BPV/BOV - Recirc vs VTA (Vent to Atmosphere)

[ZyroXZ2]

Even though I've worked on cars for awhile, this is really the first time I fiddled with altering the BPV and venting to atmosphere versus recirculation, and I wanted to fill you all in on my findings because, despite what people are saying, there is a difference.

First things first: it's really easy to test this back and forth (which is why I did this). Unplug the hose from the BPV, and plug the end of the hose (you can leave it hanging, it's not going to swing around like a horse dong). Et voila, you're a BOV, now. Plug it back in, and you're recirculating like a BPV. Makes what I'm about to tell you very easy to verify, moreso if you're modded/tuned for more boost.

VTA severely drops partial throttle and transient turbo response. It's immediately noticeable at 1/3 throttle that the car doesn't shove me in my seat. It's even more noticeable that at 1/2 to 3/4 throttle, the car does not slam me in my seat, but pushes me harder and harder as boost builds up, aka "turbo lag". Full throttle is mostly unaffected, though, as the BOVs are closed during full throttle. Fortunately, I noticed 1-2 MPG higher overall, which would make sense as there's less boost at partial throttle which requires less fuel.

BPV sees all of the previous paragraph's symptoms completely reversed. However, what I'm also noticing is that this is undoubtedly harder on the turbo during partial throttle. Basically, some of the boost pressure is being "short circuited" (if you will) right back into the turbo to spin it up even faster. This is why this car has so little turbo lag: tiny turbos running recirc. Recirculating spins these tiny things up dramatically faster and explains why partial throttle is sometimes almost full acceleration even though you're only 3/4 down on the throttle. I would bet a larger turbo would also benefit, but not to this degree since physics (bigger turbo, harder to spin up).

My recommendation is to stay on recirc. If you want aftermarket, buy one that lets you switch between the two: recirc for performance; VTA for long distance drives/commuting.

 

[Angel]

Wow great write up. I have mine venting to atmosphere. Not sure I have noticed any difference from recirculated to atmosphere.

 

[Doel]

did you plug up the port in the intake where the BPV is being recirculated when testing it as a BOV?

 

[DaBears4Lyfe]

hmmm. I vent to atmosphere and notice zero difference when i switch them to recirc. If you are noticing that big of a difference with aftermarket bovs your bovs arent closing properly when boosting causing a leak. With your test that you did, im not sure what to tell ya, but there is zero difference here with the GFBs Respons kit. They hold boost very well.

 

[ZyroXZ2]

did you plug up the port in the intake where the BPV is being recirculated when testing it as a BOV?

*rolls eyes* It's like you didn't even really read the post, lol

hmmm. I vent to atmosphere and notice zero difference when i switch them to recirc. If you are noticing that big of a difference with aftermarket bovs your bovs arent closing properly when boosting causing a leak. With your test that you did, im not sure what to tell ya, but there is zero difference here with the GFBs Respons kit. They hold boost very well.

It's the stock valves I'm playing with. That, and if they didn't close properly, full throttle would have long been noticeably worse by now (low[er] boost). Though, I've read that the GFBs can be changed with a dial, which I might do so I can switch between a few more MPGs and better partial throttle performance.

EDIT: Typo.

 

[DiabloStinger]

I've run three different brands of VTA valves, as well as the stock ones both ways. 0 difference felt on my car. However, i did allow the ECU to make its adjustments before i really laid into it each time. Your test seems solid though.

 

[ZyroXZ2]

I've run three different brands of VTA valves, as well as the stock ones both ways. 0 difference felt on my car. However, i did allow the ECU to make its adjustments before i really laid into it each time. Your test seems solid though.

I drove between 30-50 miles between BPV/BOV to compare. I would hope the ECU adjusted by then, lol

 

[SKStinger]

I drove between 30-50 miles between BPV/BOV to compare. I would hope the ECU adjusted by then, lol

This sounds like the first documented evidence of the stock diverter valves leaking. It would be less noticeable with the recirculation hoses attached but if you’re noticing a major difference with them off, I’d say the valves are leaking.

Probably also why others who have aftermarket VTA valves don’t notice a difference. They aren’t leaking boost.

 

[Jays_stinger]

Even though I've worked on cars for awhile, this is really the first time I fiddled with altering the BPV and venting to atmosphere versus recirculation, and I wanted to fill you all in on my findings because, despite what people are saying, there is a difference.

First things first: it's really easy to test this back and forth (which is why I did this). Unplug the hose from the BPV, and plug the end of the hose (you can leave it hanging, it's not going to swing around like a horse dong). Et voila, you're a BOV, now. Plug it back in, and you're recirculating like a BPV. Makes what I'm about to tell you very easy to verify, moreso if you're modded/tuned for more boost.

VTA severely drops partial throttle and transient turbo response. It's immediately noticeable at 1/3 throttle that the car doesn't shove me in my seat. It's even more noticeable that at 1/2 to 3/4 throttle, the car does not slam me in my seat, but pushes me harder and harder as boost builds up, aka "turbo lag". Full throttle is mostly unaffected, though, as the BOVs are closed during full throttle. Fortunately, I noticed 1-2 MPG higher overall, which would make sense as there's less boost at partial throttle which requires less fuel.

BPV sees all of the previous paragraph's symptoms completely reversed. However, what I'm also noticing is that this is undoubtedly harder on the turbo during partial throttle. Basically, some of the boost pressure is being "short circuited" (if you will) right back into the turbo to spin it up even faster. This is why this car has so little turbo lag: tiny turbos running recirc. Recirculating spins these tiny things up dramatically faster and explains why partial throttle is sometimes almost full acceleration even though you're only 3/4 down on the throttle. I would bet a larger turbo would also benefit, but not to this degree since physics (bigger turbo, harder to spin up).

My recommendation is to stay on recirc. If you want aftermarket, buy one that lets you switch between the two: recirc for performance; VTA for long distance drives/commuting.

I did this also ahwile ago. i would feel the car respond better with Recirc. than when VTA...

 

[ZyroXZ2]

This sounds like the first documented evidence of the stock diverter valves leaking. It would be less noticeable with the recirculation hoses attached but if you’re noticing a major difference with them off, I’d say the valves are leaking.

Probably also why others who have aftermarket VTA valves don’t notice a difference. They aren’t leaking boost.

While I'm not ruling out leakage since I haven't tested it (and also HIGHLY doubt that's the case since power at full throttle would be noticeable less and boost would read lower), it seems not everyone is keen on feeling the difference. Just based off physics alone there is a difference: recirc releases boost back into the intake tube right into the turbo. Anyone telling you there's "zero" difference just can't feel it: the behavior of air flow alone adjusts how the turbo is going to behave. I still make the same power at full throttle (as mentioned), it's only at partial throttle where the valve difference is really felt. However, an aftermarket one may actually be behaving differently than the stock ones under these loads/boost levels than the stock one. That could be for better or worse, though... but perhaps you feel "zero" difference because at the same throttle levels, the aftermarket one is completely closed off, meaning recirc and VTA are the same thing in that state.

 

[DaBears4Lyfe]

While I'm not ruling out leakage since I haven't tested it (and also HIGHLY doubt that's the case since power at full throttle would be noticeable less and boost would read lower), it seems not everyone is keen on feeling the difference. Just based off physics alone there is a difference: recirc releases boost back into the intake tube right into the turbo. Anyone telling you there's "zero" difference just can't feel it: the behavior of air flow alone adjusts how the turbo is going to behave. I still make the same power at full throttle (as mentioned), it's only at partial throttle where the valve difference is really felt. However, an aftermarket one may actually be behaving differently than the stock ones under these loads/boost levels than the stock one. That could be for better or worse, though... but perhaps you feel "zero" difference because at the same throttle levels, the aftermarket one is completely closed off, meaning recirc and VTA are the same thing in that state.

But you only vent when you let off the throttle not while building boost. If you are not holding boost when you you are on the pedal and building boost then there is an issue something is broken or not installed properly.

 

[Thwack]

I've gone VTA and felt no difference. I also logged boost levels before and after using OBD2 logger, saw no difference (in peak, i did not study partial throttle).

Plus, what you're describing doesnt make sense. The BPV's are controlled by the throttle body vacuum line, where the BPV feeds to makes no difference in wether the valve is closed or not. Also, when you vent the charged air to the low pressure airbox (or cone filter) if it had enough oomph to actually spool the turbos (it doesnt after all the volume expansion) it would still have much less resistance to just go back out the filter.

If you prefer recirculating,it feel better to you, cool go with it. But you may want to log your boost levels, you may have discovered a leak.

 

[ZyroXZ2]

I've gone VTA and felt no difference. I also logged boost levels before and after using OBD2 logger, saw no difference (in peak, i did not study partial throttle).

Plus, what you're describing doesnt make sense. The BPV's are controlled by the throttle body vacuum line, where the BPV feeds to makes no difference in wether the valve is closed or not. Also, when you vent the charged air to the low pressure airbox (or cone filter) if it had enough oomph to actually spool the turbos (it doesnt after all the volume expansion) it would still have much less resistance to just go back out the filter.

If you prefer recirculating,it feel better to you, cool go with it. But you may want to log your boost levels, you may have discovered a leak.

It won't go back out the filter as your turbo is still spinning and air is still being sucked in (vacuum). In addition, this is why I emphasized partial throttle. Transient response is hard to read from peak values. In addition, if you look at the BPV lines, one of them connects just before the boost sensor, not directly to the throttle body. The other goes into the intake manifold. It's basically designed to mechanically open when there is too much of a difference between the intake manifold pressure and the intake pipe (or basically, on each side of the throttle body). When the throttle body closes, there is a massive difference in pressure in front of and behind the throttle body, and this opens the BOV to equalize. Lastly, I think you underestimate the vent of charged air: there's a reason BOVs (especially VTA) are so loud. When it expands into the intake charge, it's basically as much air being vented as needed until the pressure equalizes from the aforementioned conditions. The higher your compression (boost), the more air this actually is. Given that people are hitting 14psi, I wouldn't say that it spools up the turbo, but it charges the intake charge somewhat for sure, and at the minimum giving you a brief moment of increased static air volume for when the next gear hits and the BOV closes back up and air is sucked in like millions of atom-sized tiny children screaming for their lives.

And yes, there is a reason most people not reading the entire post keep mentioning full throttle like I didn't clarify that full throttle there was no difference... Anyway, I think everyone's jumping in to argue rather than test it/read the post. I'm still going to consider the BOVs that can be changed with the dial. That seems quite useful for this, in fact, since my MPGs went up a tad when on VTA.

 

[Angel]

It won't go back out the filter as your turbo is still spinning and air is still being sucked in (vacuum). In addition, this is why I emphasized partial throttle. Transient response is hard to read from peak values. In addition, if you look at the BPV lines, one of them connects just before the boost sensor, not directly to the throttle body. The other goes into the intake manifold. It's basically designed to mechanically open when there is too much of a difference between the intake manifold pressure and the intake pipe (or basically, on each side of the throttle body). When the throttle body closes, there is a massive difference in pressure in front of and behind the throttle body, and this opens the BOV to equalize. Lastly, I think you underestimate the vent of charged air: there's a reason BOVs (especially VTA) are so loud. When it expands into the intake charge, it's basically as much air being vented as needed until the pressure equalizes from the aforementioned conditions. The higher your compression (boost), the more air this actually is. Given that people are hitting 14psi, I wouldn't say that it spools up the turbo, but it charges the intake charge somewhat for sure, and at the minimum giving you a brief moment of increased static air volume for when the next gear hits and the BOV closes back up and air is sucked in like millions of atom-sized tiny children screaming for their lives.

And yes, there is a reason most people not reading the entire post keep mentioning full throttle like I didn't clarify that full throttle there was no difference... Anyway, I think everyone's jumping in to argue rather than test it/read the post. I'm still going to consider the BOVs that can be changed with the dial. That seems quite useful for this, in fact, since my MPGs went up a tad when on VTA.

I am not here to argue but this got real technical real fast.

Your MPG got better when venting to the atmosphere?

 

[robz32]

Been driving with the Tial for 500+ miles and I have not notice any difference. MPG did go up slightly but that's about it.

 

[Thwack]

It won't go back out the filter as your turbo is still spinning and air is still being sucked in (vacuum). In addition, this is why I emphasized partial throttle. Transient response is hard to read from peak values. In addition, if you look at the BPV lines, one of them connects just before the boost sensor, not directly to the throttle body. The other goes into the intake manifold. It's basically designed to mechanically open when there is too much of a difference between the intake manifold pressure and the intake pipe (or basically, on each side of the throttle body). When the throttle body closes, there is a massive difference in pressure in front of and behind the throttle body, and this opens the BOV to equalize. Lastly, I think you underestimate the vent of charged air: there's a reason BOVs (especially VTA) are so loud. When it expands into the intake charge, it's basically as much air being vented as needed until the pressure equalizes from the aforementioned conditions. The higher your compression (boost), the more air this actually is. Given that people are hitting 14psi, I wouldn't say that it spools up the turbo, but it charges the intake charge somewhat for sure, and at the minimum giving you a brief moment of increased static air volume for when the next gear hits and the BOV closes back up and air is sucked in like millions of atom-sized tiny children screaming for their lives.

And yes, there is a reason most people not reading the entire post keep mentioning full throttle like I didn't clarify that full throttle there was no difference... Anyway, I think everyone's jumping in to argue rather than test it/read the post. I'm still going to consider the BOVs that can be changed with the dial. That seems quite useful for this, in fact, since my MPGs went up a tad when on VTA.

Nobody is jumping on here merely to argue with you, but what you are saying makes no actual sense, regardless of how technical you want to get.

What you're describing with the spinning turbo staying spooled goes against simple physics in your own terms. If upon vent, the turbo is still spinning and sucking up air, as you said, it's doing work on the intake air and being caused to slow. If the intake air is pressurized enough to spin the turbo (ie. do work on the turbo) it's going to take the easier path out, through the filter. A decent air filter, even at WOT flowrates, causes less than 1 PSI resistance, so that's the most intake charge you'd keep upon vent, and we're not even talking WOT, so halve or quarter that, since the resistance through the filter drops exponentially with drop in airflow.

To look at it from the other end, let's say the whole intake system stays pressurized upon vent/ shift, no air vents back through the filter to atmosphere, it just boosts back through the intake. The way pressure works is that that pressure also boosts backwards from the pressurized intake box, through the open BPV and backwards into the impeller, causing surge, slowing it down and causing stress on it. Literally the reason we have a BPV is to avoid this, we can't treat the whole intake as a closed system as you describe. If it worked like you say, it would be a much less destructive anti lag system.

I've read your whole post, I tested it, even logged it (mentioned full throttle vs partial to be more precise). I'm not getting the same results as your butt dyno. Nobody is telling you to run VTA v/s recirc, that's purely preference, but I cannot imagine a plausible scenario where your significant power drop at partial throttle is caused by going VTA. If it is acting as you describe, were I you, I'd be logging the actual boost levels at different throttle position sensor, levels, comparing the 2 and figuring out if you really are losing or not building boost, then figuring why.

 

[DiabloStinger]

^^^^
Log vs but dyno always wins in my book. I will agree that there is no discernible difference on either stinger I've owned pertaining to the valves used or how the valve vents. Now, if someone was to show me side by side logs with partial throttle load in each scenario at exactly the same speed and in the same gear, then we would know for sure.

I still believe the OP felt a difference but perhaps the cause was more foot and less valve related.

 

[ZyroXZ2]

*rolls eyes*

I give up. My butt dyno is pretty damned sensitive. I guarantee I'm not the only one feeling it. Of course, yes, it would be a pain but possible to dyno test partial throttle and graph the rate at which boost builds. If it's slower when VTA, it would prove my feel by data.

Sadly, I have no interest in actually spending money/time proving it, but I know what I felt. And no, my foot is not that inconsistent, thanks to drag racing

 

[Thwack]

In case you change your mind, no need for a dyno, you can use a $10 bluetooth obd2 scanner and torque app on phone, you can log the cars boost sensor and TPS on some partial throttle runs to make sure it's good. One of those scanners is just a general good tool to have nowdays if you don't already.

 

[gilthanis1]

One more here that has now changed the BPVs into BOVs by removing the recirc hoses. Took about 30 min, in between taking care of the dog and answering a couple phone calls.

Just took the recirc hoses off and capped the ports on the intake tubes with the 1 1/8" chair tips and hose clamps. It's not that loud at all, in my opinion. No difference in sound driving in Comfort like I normally do on my 30+ mile commute every morning. Definitely no difference in throttle response at all. Still as snappy as ever and picks up when I need it too.

Turn the dial to Sport, run up to about 4k and let off the gas and theres a nice discernible PSSHHH from under the hood, which is nice to hear. It's definitely not earth-shattering, ear-shattering, or anything else shattering....it's just there for the noise. DEFINITELY not as loud as some of the YouTube vids I've seen on removing the recirc hoses, and nowhere near the sound of a proper BOV, but those are coming.

I like it and plan on leaving it just for the noise. I like to hear sounds from my cars, that they are supposed to make, when I drive. It's a quick and easy change to give it the proper turbo sounds it needs, nothing more. Take it for what you will, that's just my 2 pennies.