Blow Off Valve FAQ: Read if you are thinking of buying one!

[GoFastBits]

Hi guys,
yzp123, the ECU measures the air passing through the airflow meter, and injects fuel for that measurement. When the BOV opens and vents the air to atmosphere, fuel continues to go into the engine (because the injectors are downstream of the throttle butterfly) but not the air. If you data log the airflow meter output during a gearshift, with a recirc valve you see the signal drop immediately upon closing the throttle, and with a valve venting to atmosphere you see the output taper down more slowly, so during the shift fuel is being injected.

Now, here's the thing. Is it really a problem? Not really. The throttle is shut, the engine is not under load when this is occurring - the engine is not operating under conditions where changes to the AFR will cause mechanical damage. If the valve is venting too much air to atmosphere, then there will be proportionally more fuel, to the point where backfiring may take place. This of course is best avoided, and can be controlled simply by controlling how much air escapes to atmosphere.

So, what are the common compaints against atmo BOVs? Stalling, backfiring, more fuel use, hurting performance, engine damage, etc etc.Well, let's address these issues:

Stalling is caused simply by having the valve open too long so that air is still escaping to atmosphere when the car drops into closed-loop idle. The solution is simply to adjust the spring pre-load so that the valve shuts before the revs drop back to about 1200RPM so it can settle into a nice smooth idle.

Backfiring is usually caused by venting too much air to atmosphere. There is more to this as other things can affect whether a car backfires readily or not including ignition timing. The solution in most cases is to increase spring pre-load or reduce the ratio of air vented to atmosphere vs recirc (can only be done on valves that allow this adjustment).

Excessive fuel usage is directly related to how much air escapes. A BOV can only cause fuel to be consumed (where it wouldn’t normally) when it vents. Now think about the percentage of time that a BOV vents for compared to how long you drive your car for. Unless you are on a race track, you would be doing very well to set the BOV off for any more than a fraction of a percent of the total driving time, and therefore the amount of extra fuel you use would be absolutely negligible and un-measurable. In fact if you got the car on boost just once more than you usually do on a trip you’d use more fuel than a BOV would be responsible for.[/font][/color]

Performance – since the BOV can only affect AFR for the time that it is venting, and it will only vent when the throttle is closed or mostly closed, it will have absolutely no negative effect on power or AFRs under load.

Engine damage – nope. You’d have to do something very, very wrong with a BOV to cause engine damage, to the point where the car wouldn’t run long enough to cause damage. Taking the BOV apart and feeding it piece by piece into the turbo intake, well, that’s a different story, but that’s about the only way a BOV could cause engine damage.

Ok, I’m sure there are people out there who have had bad experiences with atmo-venting BOVs causing all kinds of problems. I whole-heartedly believe however that in the vast majority of cases (some mods and combinations of mods can make a car more sensitive than usual) Subarus will run without issue with a correctly set-up atmo venting valve, and I’m more than happy to discuss any of these issues or topics further, relating to our BOVs or others.

Cheers,
GFB Pete

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[XanRules]

Originally Posted by GoFastBits

[FONT='Verdana','sans-serif']Hi guys,[/font]
[FONT='Verdana','sans-serif']yzp123, the ECU measures the air passing through the airflow meter, and injects fuel for that measurement. When the BOV opens and vents the air to atmosphere, fuel continues to go into the engine (because the injectors are downstream of the throttle butterfly) but not the air. If you data log the airflow meter output during a gearshift, with a recirc valve you see the signal drop immediately upon closing the throttle, and with a valve venting to atmosphere you see the output taper down more slowly, so during the shift fuel is being injected.[/font]
[FONT='Verdana','sans-serif'] [/font]
[FONT='Verdana','sans-serif']Now, here's the thing. Is it really a problem? Not really. The throttle is shut, the engine is not under load when this is occurring - the engine is not operating under conditions where changes to the AFR will cause mechanical damage. [/font]
[FONT='Verdana','sans-serif'] [/font]
[FONT='Verdana','sans-serif']If the valve is venting too much air to atmosphere, then there will be proportionally more fuel, to the point where backfiring may take place. This of course is best avoided, and can be controlled simply by controlling how much air escapes to atmosphere.[/font]
[FONT='Verdana','sans-serif'] [/font]
[FONT='Verdana','sans-serif']So, what are the common compaints against atmo BOVs? Stalling, backfiring, more fuel use, hurting performance, engine damage, etc etc.[/font]
[FONT='Verdana','sans-serif'] [/font]
[FONT='Verdana','sans-serif']Well, let's address these issues. [/font]
[FONT='Verdana','sans-serif'] [/font]
[FONT='Verdana','sans-serif']Stalling is caused simply by having the valve open too long so that air is still escaping to atmosphere when the car drops into closed-loop idle. The solution is simply to adjust the spring pre-load so that the valve shuts before the revs drop back to about 1200RPM so it can settle into a nice smooth idle.[/font]
[FONT='Verdana','sans-serif'] [/font]
[FONT='Verdana','sans-serif']Backfiring is usually caused by venting too much air to atmosphere. There is more to this as other things can affect whether a car backfires readily or not including ignition timing. The solution in most cases is to increase spring pre-load or reduce the ratio of air vented to atmosphere vs recirc (can only be done on valves that allow this adjustment).[/font]
[FONT='Verdana','sans-serif'] [/font]
[FONT='Verdana','sans-serif']Excessive fuel usage is directly related to how much air escapes. A BOV can only cause fuel to be consumed (where it wouldn’t normally) when it vents. Now think about the percentage of time that a BOV vents for compared to how long you drive your car for. Unless you are on a race track, you would be doing very well to set the BOV off for any more than a fraction of a percent of the total driving time, and therefore the amount of extra fuel you use would be absolutely negligible and un-measurable. In fact if you got the car on boost just once more than you usually do on a trip you’d use more fuel than a BOV would be responsible for.[/font]
[FONT='Verdana','sans-serif'] [/font]
[FONT='Verdana','sans-serif']Performance – since the BOV can only affect AFR for the time that it is venting, and it will only vent when the throttle is closed or mostly closed, it will have absolutely no negative effect on power or AFRs under load. [/font]
[FONT='Verdana','sans-serif'] [/font]
[FONT='Verdana','sans-serif']Engine damage – nope. You’d have to do something very, very wrong with a BOV to cause engine damage, to the point where the car wouldn’t run long enough to cause damage. Taking the BOV apart and feeding it piece by piece into the turbo intake, well, that’s a different story, but that’s about the only way a BOV could cause engine damage.[/font]
[FONT='Verdana','sans-serif'] [/font]
[FONT='Verdana','sans-serif']Ok, I’m sure there are people out there who have had bad experiences with atmo-venting BOVs causing all kinds of problems. I whole-heartedly believe however that in the vast majority of cases (some mods and combinations of mods can make a car more sensitive than usual) Subarus will run without issue with a correctly set-up atmo venting valve, and I’m more than happy to discuss any of these issues or topics further, relating to our BOVs or others. [/font]
[FONT='Verdana','sans-serif'][/font]
[FONT='Verdana','sans-serif']Cheers,[/font]
[FONT='Verdana','sans-serif']GFB Pete[/font]

wat45

[xabre1200]

Originally Posted by XanRules

wat45

LOL

[redlinerally802]

Great info, just what I was looking for. Had a few people tell me stock holds up just as well as aftermarket when going to stage 2. Have a turbo XS recirculated but guess I will stay with stock until I get my WRX pushing some serious horse.

[dynomight]

Who knew a bov could be such a hazard on a car but I guess with a proper tune, it can decrease the strain that it causes.

[ultimatkilla]

I am gonna bring this back to life.. But only for a quick question.. I am pretty sure Im leaking from my stock bypass valve(09wrx).. I wanted to upgrade to the forge recirculating valve and wanted to know if i should get tuned for it.. I only ask due to the fact I wouldnt be modifying the AFR... Or would I? Sorry for the noobness hah..

[WhatTurboLag?]

no. a bov is a bov. you dont need a tune for one, it cant be "tuned" get a good one and hope you can deal with potentially stalling out sometimes. my friends running full VTA on his car with zero problems what so ever.

[ultimatkilla]

thats interesting.. I was told if you change to vta that you need to tune for it.. Thanks

[dsmtweaker]

Originally Posted by ultimatkilla

thats interesting.. I was told if you change to vta that you need to tune for it.. Thanks

VTA cannot be tuned for.

[2Fast4U1DAY]

Originally Posted by dsmtweaker

VTA cannot be tuned for.

I was trying to explain this to another member a few days ago and the member swore that a full fuel cut when the throttle is lifted would cure the slightly rich condition that occurs at the time of the vent. I am quite the skeptic but if any one can confirm/bust this and post the logs it would clear alot of bs and arguing out of peoples threads.

[the suicidal eggroll]

Originally Posted by 2Fast4U1DAY

I was trying to explain this to another member a few days ago and the member swore that a full fuel cut when the throttle is lifted would cure the slightly rich condition that occurs at the time of the vent. I am quite the skeptic but if any one can confirm/bust this and post the logs it would clear alot of bs and arguing out of peoples threads.

The ECU already cuts fuel when the throttle is lifted, it's like that in a stock tune. The fact that any of our cars have problems with going rich on shifts with a BOV means that obviously cutting fuel when you're off the throttle doesn't fix it.

[2Fast4U1DAY]

Originally Posted by the suicidal eggroll

The ECU already cuts fuel when the throttle is lifted, it's like that in a stock tune. The fact that any of our cars have problems with going rich on shifts with a BOV means that obviously cutting fuel when you're off the throttle doesn't fix it.

Thats what I thought. The injectors turn off until the engine decels to about 1500 or so to catch itself at idle right?

[the suicidal eggroll]

Correct. I don't know the exact RPM they turn back on, but it's somewhere around 1500-2000

[7GEAR]

thank you very much, answered many of my questions

[5280HybridXT]

Ok, i haz couple questions too

So, i have a Forge BOV (came with the car/never took it off)
i wouldnt mind a stock unit, from what i have read here i might just let go of the BOV. Anyway, its always worked fine, even after i got tuned. Recently my blow off seems to be venting with a long extended "CHUSHHhhhhhhhhhhh" at very low RPMS (its never done this before)
so i took it to my mechanic over at 5250 performance in Longmont, CO.
There we desided to smoke test to check for any kind of leak within the TI hose. No leak. Next i took him for a drive, he noticed that my BOV was venting dramitically at low RPMS so then we came to a conclusion that my BOV might have "failed"

My question is.
- Can a BOV "Fail"? does the spring inside go bad orrrr?
- Can i replace the internals of the BOV orr should i just buy a new BPV.
- *(took out question via editing)*
i just cant figure out why its venting so looooooong like.

Any ideas?


P.s. let me know if you need any more information ill do the best to describe it.

[the suicidal eggroll]

Any internal failure I could think of would keep the BOV from opening at all (or it would at least open late). Maybe the spring broke? Or the seat for the spring broke? It's odd for sure. What kind of boost are you running and what car do you drive?

[5280HybridXT]

2002 hybrid WRX, 18GXT

it just doesnt sound right at all, ill be in first gear slowly going up to 3000RPMS;shift; then i get a *shuuuuuuuuuuuuuushhhhh*
gears 1-5.

im stumped.
Its starting to worry me, idk what to do next.


*sorry i forgot to add that im running low boost at 17psi

and what are some symptoms if the "Spring seat" broke. have you ever ran into something like that?

[5280HybridXT]

P.s. What could cause the spring to actually break?

So as i drove home from work i listened closely for i can describe it better to you guys, it almost sounds like when it starts to vent, it stays open.

could this be true? or am i crazy?

[the suicidal eggroll]

Originally Posted by 5280HybridXT

and what are some symptoms if the "Spring seat" broke. have you ever ran into something like that?

I have not

However, the OEM BPV is perfectly suitable for your car with your boost target. Since you can generally pick up a used one for about $30, I'd try that first, since switching to recirc would be a good idea regardless of whether this BOV is functioning correctly or not.

[5280HybridXT]

Okay, ill pick up a stock BPV.

now what if i was planning on going over 400+hp?
Im really uncomfortable driving it around when its doing this, i wish someone knew what i was talking about.

would it help if i recorded a video?

[the suicidal eggroll]

Possibly.

The thing is, the bov opening excessively when you let off the throttle at low boost isn't a bad thing, in fact as long as is staying shut when youre on the throttle it's a good thing. You WANT the bov/bpv to open as much as possible for as long as possible when you let off the throttle, without it opening while in boost. The problem isn't that it's opening excessively, it's that you're running an atmo BOV with a draw through maf, and the odd thing is that it apparently wasn't doing it before.

[5280HybridXT]

how could i test if it is opening while in boost?

& yeah im confused as of why all of a sudden it just started to do this out of no where.

what a bummer haha, when i boost it, my gauge reads that its holding pretty steady at 17psi, i dont feel much loss in power....just very skeptical about the whole situation, like something COULD go wrong since this whole thing is already pretty sketch. you know what i mean?

ill record a video once i get my IC back on, i had to replace the elbow hose because it kept popping off


thanks a lot for the help, i really appreciate it.

[dking240sx]

Hey guys just bought my car the guy I bought it from opened up the oem blow off valve were to its atmospheric! Is that gonna harm the car he plugged the recircuitlotory hose!!

[Robbie_R]

Just to clarify something. The plastic BPV came out in 05+ on the Turbo Legacy/Outbacks in North America.

I have a GFB Hybrid on my car because my friend had a lease returned Spec B and couldn't find his stock one. Fuel consumption didn't change so I'm guessing the AFRs aren't too different.

[lcjr80]

Hello guys,

I just recently bought my first turbo car. A 02 wrx. Since I was a teenager I love the sound of blow off valves. But couldn't afford a turbo car at the time. Well now I am 31 an after purchasing my house I came up with some cash to get me the wrx. My previous car was an impreza rs. Now my wrx is all stock except a greddy exhaust. I don't race just daily driving. My question is can I put a BOV with the stock motor without any issues. And which one do you guys recommend without breaking the bank. Thanks