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Old 12-12-2003, 01:21 AM   #1
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Exclamation Blow Off Valve FAQ: Read if you are thinking of buying one!

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.

Have the OEM BPVs changed over the years? Yes. 08+ Subarus have a BPV that is plastic. So if you for some reason you have a plastic BPV, it will not hold as much boost as the old metal ones. The plastic ones will only hold ~17PSI give or take.

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 rich, meaning too much fuel is injected into the cylinders. 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 and this period lasts for under one second...again, nothing to write home about.

This temporary rich condition isn’t usually that harmful. Technically, 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. The odds of either of these two conditions actually happening is very, very low though, but that's the theory.

As to blanket "you'll run rich" statements, a BOV will only run "rich" during hard acceleration and shifting as 99% of the time it stays closed.

Can my tuner or engine management tune out this rich period? Yes. There are some forms of engine management that can tune this out. Buying your engine managment soley for this purpose is a poor method of choosing an engine managment system though.

What about a 50/50 or BOVs that you can portion the VTA portion? This is a bad analogy, but if a BOV is a person in a wheelchair, a 50/50 BOV is a person in leg braces. It's not as bad, but not good enough to say bolt it up. If you find a deal on one or happen to like the sound of a particular model, go for it, but don't think you are doing your car better vs. a 100% VTA model.

Won't I be labeled as a ricer? It's your car, do with it what you like, but be aware that many people are prejudiced against BOVs just like neon underbody lights and other accessories. Be aware that a BOV can be seen as a dinner bell for street racers though. It attracts attention both good and bad.

So a BOV is not bad then right? Not so fast...most BOVs leak. Even ones that are religiously maintained, installed, and are from quality manufacturers. A leak in essence means less boost and less power or your car will be working harder to produce boost than it means, which can decrease reliability.

How do I set up my BOV to not leak/stand less chance of leaking? Read the BOV manual/instructions 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, 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.

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? Neither of those prove that the valve isnt leaking. The advice above tells you how to set it and KNOW for 100% sure.

But compressor surge will kill my turbo! Unless its really bad, it really isnt going to kill it.

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)

If you are wanting to be 100% sure you arent losing any power from a BOV, and you are running under about 22PSI, you are best off sticking with a stock unit.

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. Also thanks to my buddy Davenow for his excellent BOV post found HERE that contains additional information.

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.
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Last edited by Unabomber; 04-20-2012 at 11:22 AM.
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Old 12-12-2003, 01:30 PM   #2
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i love your posts. they are very informative. keep uo the good work!
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Old 12-12-2003, 03:09 PM   #3
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Thank you very much! The FAQ in the batter's box is the dreaded Intake FAQ. Shortys, Ram Pods, and CAIs, oh my! After that, maybe brake upgrades.
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Old 12-12-2003, 03:28 PM   #4
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cai
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Old 12-12-2003, 11:37 PM   #5
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I updated this post with the correct terminology. It initially started by using the incorrect "BOV" as a blanket for all BOVs & BPVs, but I couldn't stand having incorrect terms in a post designed for people with little experience. For the record though, BOV is commonly and incorrectly applied to every part that serves the function of a BOV, BPV, or whatever proprietary term other manufacturers use for atmospheric or recirculatory venting.
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Old 12-14-2003, 12:52 AM   #6
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Default Re: Blow Off Valve FAQ: Read if you are thinking of buying one!

Quote:
Originally posted by Unabomber
[b][size=3][
conditions.


Are there any negative effects with aftermarket BOVs? Yes.


Additionally, the lack of recirculated air can cause the compressor/turbine wheels to stall between shifts and it takes longer to build up boost again after each shift.
Wrong. As long as it vents the wheel wont stall. Not venting at all is what will cause compressor stall. The vented air goes in BEFore the throttle body, on a side that is open to the atmosphere anyway (it goes into the intake, which has a big hole on teh end with the filter on it) The air vented back into the intake is nothing more than additional normal air.

At least the way I am reading that statement its wrong.

AWESOME info post though

Last edited by Davenow; 12-14-2003 at 01:09 AM.
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Old 12-14-2003, 01:08 AM   #7
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Lightbulb I love the unabomber!

I just have to say that every single one of your FAQ posts is awesome. Can I subscribe to your newsletter? I am very interested
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Old 12-14-2003, 02:39 AM   #8
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An hour to install a BOV? Pfft, I can do the up-pipe in half that time.
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Old 12-14-2003, 09:19 AM   #9
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Davenow, Thanks for the error report! I guess I got a little carried away with the copy/paste. One thing I like about my FAQs is while they start out being mostly accurate, fellow NASIOCers post or PM me to point out corrections that I overlook. You go crosseyed working on something like this for several hours. Fixed the post based on your info.

scrmblr2, no newsletter, sorry. I do take requests though. Next is CAI and then probably brake upgrades. If you have something you want me to research, PM me. CAI will probably be in early Jan as I am going on holiday leave to hopefully the snow covered tundra of Indiana. I hope to see how my wagon likes the snow!
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Old 01-16-2004, 10:27 AM   #10
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What about the lean condition created right after the rich condition as the ecu overcompensate for the extra fuel?

I think there could be some manufacturer reccomendations, especially centered around 50/50 style and the one currently getting all the attention in 2.0 turbo forum. Although I know you try to keep these subjective, bov's are a touchy subject and I think it should be hit head on. There is no need to pussyfoot, replacing your bov is not an intelligent thing to do unless you are running boost the stock one cannot take. In that case either do the 15cent mod or buy a 50/50 or recirc.

I just get tired of reading 3 threads a day on bov's and I think this would help (actually I don't read them anymore but I still 'see' them ). Thanks for the faqs ron.



ps, I know a lot of new people to the wrx think its going to be a good mod, I completely understand this. That is why we should come right out and say, don't do it unless you want less power, slow building boost between shifts, and less drivability (in so many words). If you want to sacrifice all those things for a noise makers, than at least you were informed first...
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Old 01-16-2004, 11:32 PM   #11
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Replacing anything is not intelligent , some countries will cite you for something as silly as a wider tire saying it is unsafe.
Put me down as unintelligent from the get go with this car and everything I've done to it , but it sure does make ME happy
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Old 03-18-2004, 06:57 AM   #12
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Quote:
Originally posted by LotusDrift
ps, I know a lot of new people to the wrx think its going to be a good mod, I completely understand this. That is why we should come right out and say, don't do it unless you want less power, slow building boost between shifts, and less drivability (in so many words). If you want to sacrifice all those things for a noise makers, than at least you were informed first...
slow building boost between shifts? support this. the rest is completely wrong, but i'm curious how you support that. barring a leaking BOV, there should be absolutely no difference in spooling between the factory bpv and any bov, as both should be in the same position under throttle--closed.
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Old 03-18-2004, 08:11 AM   #13
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Quote:
Originally posted by LotusDrift
ps, I know a lot of new people to the wrx think its going to be a good mod, I completely understand this. That is why we should come right out and say, don't do it unless you want less power, slow building boost between shifts, and less drivability (in so many words). If you want to sacrifice all those things for a noise makers, than at least you were informed first...
Hats off to you LotusDrift. That was very well put. While you didn't "pussyfoot" around you also didn't flame (it is just too tempting for me to flame anyone who posts the infamous "got a BOV question" and I really must stop because it brings out all the BOV diehards and no one listens anyway). Unfortunately, those green to the Subaru, especially the younger crowd seems all to enticed with noise makers and hyperwhite lights even at the detriment of performance, visibility and their bank accounts. So, while we try our best to make a difference (being nice, and sometimes not) our simple advice or forewarning is simply not headed. I suppose we must learn to let people get a trial by fire.

Unabomber: I guess I indeed should have thanked you first off as you are a wonderful contributor to this forum. You always add intelligent things and have more self restraint than I in your replies. You lead by example in what has now seemed to fill up with noobs. Well, thanks. While everything that you posted in your FAQ is what I try to get across I can't always keep it civil so this thread will pose a nice alternative toward the urge to flame (which really should be avoided, it starts too much trouble even when you don't mean to ).

As a possible addition to the FAQ you may add that as an additional side effect/cosmetic concern of running rich is the black soot you are likely to leave on your bumper (especially with no cats). We have all seen the black soot, now you know what it is, unburned fuel!

Quote:
Originally posted by LotusDrift
Replacing anything is not intelligent
While this statement is way too open-ended as brake pads need to be "replaced" however I think we know what you meant. This statement seems incorrect to me. Stainless steel brake lines have no ill effect on the brake system I am aware of. Removing the EGT sensor and cat from the Upipe can prevent parts of these from being ingested by the turbo. These are only two small examples but you get the idea, there are many things you can replace that the factory cheaped out on to make the car less expensive or restricted and actually improve reliability. Just my .02 . Don't mean to thread steal and hope I didn't as the FAQ is what is most relevant in this thread.
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Old 03-18-2004, 09:26 AM   #14
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SubNub,

Thanks for your comments. One of the reasons I write these darn things is the positive feedback I constantly receive. NASIOC helped me in my n00b days and still does today, and these FAQs are my way of payback. I DO try to be informative and non-specific in my postings, but it can be hard not to flame. I think that level headed people will read my FAQs are see them for what I strive for them to be:

1. Quick reference
2. Strong base for additional searching
3. Non-specific as to Brand/Vendor

These FAQs don't cover it all, but I try to provide information about the main points. For example, I did research the carbon build up issue with this FAQ and the exhaust one I did. The main contributor in my research is exhaust tip protrusion. BOVs and catless running contribute, but it depends more so on the exhaust tip placement. It's hardly worth mentioning as no vendor in their right mind would say, "The XXX exhaust will leave your bumper all black and sooty". For the record though, a BOV is a contributor though.

As always, if there is something anyone would like to see covered, PM me. I am currently working on a Turbo FAQ with plans down the road for Intercoolers and Injectors.
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Old 03-18-2004, 01:44 PM   #15
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lol, any catless exhaust with the tip too close to the bumper will leave your bumper covered in soot, actually. it doesn't matter what you're using to vent if you use kartboy's longer hangers you don't have much to worry about. if you get the metal place that installs on the bumper above the exhaust tip, soot cleanup is quick and easy. the plate also protects your bumper from the occasional fireball (which EVERYONE running catless is prone to experience). tip placement is probably the one main factor in soot, as unabomber said. I don't have any trouble with soot, but i'm running a nur spec touring with the longer hangers.
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Old 12-19-2004, 02:53 AM   #16
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I'd like to know the difference between the STI BPV and the WRX BPV. I've read that the WRX has a vacuum line attached and the STI does not. Becuase of this the spring in STI valve is "lighter". Any specs would be appreciated.

Is there any difference in the maximum boost that can be held by either valve. Stated above the WRX holds to 20psi. How about the STI valve?
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Old 12-19-2004, 10:34 AM   #17
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From what I know they are 100% identical. I've heard that the STi one, perhaps it's the JDM STi one, will hold "higher boost" but am not 100% sure of which one and how much more boost it will hold. You can also mod your stocker to hold more boost. this link shows you how and I'll add that to the FAQ.
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Old 12-20-2004, 11:52 AM   #18
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thanks Unabomber!
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Old 04-23-2005, 05:38 PM   #19
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I'm going to go with jsmonet on this one... why would an atmospheric bov slow building boost between shifts?? not only that how would it create less power?? as jsmonet states... under acceleration the bov should be completely closed..... and should have no effect on "power" at all... the bov doesn't open until after you release the gas pedal......
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Old 04-23-2005, 07:03 PM   #20
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Quote:
Originally Posted by spitfirezip100
I'm going to go with jsmonet on this one... why would an atmospheric bov slow building boost between shifts?? not only that how would it create less power?? as jsmonet states... under acceleration the bov should be completely closed..... and should have no effect on "power" at all... the bov doesn't open until after you release the gas pedal......
Well you probably shouldn't...
Its not really a question of less peak power...they will make the same...but peak power isn't whats important. The argument being made it that...with all your air thrown out of the system, into the atmosphere, your turbo now has to work that much harder to repressurize after you get back on the throttle, therefore, slower throttle response, slower power delivery, plus you will have gone rich b/c the ecu thinks you still have all that air in there... There is a significant difference between venting into the atmosphere and venting back into your system after the air sensor. Its not that hard to see why there would be slight differences in spool up...he is also making the argument that because the stock value is fine a anything below like 18psi, the only reason you would go after market at those levels, is to create that neato sound. Which in most peoples opinion is a silly thing to give up even a little bit of performance for. Plus theres the going rich thing again...
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Old 10-12-2005, 02:35 PM   #21
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sent you a email unibomb, thanks
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Old 11-27-2005, 12:49 AM   #22
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nice thread
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Old 06-07-2006, 11:05 AM   #23
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Great info!

2 Questions:

1.) Are there any advantages in getting an aftermarket BOV over doing the $0.16 mod on the stock BOV?

The only reason I'm considering an aftermarket is that the moded stock surges somewhere between 5-10psi.

Will the aftermarket recirc valves do this as well?

2.) What is the difference between an adjustable BOV and a 2 stage SSQV?

I'm just looking for something that will hold 25psi of boost and not cause any surges.
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Old 06-07-2006, 03:11 PM   #24
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Quote:
Originally Posted by REX8
Well you probably shouldn't...
Its not really a question of less peak power...they will make the same...but peak power isn't whats important. The argument being made it that...with all your air thrown out of the system, into the atmosphere, your turbo now has to work that much harder to repressurize after you get back on the throttle...
Uh...the air on the intake side of the turbo is at atmospheric pressure, regardless of where your BOV vents to. Although your fuel metering can be upset by an atmospheric valve, the BOV outlet location will have no effect on the actual physical operation of the turbo. The turbo does not have to "work harder" to pressurize the intake air.

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Old 06-07-2006, 04:23 PM   #25
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1. No. The .16 mod is great or if you can find one, you can do like I did and get a JDM STi BPV which comes stock with a terribly soft spring and is supposed to hold 20-25 PSI or thereabouts.

2. They are basically the same. A two stage one just has the vent pressure at a fixed level. With an adjustable, you can custom "tune" the BOV to vent at a certain pressure.
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