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Old 01-22-2004, 07:43 PM   #1
BlueDemon
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Default Please set this issue straight, once and for all: Discussion

Please take a look at this thread here on wrxtuners.com: http://www.wrxtuners.com/forums/show...&threadid=4062 where someone is asking if 100% atmospheric BOVs make you run rich for a split second when the air is dumped.

Another guy who sarcastically calls all nasioc members "geniuses" says it's not true. I've been on nasioc now for over 2 years, and have been quite active, and have read many a thread commenting on the fact that atmospheric BOVs produce a rich condition for a split second, which is why afterfires are more prevalent with the 100% BOVs.

Please comment on this, It'd be nice to get a straight answer for this fellow who thinks we're all dumbasses.

Blue
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Old 01-22-2004, 07:54 PM   #2
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Well, he doesn't ask "if 100% atmo BOVs make you run rich for a split second when air is dumped".

He asked if they make your AFR rich, which is much less specific, and the guy said "No.", which is technically not a wrong answer to such an unspecific question

It's not rocket science -- meter air, provide fuel for it, dump metered air out of system at throttle lift instead of back into the system = rich for about 0.5s

-jblaine,
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Old 01-22-2004, 08:06 PM   #3
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...and as to the effect of this, it is very very minor in reality. We use the Forge BOVs, and we can look right at the ECU as it happens, watching AFRs... and logging it... the effect at least with the Forge isnt worth mentioning.

What IS an issue is that some atmospheric BOVs have a problem with vacuum... try this: plug the return hose on your car but just leave the elbow off the stock bov that attaches to it so that you basically convert your stock bov to an atmospheric. You'll get horrible idle and lots of backfiring. Then, put a well designed unit like the Forge on and voila - no more roughness or backfires.

Regards,
Ken
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Old 01-22-2004, 08:07 PM   #4
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Yeah, I know........ a BOV would not make your Air Fuel ratio run rich, it's the dumping of the air that does this, however that was implied in his question.

BTW, I searched and found this: 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.

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.

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.

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. If you appreciate the work I performed, click the "Report this post to a moderator" on the bottom right of this post and recommend to the Moderators that this post be added to the Newbies & FAQs' FAQ sticky at the top of this forum. Otherwise, bookmark this page for future reference.
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Old 01-22-2004, 08:13 PM   #5
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Another thing to note, according to Mike Shields at www.spdusa.com indicates that the stock BOV (see note below) helps turbo spool by blasting the compressor wheel. I suspect this is very minimal, but it definitely has *some* effect.

I have to disagree with how Unabomber sees the world of "BPV" and "BOV". A BPV can be recirc or atmospheric venting. A BOV can be as well. They're just different terms that different companies use. They're not terms meant for the different venting mechanisms.
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Old 01-22-2004, 08:17 PM   #6
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Here, send the person to another "GENIUS": Mike Shields speaks on WRX BOVs

Stop wasting your time on this moron. I am done doing so.
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Old 01-22-2004, 08:59 PM   #7
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When you back off and are on trailing throttle, the BOV opens and diverts a large portion of the air into the atmosphere, but the injectors shut off anyway, so I don't see how you can run rich. As soon as you come back on the throttle and the injectors start again, the BOV shuts instantly, and has no effect on the amount of air metered. The only problem I have had with my 100% atmo BOV is when it is set too soft so that it opens slightly on idle or cruise, where there is enough vacuum to pull the piston up and let air suck in through the BOV ports. This causes a LEAN condition because there is more air going into the engine than what is going through the afm. This also causes exhaust popping. The fix is to tighten the spring setting.

The only way I can see that it would cause rich running is if the BOV is a bit slow to close when getting back on the throttle, hence some of the metered air is escaping out of the BOV on it's way into the engine. Surely just tightening the spring to make it react faster would solve the problem. Some BOVs that are designed to recirculate, like the factory one, are open a small amount at idle and therefore should never be modified to vent to atmo 100%, because they will never fully close.

EDIT: Maybe some aftermarket ECU's that use the afm and DON'T shut off injectors on overrun may cause rich running..... Don't know of any that do this.
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Old 01-22-2004, 09:36 PM   #8
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I don't see how you can say injectors are shut off. You still have an exhaust sound when you let off the throttle. What is an exhaust sound? The multitude of thousands of explosions created when fuel and air are mixed, compressed and subsequently ignited. So how can injectors shut off and not provide the fuel? There is fuel still flowing, there has to be. Show me something that SHOWs NO fuel.

Blue
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Old 01-22-2004, 10:13 PM   #9
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Talk to any tuner who knows how the Subaru ecu operates. I don't know for sure, but most modern fuel injected cars DO shut off the injectors on the overrun until the revs drop below a certain figure just before idle, say 1500rpm or so. Some aftermarket ecu's (such as my Microtech) also have this option, and when operating, my Techedge wideband a/f ratio meter shows a fully lean condition in the exhaust i.e NO fuel. (45:1) I don't know the technical or physical reason why the popping occurs.
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Old 01-23-2004, 04:02 AM   #10
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I believe the stock WRX ecu runs ~23:1 a/f during deceleration (off throttle). The metered air that the BOV is dumping has already been accounted for by the ECU. When you get back on the throttle, the ECU sees more airflow but it still accounts for the vented air and injects fuel appropriately. As stated before, the effect is not horribly notable, but you can deffinately tell the difference once you switch back to the stock BPV.

-- Ed
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Old 01-23-2004, 12:22 PM   #11
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so basically...what everyone is saying... is that all that recirc naziness that was all the rage here was pretty silly because non-recirc bov's aren't a big deal and don't cause the motor to explode, contaminating the shores of alaska with molten radioactive turbo juice?

*whew*!

man... mentioning that you are running a bov, and have been without issues for 15k+ miles (like i have) would normally net reactions as though i'd killed a baby or something
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Old 01-23-2004, 12:43 PM   #12
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I don't think anyone is that dead set against BOV's. People simply see it as an unnecessary mod with no gains to be had except for a sound. And in some cases they cause some drivability issues.

-- Ed
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Old 01-23-2004, 12:50 PM   #13
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Have you guys ever tried to drive a turbo with compressed air from the compressor wheel side? It doesn’t work well.

The stock dump valve re-circulates the air right before the compressor inlet, but does not actually aim the air into the compressor blades. For grins someone should hook up a compressor so that it blows in the stock bypass dump tube. Shoot 15psi of air down that tube and see how slowly (if at all) the stock compressor wheel spins. For further illustration, hook a boost gauge up to the port on the compressor cover to help you measure the amount of work, or lack thereof, that’s being preformed by your bypass valve powered compressor wheel.

When the compressor is already spinning at very high speed the impact a short burst of this relatively low pressure air is going to be so small as to not even be able to measure it.

Why then does the compressor slow down when you snap the throttle shut? Because your exhaust gas pressure and velocity disappear.

One only needs to look at the world anti-lag to understand the problem and how to combat it. The old school method of anti-lag was to keep the throttle plate open just a little bit and to retard the ignition so that it fires at or very shortly after TDC. This kept exhaust gas velocity and volume up during a shift so that the turbo would keep spinning.

If all they had to do was inject pressurized air in the compressor blades I assure you they would be doing that…

A bypass valves job is to relive the pressure wave that can cause instabilities at the compressor wheel. What it does with the air, once that pressure is relived, is mostly dictated by the metering scheme of the individual car.

-Nathan
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Old 02-10-2004, 12:31 AM   #14
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ok so i'm missing something. i read an article about BOV and CBV - the same one originally posted by unabomber.

the wrx has a bov and a cbv?

so here's some questions:

take this situation: say i'm racing and exit turn 1 in third gear with my gas pedal to the floor accelarating towards the turn 2 braking point. i lift off the throttle abruptly around 7k rpm brake and downshift to 2nd. while this braking and downshifting occurs i'm now ready to get hard on the throttle and exit turn 2.

what's happening here in terms of the bov or cbv? what is desireable in terms of keeping the turbo spooled up and getting full performance when i've finished getting into 2nd and then mash the throttle again to exit as quickly as possible?

what happens then, after the turn is exited and i'm charging down a strait ready to shift into 3rd, 4th etc...?

finally, when i'm driving around town and lifting off the throttle and experience a little bit of an increase in rpm's when shifting during accelaration is that happening because of the air pressure not venting quickly enough due to the bov?

again in this situation, what is desireable? you want to keep the turbo spooled but you also want the revs to drop like a stone don't you?
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Old 02-10-2004, 09:01 PM   #15
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Sg, both the Compressor Bypass Valve (CBV) and the Blow-Off Valve (BOV) are designed to release pressure from the intake system, so the turbo doesn't spin to a stop when you close the throttle.

The CBV vents the air back into the intake, so it doesn't get missed by the engine mangement system. The BOV says, "Screw it, I like the noise," and the engine management system is thrown all out of whack (for a half-second or so) because its carefully measured air count is now all wrong.

The blowing-off of the air to atmosphere DOES result in a momentary too-rich condition. That sometimes makes the engine cough a little bit. At the track, spectators said my Evo (which was running a BOV at the time) actually belched flames out the tailpipe on shifting and lift-throttle, as the excess fuel caught on fire in the manifold on the way out.

Around town, this small stutter doesn't matter much. Accelerating in a straight line, it doesn't matter much either. And if you already were doing an abrupt off-throttle move, an extra hiccup from the motor probably wouldn't upset the car any more. But if you're balancing the car at the edge of traction in the middle of a corner, and you lift a little, and the engine bucks because of the BOV, you may not like the result. I run my car with a CBV now.

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Old 02-10-2004, 09:50 PM   #16
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search
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Old 02-10-2004, 10:15 PM   #17
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Quote:
Originally posted by batmanalan
search
You see my title? It says Scooby Specialist, and yours says Scooby Noobie. Believe me, I know when to search and how to search. This was meant as a discussion.

Blue
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Old 02-10-2004, 10:18 PM   #18
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maybe when it gets warmer than ass cold here , i'll graph my A/F w/delta dash with my stock BPV, then find a friend's BOV to try and see if there is a difference.
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Old 02-10-2004, 10:36 PM   #19
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Quote:
Originally posted by BlueDemon
You see my title? It says Scooby Specialist, and yours says Scooby Noobie.
Blue
heheehehe
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Old 02-10-2004, 10:52 PM   #20
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Quote:
Originally posted by jigga
heheehehe
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Old 02-10-2004, 10:54 PM   #21
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You see my title? It says Scooby Specialist, and yours says Scooby Noobie.

Blue

<Nelson> HAHA!!(pointing finger)

Hey, what up Hans? Hope you're doing alright man
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Old 02-11-2004, 10:10 AM   #22
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Quote:
Originally posted by finnRex
You see my title? It says Scooby Specialist, and yours says Scooby Noobie.

Blue

<Nelson> HAHA!!(pointing finger)

Hey, what up Hans? Hope you're doing alright man
YO What's up Bro. Haven't seen you posting lately? Car is running fine. What have you been up to? Been to the track lately? WOOOAH It says you live in Texas now and drive a Tundra!? When did this happen?

Blue
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Old 02-11-2004, 12:01 PM   #23
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Quote:
Originally posted by BlueDemon
You see my title? It says Scooby Specialist, and yours says Scooby Noobie. Believe me, I know when to search and how to search. This was meant as a discussion.

Blue
lmFAO!

Although this is a topic beaten to death....

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Old 02-11-2004, 08:48 PM   #24
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Hey Blue,
Yeah, I moved to Texas(back in July, actually). A job opening came up and I took it. I really didn't like Florida a whole bunch(after living there 20 years). General sense of apathy. The folks out in Texas are really really nice. You're one of the cooler folks that I've met down there.
Yeah, I traded my WRX in for a Tundra a few weeks ago(which I love, but I am jonesin' some serious speed). But I have been posting(sparingly) in the TXIC forum.
Before I traded my WRX in, I slapped a stage four on(stock IC, but VF-34 and all supporting mods) and I really miss it now I just might have to convert to the "dark side" and get a 1G DSM and put a few go-fast parts on. All in all though I gotta admit that most of the Subaru crowd is cool as all out. Not punks(not too many at least )and not all snobs. Well, I really threw this thread away...hope all is going well with you guys in the Ft. Liquordale/Miami side. Peace


Mika
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