTS are that the stock BPV handles way more than stock boost levels without any problems AND that there are lots of aftermarket fully recirculating valves (perrin) that handle more than we'd ever need.

In other words, it's a stupid mod that does #### with your MAF and gives no gain.
I don't really recommend Hybrid valves. What I am saying, is that in theory, that's what should be the best option. But if you read my final advice at the end, I did mention that a replacement BPV would be the best choice (just like Perrins). In fact, I have the Perrin recirc on order with my turbo.

And I've read many threads of individuals that performed leak down tests and reported back that the stock BPV leaks even at stock boost levels. I don't think it's fair to discount the function and role of a BPV and claim it's a stupid mod. That's why I present the description with the least amount of bias as possible and let people decide if it's right for them or not. I'm going to be running ~20 psi of boost with my new turbo, a BPV is far from a 'stupid mod' in that situation.
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#4 Old 12-28-2008, 11:21 PM
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There's always the good hammer mod too for the stocker.

I'm running 17PSI without problems on mine. "Stock" boost can mean as little as 11.5 PSI. I don't think there's any reason to upgrade till >20PSI. Many people have reported no leaking at 20PSI

I am glad to hear you went with a perrin recirc for your car though. Good part.

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#5 Old 12-28-2008, 11:41 PM
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Originally Posted by mosc View Post
There's always the good hammer mod too for the stocker.

I'm running 17PSI without problems on mine. "Stock" boost can mean as little as 11.5 PSI. I don't think there's any reason to upgrade till >20PSI. Many people have reported no leaking at 20PSI

I am glad to hear you went with a perrin recirc for your car though. Good part.
Oh, right on!

The 'hammer mod' is something I wasn't aware of. Thanks for the info.
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#6 Old 01-07-2009, 07:35 AM
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#7 Old 01-07-2009, 11:29 AM
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excellent info dumdum as usual, thanks!

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#8 Old 01-07-2009, 11:45 AM
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Thanks for the contribution, dumdum.
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#9 Old 01-08-2009, 09:00 PM
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this is the stock bpv right?
can you explain to me how this works

and big thanks Boost Addict for this great article now i understand bpv, bov, and hybrid bov alot more now
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#10 Old 01-08-2009, 09:49 PM
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Glad I could help, man!
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#11 Old 02-13-2009, 02:32 PM
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is this only for subaru or all cars in general?
cus i'm also wondering about evo's
i wonder if evo's also use bypass vavle
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#12 Old 02-15-2009, 10:52 PM
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Stock the EVO has a BPV also, but dont seemto have as much trouble when switching to the annoying BOV.

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#13 Old 02-16-2009, 08:33 AM
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BOV is technically illegal. Venting anything under the hood, even air, is not allowed.

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#14 Old 02-16-2009, 06:42 PM
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Why use a BOV though? There must be a use for it, im just curious. Even if it is technically useless for street use is there a benefiet from it for running even higher boost pressures?
By the way thanks for the info on bypass valves boost addict, very interesting
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#15 Old 02-24-2009, 06:20 PM
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Why use a BOV though? There must be a use for it, im just curious. Even if it is technically useless for street use is there a benefiet from it for running even higher boost pressures?
By the way thanks for the info on bypass valves boost addict, very interesting
"What is the function of these valves? A turbocharger compresses air with the help of exhaust gases... Just because you let your foot off the gas, that doesn't mean boost building has stopped. That excess boost has to go SOMEWHERE."

This is a direct quote from my original post. To paraphrase, the purpose is that the additional boost needs to be let out when the throttle plate is shut. The throttle plate shuts when you remove your foot from the acceleration pedal. However, since a turbocharger operates mechanically by exhaust gas, you can't really stop it from spinning and building boost. So in theory it is designed to vent out the excess boost, then once the throttle plate opens again, the valve is shut and boost building begins again.

Hopefully that clarifies your question.
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