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Discussion Starter · #1 ·
I know it's taboo. "BOVs are the un-silent killer" "BOVs r dum" "Don't buy a BOV"
Here's my thought.

I have a K&N Typhoon intake. It looks cool and sounds cool. It really really sounds sweet honestly; the intake is pretty loud, the turbo sounds a little louder, and you can hear the stock BPV. Hearing this got me thinking.

(I'm saying this making the assumption that the stock BPV lets out all pressure, but please correct me if I'm wrong. From what I've seen I believe it does.)

The stock BPV vents to the turbo inlet, getting metered air back into the system. Except I have a high flow intake(supposedly). The BPV creates positive pressure in the pre-turbo side of the intake. Wouldn't this air just flow the opposite way out the air filter? Suppose I'm at 14 psi and shift, leaving the intake manifold at 0 psi. All this air goes pre-turbo, but this is basically venting to atmosphere, because the high flow filter would let it right out. If anything the pre-turbo pressure would go from a slight vacuum to atmospheric pressure, so what's the point? Wouldn't it be best to have a BOV or BPV that only drops intake manifold pressure to maybe 5 psi? That way it doesn't make the turbo flutter(probably), doesn't put (too much)pressure on the throttle body, and you still have some pressure for after the shift. Because the compressor wheel will slow, the pre-turbo air will return to atmospheric pressure, or close to it. At that point, what does it matter where the excess air goes? If anything, a 50/50 BPV/BOV would surely get the pre-turbo inlet up to atmospheric.

Is this crazy talk? Hearing my BPV is nice, and I kind of wish it was a little louder. This got me thinking of the pros and cons, and that's how I ended up with the conclusion that it doesn't matter. Then watching some old WRC videos I noticed the flutter and realized that with no BOV there'd be hardly any turbo lag, leading to the thought of pressure left in the manifold. Obviously for a team who has many cars and engines this is a great idea, because who cares if you send a compressor wheel through the throttle body? ;) . Plus their turbos are much more expensive, but back to the main subject. I at least figured it was a good topic for discussion, though I'm sure this is more like what it is -->:deadhorse: For someone with a FMIC this would be really beneficial, because of the amount of volume of air post-turbo, due to all the extra piping.

So then, what's so bad about a BOV?
 

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If you go that route why not Boomba BPV. I mean running a BOV just confuses the motor since its gonna expect metered air that just isn't there and may cause a rich condition between shifts. Btw are you running an AP and tune with that intake?

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Discussion Starter · #3 ·
Honestly haven't looked at any yet. Also, I'm in the middle of doing an opensource etune at the moment, no AP. How confused would the motor really be? With the BPV the air is flowing backwards across the MAF sensor, assuming the filter doesn't hold much pressure. There's not gonna be any air that it's expecting there as it is with a SRI or CAI. With the OEM airbox it could hold a little pressure, since air is forced in at the air scoop, but even then there are drain holes and bad tolerances on the plastics, so it's still letting out air.
 

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.... The BPV creates positive pressure in the pre-turbo side of the intake. Wouldn't this air just flow the opposite way out the air filter?No Suppose I'm at 14 psi and shift, leaving the intake manifold at 0 psi. All this air goes pre-turbo, but this is basically venting to atmosphereNo, because the high flow filter would let it right out. NOIf anything the pre-turbo pressure would go from a slight vacuum to atmospheric pressure,not sure even this is correct so what's the point? Wouldn't it be best to have a BOV or BPV that only drops intake manifold pressure to maybe 5 psi? That way it doesn't make the turbo flutter(probably), doesn't put (too much)pressure on the throttle body, and you still have some pressure for after the shift. Because the compressor wheel will slow, the pre-turbo air will return to atmospheric pressure, or close to it. At that point, what does it matter where the excess air goes? If anything, a 50/50 BPV/BOV would surely get the pre-turbo inlet up to atmospheric.


So then, what's so bad about a BOV?same as always - improper fueling because it now does not match the metered air.

When you shift, the engine is still sucking air constantly as long as it is rotating. Fueling supposedly stops, but sucking in air and pushing it out the exhaust NEVER stops when the engine is rotating.
That is why I say i am not sure even the inlet goes to atmoshperic at that point; one would have to measure it. But my gut feel is that "ram air" into the inlet NEVER is so good as to supply any
positive pressure into the inlet even in the case you mention. In fact I think the whole inlet tract is designed so as to always be at negative pressure, even when shifting/letting off as you describe.
Otherwise, how could fueling and airflow be controlled at all by the engine management system?????
 

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Your intake is in a vacuum. You would have to fill the volume of the intake with enough air to make positive pressure. Considering even after the bypass valve vents into the intake a vacuum is being pulled from the engine and turbo I highly doubt you will unless you are running crazy high pressures and volumes of air such as compound turbos.
 

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Discussion Starter · #6 ·
When you shift, the engine is still sucking air constantly as long as it is rotating. Fueling supposedly stops, but sucking in air and pushing it out the exhaust NEVER stops when the engine is rotating.
That is why I say i am not sure even the inlet goes to atmoshperic at that point; one would have to measure it. But my gut feel is that "ram air" into the inlet NEVER is so good as to supply any
positive pressure into the inlet even in the case you mention. In fact I think the whole inlet tract is designed so as to always be at negative pressure, even when shifting/letting off as you describe.
Otherwise, how could fueling and airflow be controlled at all by the engine management system?????
Suppose we estimate 1 cubic foot of air before the tb and after the turbo. Once the BPV opens, it flows into the turbo inlet and expands to about atmospheric (we'll say that just for easy numbers). This means that 2 cubic feet of air are flowing into a pretty small space, especially if you have a SRI. Does it really seem like the turbo would be able to pull back in those 2 cubic feet, just as they are flowing out? Any positive pressure in the intake inlet (unused by turbo) will be let out the filter. Fueling is being cut, air is being cut, engine RPMs are slowing. 2 cubic feet seems like a lot at that point. Other things to think about is that once you get a bigger TMIC or FMIC, you make the initial volume larger, in turn making secondary volume larger. Likewise, upping the initial PSI will make the secondary volume larger. I get where you're coming from, but it just doesn't seem like it would use all that air. BUT this shows why a 50/50 or aftermarket BPV makes a difference. So actually, yes this proves why a BOV doesn't work as well.

The stock BPV vents to the turbo inlet, getting metered air back into the system. Except I have a high flow intake(supposedly). The BPV creates positive pressure in the pre-turbo side of the intake. Wouldn't this air just flow the opposite way out the air filter? Suppose I'm at 14 psi and shift, leaving the (intake manifold)post-turbo, pre-tb at 0 psi. All this air goes pre-turbo, but this is basically venting to atmosphere, because the high flow filter would let it right out. If anything the pre-turbo pressure would go from a slight vacuum to atmospheric pressure, so what's the point? Wouldn't it be best to have a BOV or BPV that only drops (intake manifold)post-turbo, pre-tb pressure to maybe 5 psi? That way it doesn't make the turbo flutter(probably), doesn't put (too much)pressure on the throttle body, and you still have some pressure for after the shift. Because the compressor wheel will slow, the pre-turbo air will return to atmospheric pressure, or close to it. At that point, what does it matter where the excess air goes? If anything, a 50/50 BPV/BOV would surely get the pre-turbo inlet up to atmospheric. Wrong terms, corrected myself
 

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You can try and justify this in your mind all you want. But it’s not blowing air out the end of the filter. There is not that much air being moved back.

The intake pre turbo is negative atmospheric pressure. The second the throttle slaps closed and the bypass releases, you still have velocity from the turbo pulling air out of the intake coupled with the draw of air from the pumping of the engine.

If you got positive pressure pre turbo it would blow the intake completely off the car.
 

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Discussion Starter · #8 ·
You can try and justify this in your mind all you want. But it’s not blowing air out the end of the filter. There is not that much air being moved back.

The intake pre turbo is negative atmospheric pressure. The second the throttle slaps closed and the bypass releases, you still have velocity from the turbo pulling air out of the intake coupled with the draw of air from the pumping of the engine.

If you got positive pressure pre turbo it would blow the intake completely off the car.
Okay... a little extreme there about blowing the intake off. I will admit that when I wrote this yesterday it was after an exam and I got a little convoluted, because while writing I forgot about the turbo continually spinning... :face: yikes. BUT, the math is there. Especially if the pressurized air expands and equalizes to less than atmospheric pressure or vacuum. The volume just goes up on the other end. Granted, like I said, it's hard to say without actual numbers, like cubic feet per second at that moment.

Anyway, aftermarket BPV. Theoretically this is a good idea, right? As long as the thing doesn't leak, and as long as it's able to keep a constant pressure rather than venting pressure to 0. Leaving the IC charged to about 5 psi would surely help reduce lag, without having a terribly harmful effect on the turbo.
 

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Discussion Starter · #9 ·
Your intake is in a vacuum. You would have to fill the volume of the intake with enough air to make positive pressure. Considering even after the bypass valve vents into the intake a vacuum is being pulled from the engine and turbo I highly doubt you will unless you are running crazy high pressures and volumes of air such as compound turbos.
Good point. Didn't see this earlier. Still though, would there be no pressure variance as the BPV opens? If the area in the engine bay next to the filter is just as negative, any pressure raised on the inside would flow outwards.

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There is ZERO mechanical benefit to upgrading to an aftermarket valve unless the OE one you have can't handle the pressure that you're seeing. Aesthetically, an aftermarket valve may be a little more audible or add some "bling" to your engine bay, which may be a "pro" for you, but do not think of this as any kind of a performance enhancement modification if your OE valve works fine.
 

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KeatonHo said:
If that's the case, what does the OE valve vent to? Does it empty to 0 psig, or does it maintain some pressure inside?
The reason a diverter valve (whether vented to atmosphere or recirculated) is used in turbocharged systems is to avoid the turbo trying to press air into an engine that has the throttle plate closed (and causing compressor surge). Boost pressure is controlled by the hot/exhaust side of the turbo, not the cold/compressor side of the turbo. Air from a recirculated diverter valve is being plumbed into the cold side of the turbo (to be compressed again). Essentially, a recirculating bypass valve is the engine's attempt at being "green"...
 

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What you're looking for is a software fix, not a mechanical fix.

There is already code to allow what you want to be implemented on your vehicle. Cobb and Ecutek call this "flat-foot shifting". The mechanical solution you are trying to achieve will not work.
 
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