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Old 07-29-2007, 06:28 PM   #101
02bluesubaru
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im about to get an 02 do i need to get a better bov?
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Old 07-30-2007, 12:13 AM   #102
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is there a way to modify your stock bov to be atmospheric instead of reculating the air?
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Old 07-30-2007, 12:19 AM   #103
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Quote:
Originally Posted by Unabomber View Post
Blow Off Valve FAQ

Term usage: "Blow off valves" go by several names, among them are compressor bypass valve (CBV), air by-pass valve, bypass valve (BPV), blow off valve (BOV), Diverter valve, and possibly a few others. BOV is the common and incorrect term that lumps true blow off valves and bypass valves under the same term. For the sake of correctness, this post will refer to either aftermarket BOV, aftermarket BPV or OEM BPV as these are the most correct terms.

What is the function of a blow off valve (BOV)? To release pressure from the intake tract of a turbo car when the throttle closes. It is a vacuum-actuated valve designed to releases the air to the atmosphere.

What is the function of a bypass valve (BPV)? To release pressure from the intake tract of a turbo car when the throttle closes. It is a vacuum-actuated valve designed to recirculate the air back into the intake before the turbo inlet, but after the airflow sensor.

What is the purpose of a BOV/BPV? When the throttle closes and the intake system is under pressure, the high-pressure air entering the motor will bump into the closed throttle plate, and in the absence of a BOV/BPV, a pressure wave will travel back to the turbocharger. The result is that the compressor wheel will stall (a phenomenon known as “compressor surge”) and slow down very quickly. This is hard on the bearings and decreases the turbo’s lifespan, but it also means the turbo will take longer to spin up the next time the throttle is opened.

Are aftermarket BOVs necessary with Subaru turbos? No. The OEM BPV is perfectly fine up to 20psi of boost. For applications using higher boost levels, an aftermarket BOV/BPV should be considered.

Can I mod my stock BPV to hold higher boost? Yes. This link gives detailed instructions on how to do so.

Is the STi BPV better than the WRX BPV? No. They are the same. However, the JDM STi BPV will hold more boost as it is physically different than both the USDM STi BPV and the WRX BPV. The specific PSI rating of the JDM STi BPV is unknown, but users have reported it is good up to 25 PSI.

Is an aftermarket BPV better than the stock BPV? No. Unless you are considering an aftermarket BPV solely for the purposes of holding higher boost levels. An aftermarket unit should sound just like the OEM unit.

Which manufacturer is best? This topic is highly debated. There have been no reported consistent "bad" aftermarket BOVs. Obviously, there may have been bad ones sold, but not enough to report as "bad" overall.

What are the different types of aftermarket BOVs/BPVs? Different manufacturers use different methods. There are three basic types:
1. Aftermarket BPV: Similar in function to the OEM BPV where 100% of the air is recirculated.
2. Atmospheric BOV: 100% of the air is vented to the atmosphere.
3. Hybrid BOV: These depend on the manufacturer and end user settings. These can either be adjustable or manufacturer set for different percentages of atmospheric/recirculation dumping. They can also be set to work as recirculation during lower boost conditions and 100% atmospheric during higher boost conditions.

Are there any downsides to aftermarket BOVs? There have not been significant amounts of problems with BOVs. Aftermarket BOVs can and do require some light end user maintenance to keep them performing perfectly. For aftermarket hybrid BOVs that have end user defined settings, there will be an initial period of adjustment to obtain the desired recirculation/atmospheric ratio. As well, most aftermarket BOVs will require "tuning" (usually via supplied washers, a screw, or other mechanism on the BOV itself) to allow them to idle correctly and blow off at the right time.

Are there any negative effects with aftermarket BOVs? Yes. The downside of releasing the air to atmosphere is that it has already been metered by the mass air sensor, and when it blows off, the ECU will be injecting the wrong amount of fuel into the cylinders. The engine temporarily runs extremely rich, meaning too much fuel is injected into the cylinders.

This temporary rich condition isn’t usually that harmful, but it can cause bucking or hesitation on lift-throttle. If the condition is really bad, it can eventually foul spark plugs and even clog the catalytic converter as unburned fuel on the catalytic converter burns very hot, and too much of it can melt the cat.

Can my tuner or engine management tune out this rich period? No.

Where do I buy aftermarket BOVs/BPVs? Every Subaru/Import performance store sells BOVs. For purchasing, support your local economy or the NASIOC Vendors.

How hard is it to install aftermarket BOVs/BPVs? Allow around 1 hour for install time. Professional installation, depending on your area, is around $75. This is one vehicle modification that is very simple and can be successfully accomplished by even the greenest shade tree mechanic.

How do I install BOVs/BPVs? Refer to the BOV/BPV manufacturer's instructions. For BOVs/BPVs without instructions, below is a link to one of the better known installation instructions:
scoobymods.com instructions (with photos)

Editors Note

My thanks to Dan of Mach V Motorsports for writing an excellent BOV article that provided a lot of the information contained within this FAQ. He also provided additional information that was helpful in the formulation of this FAQ.

This post was created because I wasn't able to find a good blow off valve FAQ. I came up with the text based on LOTS of searching here. Upon reading this you should have an idea of whether a blow off valve best suits your needs or not. The manufacturer is up to you.

If you find an error in this FAQ, please PM me with factual details and I will update this post. Responses such as, "I have XXX's blow off valve and it's great!" or "XXX's blow off valve broke after 1 month" are not appreciated here, that is what the Car Parts Review Forum is for.
cause the last two posters can't read.
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Old 08-04-2007, 02:26 AM   #104
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So is it better to get a BOV or not? I know some people have it on their subys but some people say its bad?
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Old 08-04-2007, 11:42 AM   #105
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not

Are there any negative effects with aftermarket BOVs? Yes. The downside of releasing the air to atmosphere is that it has already been metered by the mass air sensor, and when it blows off, the ECU will be injecting the wrong amount of fuel into the cylinders. The engine temporarily runs extremely rich, meaning too much fuel is injected into the cylinders.

This temporary rich condition isn’t usually that harmful, but it can cause bucking or hesitation on lift-throttle. If the condition is really bad, it can eventually foul spark plugs and even clog the catalytic converter as unburned fuel on the catalytic converter burns very hot, and too much of it can melt the cat.

Can my tuner or engine management tune out this rich period? No.


come on people
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Old 08-05-2007, 03:13 AM   #106
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Is a hybrid BOV with, say, 50/50 recirc/release better to use? Is it still possibly damaging?
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Old 08-05-2007, 10:08 AM   #107
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Quote:
Originally Posted by MarineSniper View Post
Is a hybrid BOV with, say, 50/50 recirc/release better to use? Is it still possibly damaging?
EVERY blow off valve and any type, manufacturer, country, style, or *insert adjective here* all suck.
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Old 08-05-2007, 11:58 AM   #108
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Quote:
Originally Posted by GC8V View Post
So is it better to get a BOV or not? I know some people have it on their subys but some people say its bad?
Are you joking? I seriously can't tell whether or not you are joking or whether you are incapable of reading the quote directly above your post. In the event that you aren't joking, .... <insert random statement of total disapproval here>

Last edited by theicewall; 08-05-2007 at 12:03 PM.
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Old 08-05-2007, 11:59 AM   #109
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Quote:
Originally Posted by MarineSniper View Post
Is a hybrid BOV with, say, 50/50 recirc/release better to use? Is it still possibly damaging?
If you must, a 50/50 is LESS BAD than a 100%Atm, but less bad still implies bad.

edit- a 75% recirc and 25% vent is even less bad, but I forget off the top of my head whether Forge or Worx (I think the latter) makes an adjustable valve utilizing spacers that would allow for this. Keep in mind that I am still telling you that it's bad and a waste of money.
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Old 08-05-2007, 12:10 PM   #110
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but like well isnt it cool to have a car that sounds like fast and the furious... god i hate bovs
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Old 08-13-2007, 11:59 AM   #111
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Default Stupid Question here... LOL

Right now I'm in Stage 2 with Cobb AP. I hava a Forge Recirc. BPV. I'll be getting protune soon, now, I need that Recirc BPV on the way or the stock one will do the SAME THING at around 17-20PSI from the protune?

I think the stock one is OK but because I'm close to 20PSI range, I dont want to sell a BPV to get it again when protune...
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Old 08-15-2007, 03:30 AM   #112
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Hey i just wanna put this info up since i didnt see it however i did not read page 3 and 4.

Perhaps you might wanna add the info that if you relocate the MAF you can run a 100% Atmospheric BOV.

I just got a front mount and a couple of my friends mentioned this to me, therefor i felt like sharing. I will be attempting a 100% atmospheric BOV with my new SSAutoChrome front mount intercooler which is pretty much around the same price as a used sti top mount intercooler. All you need to do is get the BOV Mounted before the MAF sensor, Therefor if the BOV lets the air out your maf will get the correct reading and inject the proper amount of fuel. :-D
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Old 08-15-2007, 11:31 AM   #113
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Default True!

^ I was wondering why no one had mentioned that. Not sure if it would 100% correct the problem though...

I hear a lot of hating on BOVs and how much they *cough* blow *cough* but it should be worth mentioning that they don't always suck...

It's just that our cars use a MAF setup. A lot of full blown race setups use MAP with VTA... maybe because you are getting cooler air from your intake rather than the already compressed air out of your BPV...

Just my 2 cents...
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Old 08-19-2007, 01:53 PM   #114
WRX5SpeedSVX
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Default Reloacating maf?

Quote:
Originally Posted by jet2686 View Post
Hey i just wanna put this info up since i didnt see it however i did not read page 3 and 4.

Perhaps you might wanna add the info that if you relocate the MAF you can run a 100% Atmospheric BOV.

I just got a front mount and a couple of my friends mentioned this to me, therefor i felt like sharing. I will be attempting a 100% atmospheric BOV with my new SSAutoChrome front mount intercooler which is pretty much around the same price as a used sti top mount intercooler. All you need to do is get the BOV Mounted before the MAF sensor, Therefor if the BOV lets the air out your maf will get the correct reading and inject the proper amount of fuel. :-D
Where do you mount the maf @? Is there a kit or is it pieced together? after taking 45 min to read this thread, it looks as if anyone still wants a whoosh sound on a stock or close to stock setup and not compromise drivability, this or the K&N typhoon intake kit are going to be the ways to go.

Last edited by WRX5SpeedSVX; 08-19-2007 at 02:14 PM. Reason: still learning.
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Old 08-19-2007, 02:14 PM   #115
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after even more reading both of those can affect drivability. what the heck!! Is there anything you can do to get more turbo noise besides removing the snorkal resonator thing.
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Old 08-19-2007, 06:16 PM   #116
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In all honesty the resonator removal doesn't add that much sound. You can hear it more but not like WOW that's louder.
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Old 08-19-2007, 07:48 PM   #117
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whats up with compressor surge?
i cant find the thread i'm looking for. I'm getting surge pretty bad... its not as high boost levels but it still worries me.
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Old 08-20-2007, 10:34 PM   #118
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^^^ Often it happens at lower boost levels when the sudden change to vacuum isn't enough to open a valve who's spring is a little on the tight side. It isn't nearly as likely to occur at full boost, but then you're more likely to have the valve leak. I'm basically referring to any adjustable or aftermarket valve you'd consider buying.
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Old 08-24-2007, 03:21 PM   #119
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Great Post.. ty
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Old 08-27-2007, 12:50 AM   #120
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This is probably a dumb question but if BOV's are so bad why do so many rally cars run with them? I have heard them in rallies through out the country and on WRC cars.
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Old 09-01-2007, 06:19 PM   #121
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Because rally cars have stand-alone engine management that utilizes a whole heck of a lot more inputs, sensors, piping, and most importantly it uses density. You're wrx is nothing... nothing... like a wrc car.

Don't compare your engine to this.
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Old 09-04-2007, 05:59 PM   #122
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Thanks for the answer, and I'm def. not comparing my engine to that. I only wish I had one of those. I still have the stock bpv, I was just looking for a specific reason, so thanks.
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Old 09-09-2007, 11:25 PM   #123
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pardon me, noob qn... if the BPV routes the excess air back into the intake (assuming it's a cone filter short ram intake setup), doesn't most of the air escape via the cone filter and hence the pseudo-BOV 'phishhhhh' sound during gear shifts? if this is true, doesn't it cause the engine to run momentarily rich since most of the air escapes and doesn't enter the engine?
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Old 09-10-2007, 01:12 AM   #124
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Quote:
Originally Posted by Unabomber View Post
EVERY blow off valve and any type, manufacturer, country, style, or *insert adjective here* all suck.
TIAL doesnt, neither does the HKS race valve (which is right abouts $500)
The TurboXS Type-H works real well if you spring it really hard, but for some reason the TurboXS RFL (which is supposedly a type H with a horn) doesnt. (at least the ones I tested didnt)
Im probably the one of the only people on nasioc that is really experienced with this subject. Im talking firsthand experience directly testing them. I tested 30+ valves to see what leaked/surged/didnt work worth a damn a couple years back. TIAL was the only one that didnt leak. I have seen other cars with the HKS race valve and from what I could see at the time, they held rock solid.





Speed density tune FTW. No running rich from a bov.

Last edited by Defiant Autospeed; 09-10-2007 at 01:17 AM.
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Old 09-10-2007, 01:14 AM   #125
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Quote:
Originally Posted by Rexer06 View Post
pardon me, noob qn... if the BPV routes the excess air back into the intake (assuming it's a cone filter short ram intake setup), doesn't most of the air escape via the cone filter and hence the pseudo-BOV 'phishhhhh' sound during gear shifts? if this is true, doesn't it cause the engine to run momentarily rich since most of the air escapes and doesn't enter the engine?

It puts it back directly in front of the turbo inlet. Its seriously like 3 inches away from the inlet snout on the compressor. Its also after the MAF sensor, so you are still getting the air that is already metered.
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