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[p7r4e7y]

This is the big BOV thread. If you didn't find this thread, you didn't search.



Shamlessly "borrowed" from Unabomber in Nasioc.

Originally Posted by Unabomber

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.

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.

Which Turbo XS BOV is this? There are 2 different ones I believe and do you need any flanges or anything?

Oh yeah any pictures??

[BigRich]

Has anyone bought and installed this BOV? Is it a generic make or something? Go Fast Bits? Never heard of them. But it looks just like the Blitz BOV and sounds pretty cool according to vividracing.com's mpeg demo.

Thanks for any info.

[God]

Never heard of them at all. No idea.

[Byrner13]

The Go Fast Bits BOV is made by MRT in Australia. I have it and I love it. Check em out. You'll like it.

[BigRich]

If they are made by MRT, then I'm in... Thanks for the info.

-- Rich

[Zahnter]

I'm pretty sure Go Fast Bits is it's own company and MRT is just a distributor. MRT likely sells their own BOV.

I may be mistaken.

[Byrner13]

You may be correct Zahnter, I was always under the impression that they were manufactured by MRT. Huh. Anyways, I still love it. Thanks for the heads up. Matt

[Valien]

Yes to the above. They are the MRT made BOV's. That's the BOV that i'll be getting after my 1k breakin.

Looking forward to it.

I've heard nothing but good from those that have it.

I've seen and heard the TurboXS one and it's nice, but I didn't care for the purple color and the 100% atmospheric. The MRT one can do a 50/50 vent which I want.

~V

[Austin]

I seen one of these and one of the TurboXS bov's. Both set up for full vent. IMHO, GFB was just right and the TurboXS was a little too loud, for my tastes.

[Sleeper_WRX]

I have the GFB and it is awesome. I originally had it on 100% vent and it sounded great. Then I tried it 50/50 and it sounds good as well. I'm still running 50/50 now. Check it out...Dan at Vivid is a great guy to deal with...really cool.

[BigRich]

Ordered my BOV from vividracing.com over 2 weeks ago and still waiting!!! Anyone else have problems waiting for shipments from them?

[Sleeper_WRX]

I don't know why you would be having this problem....I got mine like 4 days after I ordered it. Have you talked to dan yet?

Nick

[BigRich]

I sure have... Even though the website said they were in stock when I purchased it, he said that they have not arrived yet. Arrrgh. And my CC was charged already. Most places don't charge unless the item has been shipped. Just want to get what I paid for and not feel as if I made a $216 donation to charity.

[Austin]

Nothing makes as angry as a vendor who charges my card, only to learn a week later that he doesn't even have it in stock yet. BAD VENDOR!