BOV's wank or not? - serious question

[Bill Surewood]

Ok I am getting to the stage where i am making all my intercooler piping for my project.

I'm looking to run 18-20 psi through a GT3040 turbo (rated at 600HP) into a GTiR SR20, will i need a BOV?

I am no longer running quad throttles, a single 70mm unit has gone on.

I have a older styple Blitz but i would rather just go with none.

Thanks in advance

[tandy ass]

It will help, recirculate the vented air AFTER the intercooler, as close to the throttle body as possible, back to the intake of the turbo.

[Bill Surewood]

Thanks for that bozz, that seems to be the go if i do run one.

I just dont like the idea of them, I have heard of many a story of BOV's getting stuck slightly open and the like (it's even happened to me).

I'm more concerened with damage to my 3k+ turbocharger if i do decide to delete the BOV

[Bill Surewood]

As far as i can see an RS200 does not run a BOV and runs the boost i wish to run from factory with a similar sized turbo, or am i mistaken?

http://www.autocollections.com/image/cars/1986%20ford%20rs200%20r%20eng.jpg

http://www.blackhawkcollection.com/image/cars/1986%20fords%20rs200%20evo%20eng.jpg

[Zac]

I thought the BOV only really came to the fore in the Ford Sierra touring car days? The RS200 is before those. That said, I have video of a Porsche 956 sports car that has a BOV!

[Forg]

Don't pretty much all factory turbo cars have a BOV, except for the really old ones (eg. like an RS200)?

[Bill Surewood]

more for noise i believe forg, to get rid of the flutter. I'm sure i read that in the factory 200SX workshop manual

[Stix Zadinia]

alot of rally cars (WRC, RS2000 etc) don't have BOV's because they can afford backpressure onto the compressor wheel, and being able to rebuild their turbos/engines etc every so often.

according to my extremely limited research, power is actually lost through a BOV by not having air pressure when you reapply your foot to the throttle..

so i guess it comes down to turbo longevity vs power. there are a few adjustable BOV's where you can vary the amount that is released vs recirculated and they may be the way to go to get more performance. i know in VW/Audi Turbo setups if you modify them by fitting a BOV it interferes with the ECU etc, because it needs a specific amount of recirculated air to measure fuel mixtures (or something to that effect)

i would probably recommend an adjustable GFB branded one, all the US guys with 1.8T GTIs recommend them, and seem to have very good results (good for an aussie product!)

hope i'm not sounding out things that everyone already knows...

[GTSBoy]

Welllllllllllll,

The problem is that there are three reasons for anyone (including an OEM) to fit a BOV.

1) Turbo damage (ie spinning the nut off the comp end of the shaft, and possibly the thrust forces from rapid changes in direction of thrust on the shaft.)

2) As a compressor bypass. This in itself has two subreasons....a) to prevent the turbo from slowing when the throttle slams shut, and b) to allow induction air to go around the compressor when driving the car off boost.

3) Noise concerns.

The problem is that some of these are legitimate reasons, under some circumstances, and others are reasons that general public people like you and me have come up with at some stage in the past, that have somehow managed to become thought of as the "real" reason for having a BOV.

My 2c coming up.

It would seem that a lot of knowledgable people have declared reason 1 to be a furphy. The number of instances of turbos coming apart in really high boost racing applications would have been too small to justify the development of BOVs for that purpose, especially considering that the types of race cars using turbos at the time would have had maintenance schedules from hell, including very regular turbo tear downs. For OEM applications, there were very very few cars (if any at all) that ran anywhere near enough boost to worry about turbo damage from full bottle gear changes. So it can hardly be the trigger for the development effort either.

Reason number 3 might be a concern, except that when BOVs were devloped, the noise of compressor stall would hardly have needed to be dealt with in order to make the car comply with noise limits, because it was so long ago, when nobody had limits that tight in place. It might be one reason for fitting a BOV to a modern car like the S15, but really, a BOV makes a lot of noise. If you take a typical recirculating BOV equipped car (say a Skyline like mine) and fit a pod filter to it, the BOV noise is really very loud, yet you cannot hear it at all with the factory airbox in place. The same is true if you deactivate the BOV. With teh factory airbox in place, you hear no compressor stall worth talking about, but with a pod, the chuffing is very loud. So an OEM would just design a slightly better sound damping inlet, rather than go to the effort of fitting a BOV and it's plumbing in order to control some noise.

Reason number 2 is where the interesting stuff lies. It is certainly true the some car/engines tested show faster accel times with a BOV, which must be from keeping the turbo spinning fast. Others tested definitely are quicker with no BOV....so maybe there are differences between specific engines and the rest of their support systems, or even just in the relative sizing of the turbo to engine matching, that means on the one hand you can get better performance with and on the other without.

It is certainly true that some testwork done with RB20s shows that when the factory BOV is de-activated, so that there is no BOV working at all, that the respone from off-boost to on-boost is slower than with a functioning recirculating BOV. The reason for this is pretty simple. When the car is off boost, the BOV is open, and the restriction in the flow path through the BOV is less than the restriction through the turbo's compressor. So the engine can breathe a smidge more freely through the BOV, and hence ingest a bit of extra air, make a bit of extra power before it is on boost, making a bit of extra exhaust gas to spool the turbo up a little bit earlier. And really, getting the turbo going even a small fraction of a second earlier is worth a great deal. As a compressor bypass, a recirculating BOV is certainly a good idea.

So I agree with BOzz.

cheers

[tandy ass]

Just to add to GTSBoy's comments, there are a few other benefits.

A thread I asked about a while ago got me testing it and verified the results.

The compressor side of the turbo squeezes the air, whenever you squeeze air, it gets hotter as it is more dense. This is the exact method how the hot side of air conditioners work. The purpose of the intercooler is to remove the heat from the compressed charge going into the motor. Now we have a compressed, cool charge. You snap the throttle shut and this cool compressed charge of air gets vented back to the intake, expanding to atmospheric pressure, absorbing heat and entering the turbo at BELOW intake temperature, sometimes considerably below atmospheric temperature, following the same principle of how the cold side of an air conditioner works.

I feel the biggest benefit of a recirc BOV is eliminating the flutter on an engine that uses a closed loop air mass meterering system. RB20/25 people can easily reproduce this - unplug your factory BOV line from the plenum and block the hose with a bolt or screw, open your airbox/pod and put your hand near the intake of the AFM as you open the throttle, give it a 5-6 thousand RPM rev and snap the throttle shut. Feel the air going backwards then forwards then backwards and so forth? Now hear the engine almost stall? The AFM cannot distinguish whether the air is going forwards or backwards, it simply meters air going past it. So all of this air going in and out is being registered as air going into the motor, the ECU is squirting fuel into the motor for the air it believes is being consumed by the motor. Hence it running extremely rich, killing fuel economy and carboning up your motor. The same thing happens if you're mentally retarded and fit a venting BOV to these types of motors.

I believe the biggest reason OEM's fit a recirculating BOV to a force fed motor equipped with an AFM would be emissions.

[Nero]

emissions and reliability are huge reasons why OEM consider such things

[Bill Surewood]

Thanks guys so far, so it appears emissions play a huge part in OEM fittment of BOV's

Has anyone got any hard evedence of lack of BOV turbo falure, due to back spin etc.

Quite curious now

[Jim]

I agree with GTSboy, Bozz and Nero :p

I have seen a few damaged turbos that were shown to me back in my early turbo days when I asked Mitsu uber-guru Ron Masing the same question, he showed me one off his starion group C car where the compressor nut had come off, and another with missing and mangled fins. He said both these occured when he did not run a BOV on the starion, and once fitted the problem never reaccured. I'm a firm believer in them from my own mucking arounds with the Fridge of Fury, HOWEVER any loud BOV will be making it's wooshy noise by creating a restriction, and restrictions are bad when you are trying to move a large mass of air quickly. A quiet BOV most likely works better than a noisy one.

Recirculating BOVs are a must for an air flow metered car.

You can probably get away with not using a BOV on an auto car because you generally dont snap the throttle shut on gear changes.

[ox!gen]

It's not so important if the bov is restrictive or otherwise, as long as it flows wnough to stop compressor surge. most bovs will show a pressure spike (great autospeed article on this if i remember rightly) just as large as without a bov, but because they vent, they don't surge, and with no surge, there's no rapidly chaging load on the bearings. wether or not that rapidly changing load is enough to war out the turbo any quicker isn't proven, but the pressure spike that people would have you belive breaks fins is no smaller with or without a bov.

[Zac]

Oxygen kinda mixed up some terminology there, but basically you are talking about rapid changes of shaft rpm?

[GTSBoy]

Oxygen is, yes.

Because the pressure rises to about the same value, we go up to the same level on the compressor map. But because the BOV allows the compressor to keep some airflow, we don't go as far to the left of the comp map, hopefully staying to the right of the surge line, or at least not going so far to the left that we actually go past the y axis (which is the negative flow that Bozz mentions.

cheers

[Zac]

Yeah that's what I meant, I just wanted to decipher his post a bit :)

[ox!gen]

Actually, I was talking about rapidly changing thrust on the thrust bearings in the turbo. having no bov actually doesn't slow the compressor as much as having none, since it aerodynamically stalls, not rotationally stalls. If it aerodynamically stalls, it's doing no work, so it slows less with a wall in front of it.

what I was saying, is that the surging caused when it flutters creates rapidly changing thrust loads on the bearings in the turbo, which, while being smaller than the initial pressure spike, are supposedly really bad fo bearings compared to constant large loads.

In my car I run no bov because it stalls the turbo less on liftoff (I really carefully looked into it and there's a LOT more boost available on my car after a change without a bov than with, although it seems to very engine to engine.) I do occasionally worry about the damage the flutter causes to the bearings in the turbo, but there's no proof that that amount of wear is significant to the life of the turbo, and even less reputable evidence that Its going to break blades. I'd think it would be the initial large pressure spike that supposedly breaks blades, which a bov does NOTHING to reduce. that only leaves the bearings, which, I'm not convinced are that hard done by by surge.

so far it hasn't blown up...

[dsmith]

So with an AFM equiped car, why is it essential to have a recirculating BOV and NOT an atmo venting one?

Would I be correct in saying that with an atmo bov the car will run bulk rich on gear changes because the ecu is expecting a whole heap of air to be there when it has infact been vented out of the system so it adds a heap of fuel that it doesn't need?

Besides running rich, is this a problem?

Does running rich in itself pose a problem?

[Maadmike]

So with an AFM equiped car, why is it essential to have a recirculating BOV and NOT an atmo venting one?

Would I be correct in saying that with an atmo bov the car will run bulk rich on gear changes because the ecu is expecting a whole heap of air to be there when it has infact been vented out of the system so it adds a heap of fuel that it doesn't need?

Besides running rich, is this a problem?

Does running rich in itself pose a problem?

The air goes through the AFM, so it meters for all that all, and the ECU tells the injectors to put the corresponding amount of fuel in. When you vent to ATMO the ECU is telling the injectors to put fuel in for air that just went 'psssht' out of the BOV, instead of into the engine. Hence the reason you see the black cloud of smoke changing gears, its because there is way too much fuel being put into the engine for the amount of air that is actually making it into the engine and it is running rich.

[Datto-Zed]

Darren: On some engines (eg RB20) it causes some big drivability problems. When Danny's R32 had the atmo valve, it'd stall easily after giving it a hit, and the biggest problem was when it would vent as you modulated the throttle around a corner. Everytime it would vent and consequently run mega ritch, it was as good as cutting all power. As you can imagine the power going on-off-on-off-on around a corner is pretty unsettling! Took him all of a week to get pissed off with it and order a plumb back valve.

[dsmith]

Im getting over the "ppsshtt" thing now, all it does is give my cover (who would thing a Laser is turbo?) away to commodore drivers. :lol:

[Tripharn]

Im getting over the "ppsshtt" thing now, all it does is give my cover (who would thing a Laser is turbo?) away to commodore drivers. :lol:

Yea, I have the same problem.. plus the intercooler hanging out the front :)

So, what was the final decision? To BOV or not to BOV?

Also, is this a decision to go completely without or with a venting BOV?
Or are you considering a recirculating BOV (by pass valve.. whatever) as well?

[RB30-POWER]

Some good arguments for and against here

http://ausrotary.dntinternet.com/forums/viewtopic.php?t=17113&postdays=0&postorder=asc&highlight=bov&start=0

[niggseh]

Putting AFM cars aside.....
What if you were to have a "TUNED" BOV, where it vented enough air to stop the supposed turbo damage and yet still held enough air for the backpressure needed to spool quicker during gear shifts? (I assume ppl lose turbo response due to long IC piping and large FMIC. So pressure needs to stay in for the turbo to maintain response.)

Having run a ATMO BOV then NO BOV.......I could hardly tell the difference. Actually, I think it was a tad slower running no BOV. (Using the argument, air comming back from the throttle body slows down the compressor blades)
My setup was a T28BB turbo with short IC piping and a relativly small FMIC. I think this elminated the need for pressure build up required for quicker spool times in between shifts. And being a Ball Bearing turbo.....it had no trouble spooling at low RPMs anyway.
I've seen some new turbo's with gaps in between the compressor blades and housing (picture 2 circles, one on top of the other. First one smaller than the second one.), I was told it's to reduce or eliminate compressor surge*. So the air passing back through the turbo can flow freely and not OVER the blades causing damage and/or lag.

*I hope I'm using this term in the right context

So to answer the question, NO BOV or BOV. I think it all depends on the application it's applied to. Taking into consideration amounts of boost, turbo design/componets and intercooler setup.
I got my BOV to only open when above 7-8psi and more than 3,500rpm. Anything under and it would flutter, sometimes you could hear it flutter through the BOV too, meaning only small amounts of air being vented...........I think :s

above claims are from my limited knowledge and experience, feel free to correct me
P.S Here's me comming off the gas @ 2,000rpm in 1st.
http://maxe.ausgamers.com/images/wachacha.wav
Thorpy says Flutter is fully sik yo

[niggseh]

forgot to add, I was running 1.2bar through that tiny snail. :(
Top end............what top end?

[kamikaze]

i have atmo bov on a sr20det fwd . it very rarely stalls but it does some times.
also i have tightened the bov a little which reduced stalling and also acts a normal bov

but i think it is causing my car to back fire every now and then.

[The Pupat]

Putting AFM cars aside.....
What if you were to have a "TUNED" BOV, where it vented enough air to stop the supposed turbo damage and yet still held enough air for the backpressure needed to spool quicker during gear shifts? (I assume ppl lose turbo response due to long IC piping and large FMIC. So pressure needs to stay in for the turbo to maintain response.)

Having run a ATMO BOV then NO BOV.......I could hardly tell the difference. Actually, I think it was a tad slower running no BOV. (Using the argument, air comming back from the throttle body slows down the compressor blades)
My setup was a T28BB turbo with short IC piping and a relativly small FMIC. I think this elminated the need for pressure build up required for quicker spool times in between shifts. And being a Ball Bearing turbo.....it had no trouble spooling at low RPMs anyway.
I've seen some new turbo's with gaps in between the compressor blades and housing (picture 2 circles, one on top of the other. First one smaller than the second one.), I was told it's to reduce or eliminate compressor surge*. So the air passing back through the turbo can flow freely and not OVER the blades causing damage and/or lag.

*I hope I'm using this term in the right context

So to answer the question, NO BOV or BOV. I think it all depends on the application it's applied to. Taking into consideration amounts of boost, turbo design/componets and intercooler setup.
I got my BOV to only open when above 7-8psi and more than 3,500rpm. Anything under and it would flutter, sometimes you could hear it flutter through the BOV too, meaning only small amounts of air being vented...........I think :s

above claims are from my limited knowledge and experience, feel free to correct me
P.S Here's me comming off the gas @ 2,000rpm in 1st.
http://maxe.ausgamers.com/images/wachacha.wav

Compressor surge is caused by pressure waves in the air rather than chop from the blades. the compressed air bounces from the throttle body and the turbo due to the restriction in flow that they cause to the system.

[Big George]

Thanks guys so far, so it appears emissions play a huge part in OEM fittment of BOV's

Has anyone got any hard evedence of lack of BOV turbo falure, due to back spin etc.

Quite curious now

I have minced 2x turbos on 2 different engines before I "believed in" BOV's.
The first one was a T2 oil cooled on a E15et which ran 14psi and with the pop-off valve removed from the plenum and plugged up. The turbo shaft split in two and started burning a lot of oil.

The second one was a VG30 turbo on a SR20det, this turbo has a ceramic exhaust wheel. The engine did not have a BOV and I changed from 1st to 2nd gear on 10psi, the compressor stalled (as it does on every gear change) and the exhaust wheel sheared off and visited the cat.

See attached pictures.

[RB30-POWER]

The pop off valve is only to protect the motor if you raise boost, its designed to leak the excess pressure, as to avoid engine damage.

It won't have anything to do with gearchanges.

As for the VG30 turbo, its a ceramic turbo wheel and these are known to fail with raised boost and age. They fatigue really easy.

I don't think a BOV would help either of these cases.

I think the turbo's just had to much age and were being run over factory boost pressure, which caused the failures. One thing we can say, we will never ever know what actually contributed to the failure for sure :)

[Big George]

The old fatigued turbos are more likely to break when they are stressed, so that simply proves the point that any kind of stress on the turbo *can* break them.

Anyhoo, if you asked me a few years ago i'd swear BOVs are for wank purposes, but they are a safety device originally designed to protect components. Some people decided they liked the sound so the concept seemed to run away and get out of hand. Now you can get supersonic and a whole variety of noisemakers....

I'd prefer the benefits of the safety device, plumbed into an external sound deadener device like a BOV muffler so it doesn't make that annoying sound all the time.