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Old 12-30-2016, 03:59 PM   #801
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Join Date: Dec 2016

Nice write up. Thanks for you time.
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Old 01-04-2017, 10:29 AM   #802
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Join Date: Dec 2016

great faq, I needed this info
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Old 07-29-2017, 11:12 AM   #803
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Default Im a noob.

Originally Posted by Davenow View Post

Based on the recent influx of BOV related posts, and the ensuing garbage that many people have posted in reply to these, I have decided to make up a small FAQ to try to squash some of the rumors and just straight up bad information out there.

Yes, Unabomber has an FAQ on this. The fact is that it doesnt "get to the point" fast enough for most people to get the point. This is written more bluntly, addressing some issues head on.
We dont need 329872389472387 bits of data and links to valves. We need misconceptions fixed, and we need people to stop repeating the same bad info over and over. Or at least hopefully slow it down a little bit

First, some vocab, since some dont know what it all means

VTA=Vent to atmosphere
-this is the noisey style that is the most popular. 90% of BOVs out there are fully VTA (meaning they vent 100% into the air)
A BOV that dumps the vent back into the intake stream. Before the turbo, after the MAF sensor.
Vents half into the intake, half to atmosphere. Makes some noise, doesnt swing as rich. Typically these are set up sequentially, so that on light vents, it dumps 100% back into the intake, this is so that at light boost/part throttle you get no rich condition, and no/less noise. At higher boost, the valve opens all the way, and dumps into the intake and atmosphere.

Now lets clear a couple things up. If it seems like I am pro-bov, just read everything. I get to the down sides further down.

1. BOVS make you run rich.
--No they do NOT! Here is why. When you are idling, driving around, and under boost, the BOV is CLOSED just like stock. The only way to be running rich is if the BOV was somehow letting air out post maf, WHILE RUNNING, constantly. The exception to this would be a valve that is leaking REALLY badly, and in a case like this, you will know there is a problem right away anyway, as the car will probably barely run. For it to leak this badly, it would have to be physically damaged or just REALLY f'd up

2. But what about when you shift? Arent you rich then?
--Yes. BUT, you need to understand a couple things.
First-The rich condition is not THAT rich. On most tunes the target A/F under boost is @11.1-1 or so. Say you are at 11.1-1, then you shift and it vents. It will swing rich, typically to around 9.5-1. That is NOT that rich.

Second- The rich condition does not last very long. On a typical datalogger, the rich condition takes up no more than 2 rows. You are rich for well under a second.

3. I heard its bad for my cats!
--Its not great for them, but its not really enough to shorten their lifespan in any meaningful way. Exactly how long are you planning to keep the stock cats or even keep the car anyway? Seriously, there are guys running around with 100k+ on their car, totally stock exh, with a bov. No cat problems. If you have a VTA blow off valve, and you lose a cat, you would have lost it anyway, because either the cat was bad to begin with, or your vehicle has some other issue

4. But people will think I am a ricer!
--Did they pay for your car? Are they making your loan payments? Will them thinking this stop you from getting laid? Exactly, then tell them to die in a fire and enjoy your BOV if that is what you want.
It should be noted however, that if you are the type of person that doesnt like people trying to race them, you may want to stay away from a BOV simply because so many ricers think that everyone with a BOV wants to race.

5. I heard that if I want one, I should do a 50/50 venting BOV.
--These are somewhat pointless. You still get the rich condition, just not quite as badly. However, it isnt that bad anyway to begin with. These are however good for people that want a little noise when they are on it, and little to no noise when they arent on it hard. Dont fool yourself into thinking that you wont run rich (because you wont "run rich" with a fully VTA valve either), you still dump post MAF air when it vents, so you still get a rich spot when it vents.

6. But I need one because I run more boost than stock.
--In most cases this is not true. GENERALLY the stock valve can be trusted till @20PSI. Some leak a little bit after about 18PSI, but 90% hold rock solid till about 22PSI, and 99.99999% hold solid at 20PSI. If you are planning to run more than 24PSI, you may want to consider going aftermarket. Over 26PSI, you need to either mod your BOV, or go aftermarket.

7. So BOVs are fine? I can run one with no real downsides?
--Not always. MOST aftermarket valves leak. A leaking BOV, DOES slow you down, and can over time, damage things. To make matters even worse, 99% of people who buy BOVs, dont know how to, or bother to, take the time to set it up properly. So even if they bought a quality valve, it may be leaking.

8. I have had XXXX brand valve for years, it has never leaked.
--How do you know? By looking at your boost gauge? Looking at the boost levels in your datalog? Niether of those prove that the valve isnt leaking.

9. Ok so how do I set my BOV up, so that it doesnt leak/stands less chance of leaking?
--Read the manual first off. Learn how to adjust it. If it doesnt have a manual, just look at the valve. You can almost aways figure out how to adjust it. Some have screws and such, some use washers under the spring. Others use replacement springs.
Now that you know how to adjust it, here is what you do.
First off, start the car, get out and open the hood. If you can see the piston of the valve (look in the hole it vents out of if its a VTA valve), check to see if it is moving at all at idle, or worse, partially open. You want that piston rock solid not moving at idle.
Now give it a little gas. It shouldnt move when you step on it, and when you lift, it should pull up a bit. Depending how hard you hit the gas, it will either JUST move, or fully open.
If its doing anything but what it should be, adjust it a little tighter.

Now go drive the car

When you shift at light throttle, is it venting? If so, is it a pure straight vent, or do you get a little bit of a "chufchchchc" noise right before it vents? That noise is compressor surge. Which isnt as bad as some people like to think and say. Ideally, you are looking to hear a little surge then a vent. Or no sound at all. Either is ok. Now drive it hard, does it surge a little or just straight vent?
Get out of the car and tighten the valve and drive it again. Repeat this until you get a LOT of surge when you lift under boost. Now start loosening it until you JUST start to hear no almost surge.
What the surge means is that it fought opening just a little bit. If you hear a LITTLE surge just as it begins to vent, that means that the valve is holding itself shut as tightly as it is capable of, without being so tight that it doesnt vent properly.

10. But OMG, compressor surge is horrible for your turbo!!
--Unless its really bad, it really isnt going to kill it. I had a conversation with Corky Bell (Author of MAXIMUM BOOST) during a BBQ at his shop about this exact subject and he assured me that compressor surge was nothing more than a bad sound, and that his early miata turbo kits didnt even use one, and never had problems with losing the turbo. This goes against everything I had previously heard, but as one of the godfathers of turbocharging I am going to take his word for it. This guy knows more about the subject than 90% of nasioc all added together.

Bottom line is this.
If you are wanting to be 100% sure you arent losing any power from it, and you are running under about 22PSI, you are best off sticking with stock. If you REALLY want the sound, then stock with a GOOD valve. You would be suprised at how many big name and very popular valves are junk.

Generally you can trust
TurboXS, APS, Perrin, TIAL (best one out there), the HKS RACE valve (not the SSQV) and a couple others. There is one very popular brand that makes a lot of 50/50 style valves, I am not going to name names, but searching my name and BOVs should bring it up. Its 3 letters and they are known for 50/50 valves.

How do I know this? Like what am I using to base my opinions?
I spent a good long time several years back testing valves. As many as I could get my hands on. Buying, borrowing, trading, you name it. All in all I had easily 30+ valves on my car. Multiple examples of the same valve whenever I could, to help eliminate the chances of getting just an extra bad (or extra good!) valve.
I was shocked at what I found. Almost all leaked.
Great post, very informative. Im a new 07 wrx sti owner and Im looking for tips and dos and don'ts. Any tips would be appreciated. Unsure of what mods previous owner did other than short ram K&N in take. Thanks in advance!

Last edited by dparson; 07-29-2017 at 11:13 AM. Reason: Spelling
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Old 08-15-2017, 11:25 AM   #804
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Member#: 472553
Join Date: Aug 2017
Location: Upstate NY
2016 WRX

Default Wrx 15+ Bov

Anyone have a BOV on a 15+ WRX?

If so, I am looking for type and how well they function (if there is any noticeable leaks or other issues).
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Old 10-17-2017, 02:59 AM   #805
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Member#: 475506
Join Date: Oct 2017
Location: Michigan
2016 WRX STI


Originally Posted by wrx518 View Post
Anyone have a BOV on a 15+ WRX?

If so, I am looking for type and how well they function (if there is any noticeable leaks or other issues).

I have a Ď16 STI and I have a Go Fast Bits TMS Respons Hybrid Blow Off Valve

Iíve had it for about 8 months and itís leaking. My tuner suggested I get a Tial. He said theyíve never have any problems with them leaking.
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Old 02-11-2018, 12:55 AM   #806
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Member#: 466673
Join Date: Apr 2017
Location: Canada
2009 STi

Default SD and BOV

I dont think you covered thisbut anyway my question is

My plan in the future is to go rotated with front mount and Speed Density Tuned. I am doing alot of research at the moment but can't seem to find the answer for my question.

Is it okay/better to run BOV with a Speed Density?
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Old 03-06-2018, 08:21 AM   #807
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Member#: 466652
Join Date: Apr 2017

I have a GFB Respons that makes my exhaust op when I open it lol love it!
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Old 03-06-2018, 09:55 PM   #808
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Member#: 483009
Join Date: Mar 2018
Chapter/Region: E. Canada
Location: Mississauga ontario
2000 Wrx bugeye


I have gfb respons as well itís awesome
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Old 03-26-2018, 04:22 PM   #809
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Member#: 483862
Join Date: Mar 2018

I have a HKS Super SQV4 that I am looking to install on my 20018 WRX. I have a appointment to have it installed and then tuned. My question is, Would getting a EIDS system for it help with the fuel issues being read from the ECU or is this a problem the tune will fix? Also, With the tune will it resolve the potential stalling issues associated with engine running rich?

I am wondering because I had one shop basicall tell me I was an idiot for getting this valve in particular and another shop that was willing to do the install and tune but said they would do their best to eliminate the potential for stalling. Just looking for solid advice from someone that might have one and what issues you have run into or have heard of people running into. Appreciate it
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Old 04-06-2018, 10:28 PM   #810
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Member#: 465158
Join Date: Mar 2017
Location: Boston
2016 Subaru WRX STi
Pure Red


Great info and well put together I use a go fast bits hybrid adjustable BOV and my car been running fine I followed these steps also, hereís a video install and how to set it up might help someone.

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Old 04-19-2018, 11:26 AM   #811
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Member#: 484874
Join Date: Apr 2018
2006 Subaru STi
WRB / Gold

Default bovs

Good read thanks for the info.
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Old 05-24-2018, 07:48 AM   #812
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Member#: 486328
Join Date: May 2018
Location: Charleston West Virginia
2003 WRX


Lots of Good info here! Thanks for taking the time to write this up. I will be saving a link to this thread👊
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Old 06-04-2018, 11:17 AM   #813
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Thank you good sir for this info!
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Old 06-12-2018, 03:19 PM   #814
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Join Date: Mar 2018
Default don't do it

I have only heard bad things regarding Subaru's and aftermarket blow off valves. Our scoobies just don't like them. Stock is rated until 300 hp as well I believe, so no rush for an upgrade.
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Old 06-30-2018, 05:19 PM   #815
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Old 08-11-2018, 07:14 PM   #816
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Location: Bend, OR
2018 STI


Very helpful for someone new to the community!
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Old 09-04-2018, 02:59 AM   #817
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Thank you for the great info!
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Old 09-13-2018, 10:42 PM   #818
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Member#: 251046
Join Date: Jul 2010
Chapter/Region: MAIC
Location: Mostly at my home
2004 WRX Wagon
Stage 5.1.2a.2


I just got TurboXS' new and improved hybrid BOV, and it is a quality piece. No longer is the piston a tolerance fit, it has o-rings. The top nipple is 90 degrees and rotates 360. No more routing issues! When it recirculates, it's fairly quiet (to almost nothing if you're really gentle) which I like because I like to be able to drive without drawing attention.
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Old 01-07-2019, 10:37 PM   #819
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So if i remove a bov and replace it with the stock bpv I assume I would have to go back and retune it or could i just leave it as it is
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Old 01-08-2019, 10:50 AM   #820
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Chapter/Region: SCIC
Location: Compton, CA
2014 WRX Sedan Lmtd


anyone running Verticooler on USDM WRX? I have the Cobb BPV XLE, but the screw to adjust the spring pressure digs into the brake reservoir. I wanted to upgrade to something I can run tighter, I also suspect its leaking now. Can anyone recommend a good BOV that will fit in tight areas?
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Old 01-15-2019, 05:37 PM   #821
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Join Date: Jan 2019

What are some bad BOVs to avaoid?
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Old 01-16-2019, 01:01 PM   #822
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Member#: 491095
Join Date: Sep 2018
Chapter/Region: SWIC
Location: New Mexico
2009 STi Blue


i ran my ssqv4 recirculated and am now running non and logging it...no issues so far either way...i like it vta atm!
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Old 07-10-2019, 07:25 PM   #823
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Member#: 489481
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Location: Stryker Ohio
2008 Impreza WRX STI
Lightning red

Default Very nice

I appreciate the good info. Local guy (learned was basically a scam) made it seem like a somewhat leaky bov was the same was a gaping hole
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Old 11-12-2019, 12:35 PM   #824
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Default Thanks

As a new member of this forum I am super grateful for this FAQ what a great explanation and inside this gives me!
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Old 01-17-2020, 01:33 AM   #825
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Chapter/Region: BAIC
Location: East Bay,Ca
2004 Impreza Wrx
Rally Blue


Originally Posted by Davenow View Post
Because you havent?
*Eats popcorn*
This is good stuff
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