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Toymods member
I supported Toymods
Location:
Rydalmere, NSW
Registered:
May 2002
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 BOV's & Wastegates
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Sat, 01 June 2002 13:22
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Hi people...
At the risk of looking completely dumb (as I know next to nothing about the subject of turbo applications),  Can someone (or ALL of you if you like) please explain the functions and use of Blow-Off Valves or BOV's and Wastegates, and how they work on a smallish-sized turbo like a T-28 Garrett.
Thanks.
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Location:
North Brisbane
Registered:
May 2002
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Re: BOV's & Wastegates
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Sat, 01 June 2002 14:01
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boost is created by by exhaust gas leaving the engine and turning a turbine (rear one), which is connected to a compressor (front turbine) this compressor sucks air and compresses it (funnily enough), to stop the turbo making as much boost as it possibly can (fun but not good) you must detour some exhaust gas around the rear turbine, this is what a wastegate does, it opens at a preset boost level and prevents the turbo from making anymore boost....
when you close the throttle quickly after boosting it up (gear changes) the turbo is still making boost (engine still outputing exhaust, so it's still spining) so all this pressure is put on the throttle body, and the turbo itself, this pressure causes premature ware of the turbo, (plus venting it reduces lag after gear shifts)...
so yeah, in any descent turbo car, both of these things are a very good idea...
anyone wish to correct me feel free, i'm still learning just like everyone else...
c-ya,
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Location:
sydney
Registered:
May 2002
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Re: BOV's & Wastegates
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Sat, 01 June 2002 16:49
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that pretty much sums it up..
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Toymods member
I supported Toymods
Location:
Rydalmere, NSW
Registered:
May 2002
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Re: BOV's & Wastegates
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Sat, 01 June 2002 17:32
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Ok, so can one be used with the other, or does one do the other's job?
This is the sticky point with understanding these two...
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Location:
Brisbane
Registered:
May 2002
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Re: BOV's & Wastegates
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Sun, 02 June 2002 00:36
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If you want to keep your motor in one piece you NEED a wastegate.
The wastegate is used so that the turbo doesn't keep boosting and boosting; as rev's get higher there is more exhaust so the turbo would spin faster. The wastegate acts to send the excess exhaust away from the turbo giving the turbo only enough exhaust to make 'X' boost.
The BOV on the other hand takes car of what happens during gear changes ect, when you change gears you take your foot off the throttle giving the engine less fuel so it slows down a bit, without a BOV the air coming from the turbo would be rammed into the engine (which is trying to slow down...) and slam back into the turbo (ouch!), the BOV vents the pressure created by the turbo to stop this.
If what your asking is "Can I only use a BOV and make it act as a wastegate as well?" I'm not sure, I would like to know myself!......sorry(just a guess but i'd say it's not efficient?)!
Cheers
Wilbo
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Toymods member
I supported Toymods
Location:
Rydalmere, NSW
Registered:
May 2002
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Re: BOV's & Wastegates
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Sun, 02 June 2002 04:11
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Thanks Wilbo, that answered my question - although it would be fascinating to see a BOV taking care of a Wastegate's job as well... 
Ok, so what size wastegate for a T-28 Garrett? 
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Location:
Brisbane
Registered:
May 2002
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Re: BOV's & Wastegates
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Sun, 02 June 2002 05:01
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Probably best to get in contact with YOGI (he has a 4A-GTE with a GT-30) he will know from his experiences what will work best.
Cheers
Wilbo
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I supported Toymods
Location:
Perth WA
Registered:
May 2002
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Re: BOV's & Wastegates
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Sun, 02 June 2002 06:56
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You probably wouldn't want a BOV to control boost, for a few reasons:
1) The turbine would be spinning wildly to create boost that is just vented to the air. This would mean premature bearing and turbine wear.
2) The charge air would be compressed and heated by the turbo, before the excess is vented by the BOV. The remaining charge air is low pressure, and hot - detonation time.
3) Possibly higher exhaust back pressure by putting all the exhaust through the turbo, rather than diverting it down a fat pipe to the exhaust. This would also drive engine/turbo/exhaust valve heat up.
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Location:
Arthurs Creek, Victoria
Registered:
May 2002
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Re: BOV's & Wastegates
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Sun, 02 June 2002 12:24
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Ignoring the BOV side for a minute, if your using an originally OE t28 turbo, you'll probably find that it has an inbuilt wastegate, identified by a little flap on the outbound side of the exhaust housing and a brasy coloured cylinder mounted on the side of the turbo with an arm going out of the centre of it.
If you have one of these, you don't need an external wastegate, however, that said, you can up the boost on an internal wastegate, by using what is called a t-peice boost controller, which fakes the amount of boost the internal wastegate is seeing, there by causing it to open at a different level from normal boost.
Hope some of this makes little sense.
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Toymods member
I supported Toymods
Location:
Rydalmere, NSW
Registered:
May 2002
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Re: BOV's & Wastegates
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Sun, 02 June 2002 13:01
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Yeah, that's all good, but what if I'm running on stock internals, and the max boost I can use is only about 9-10PSI? It'll be going on a stock 3S-FE engine (yes, the RAV4), and I just need to know some of the basics of what these T-28's can do, and what I have to limit it to do (due to the stock engine).
AVO will be looking after the turbo aspect of the project - I just wanted to know what they'll do (the wastegate & BOV) and how they'll do it. 
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Location:
Arthurs Creek, Victoria
Registered:
May 2002
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Re: BOV's & Wastegates
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Mon, 03 June 2002 00:33
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Ok, well if your only running about 9 psi, which should be right given the internally stock motor, then they will use the internal wastegate. Therefore there will be no external wastegate sitting on your exhaust manifold.
As for Blow off valves, I'm sure unless you request it, AVO will put on a plumb-back BOV, because they want to avoid confusing the computer and want to make the car as street legal as possible, whereby not using an atmospheric BOV which creates too much unwanted noise and can break EPA rules for some reason.
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Toymods member
I supported Toymods
Location:
Rydalmere, NSW
Registered:
May 2002
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Re: BOV's & Wastegates
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Mon, 03 June 2002 04:34
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Yeah I think it's the EPA rule that governs "noise pollution", and trucks and their exhaust brakes fall under the same category.
Only problem with that is in some places (where a truck route and a residential area meet, for instance), there will be signs out saying "Please Minimise Compression (or exhaust) Braking"; there isn't anything that says "atmospheric BOV's are an EPA offence (or illegal?)"  Personally I don't know why someone would want to ban atmospheric BOV's (or limit their use) - I think they sound great.  For safety reasons you can't immobilise a truck's compression exhaust, nor would you want to as it's the only thing that gets them to the bottom of hills safely (in low gear, without foot-brakes). So in this instance you can't take the exhaust brake off the truck, but you can take the BOV off the turbo; and for the same reasons (well, getting the most out of your investment) you wouldn't want to take it off.
Alright, so what does a plumb-back BOV sound like in comparison to an atmospheric BOV? 
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Location:
Arthurs Creek, Victoria
Registered:
May 2002
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Re: BOV's & Wastegates
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Mon, 03 June 2002 05:57
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a plumback you shouldn't hear anything at all, if anything you will hear a little woosh, a tiny one, which is just the noise from the air going from the BOV through the pipe and back into the turbo compressor inlet. The idea of a plumback BOV is to recycle the air that has already gone through the compressor, and put it back into the compressor, and make no noise in the process.
I think it's also the fact that the air has gone through the intercooler that the EPA doens't like when using an atmo BOV.
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Location:
Adelaide
Registered:
May 2002
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Re: BOV's & Wastegates
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Mon, 03 June 2002 06:33
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Turbosmart have released a semi-plumb back BOV, (dont know the official name) that vents a small amount to the atmosphere and plumbs back most charged air. This gives the sound (obviously not as much as a full atmospherer BOV) without stuffing up with the AFM.
However doesnt your RAV4 have a map sensor? If it does a plumb back is not nessessary as it reads pressure directly from the manifold and not before the turbo.
Just out of interest I did once see a sigma GSR without a wastegate and a BOV controling boost pressure. Not too sure, but i think the turbo only lasted a few months (maybe a tad longer) because shaft speed was astronomical. I suspect it was an expensive turbo too as usually only aftermarket pricey ones come without an internal wastegate... 
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Toymods member
I supported Toymods
Location:
Rydalmere, NSW
Registered:
May 2002
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Re: BOV's & Wastegates
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Mon, 03 June 2002 11:37
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Ok I'm a bit lost right about here... an AFM is what exactly? (not being a mechanic).... And a MAP sensor??
I got not idea about these, and I'm here to learn.
Just on the Sigma GSR's, I believe most of them ran either T03's or T04's... I'm not 100% about it, but it rings bells. I know a few of them that run T3/4 hybrids. 
I might have a look on the Turbosmart website about these semi-plumb-back BOV's. 
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Location:
Brisbane
Registered:
May 2002
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Re: BOV's & Wastegates
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Mon, 03 June 2002 11:53
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RE: AFM's and MAP
These are the two ways in which your car works out how much air your motor is getting to find out how much fuel it needs to give to achive the correct air/fuel mixture!
An AFM is an "Air Flow Meter", it uses a flap to work out the volume of air the motor is reciving (They look like a black/silver box with a filter on one side and the other connected to the inlet manifold).
(venting air through a BOV may effect the AFM readings and this could confuse the computer).
MAP senses the "Manifold Absolute Pressure" to work out how much air the motor is getting, the higher the negitive pressure the more air the engine is getting (sucking in), hence the more fuel it should be given to achive the correct mixture.
I pretty sure you have a MAP engine so you don't really need to worry about the venting BOV thing....I think? (not the real turbo guru!)
Cheers
Wilbo
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Location:
Arthurs Creek, Victoria
Registered:
May 2002
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Re: BOV's & Wastegates
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Mon, 03 June 2002 11:56
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It is absolutely correct about the hybrid BOV. It does as Robertox above said.
As for the GSR, don't worry about the idea of having a BOV controlling the boost rather than a Wastegate. it will cause you grief.
An AFM is an Air Flow Meter. It sits before the turbo and monitors the amount of air going into the motor and relays this back to the cars computer. Another way of monitoring the amount of air going into the engine is by using a Map sensor (I belive it stands for Manifold Absolute Pressure? Although I might be wrong about the A part) and that as it says, monitors the pressure in the manifold. From this the cars computer can work out how much fuel to inject and stuff like that. Your car can use either of these, or both, but it is very unlikely that a car uses both I think. I don't have any idea what your engine would use, but if you don't have an AFM then it's not really an issue not having a plumback BOV.
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Toymods member
I supported Toymods
Location:
Rydalmere, NSW
Registered:
May 2002
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Re: BOV's & Wastegates
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Mon, 03 June 2002 12:55
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My 3S-FE engine setup has a big black box mounted on the right of the engine bay (as you look at it, actually the left side of the car), in the bottom of which is the air filter, and the top has a plug with a thick black wire extruding from it (¼" thick), which I'm guessing is the AFM. This box is linked up (via air inlet path) to the silver inlet manifold. I don't know where the black lead goes to. Goddamn, I'm so used to RWD engine config's that I'm getting lost with East/West wngines...
Tell you what - I'll be at this meet on the 8th of June. Whoever is replying to me and will be at this meet can show me around the engine on the day. Deal?
From there I know what will be going on and where, and the function of every part before AVO gets their grubby little nubbies on the RAV.
So, can I have a show of hands please? 
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Location:
Adelaide
Registered:
May 2002
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Re: BOV's & Wastegates
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Tue, 04 June 2002 03:37
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What you described sounds exactly like an AFM... so plumb-back it is. Altough you can have a atmospheric but because the engine reads air going in (and then out the BOV which the computer is assuming goes into the engine) when you close the throtle you will blow a lot of smoke because the computer will run it too rich... Also at night it will probably blow flames too when you snap the throttle closed which can sorta look cool but will most likley fry your cat, and muffler in time and get you majorly defected and make the engine lumpy and all sorts of problems that arent really worth the wank factor of having a fire breathing RAV4...
Oh yeah, the A in MAP does indeed stand for 'absolute'
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Location:
Arthurs Creek, Victoria
Registered:
May 2002
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Re: BOV's & Wastegates
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Tue, 04 June 2002 03:46
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What meet on the 8th is this? and I'm assuming given the fact that you are talking about AVO all the time that you are in melbourne?
I'm in melborune to, but I doubt I would be able to make the 8th meet anyway.
But what you said sounds right, it is the AFM at the front of the motor.
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Toymods member
I supported Toymods
Location:
Rydalmere, NSW
Registered:
May 2002
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Re: BOV's & Wastegates
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Tue, 04 June 2002 07:24
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No I'm not in Melbourne, I'm up here in Ballina, but I'll be in Sydney on this weekend, where you guys (coincidentally) will be having a "Weight Watchers" cruise at the car park of the Bull 'n' Bush - it's in the Cruises & Events section anyway. Will anyone that's posted on this thread be going?
Let me know if you are. I'll be there in the early afternoon.
Cheers.
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Toymods Board Member
I supported Toymods
Location:
Turramurra, Sydney.
Registered:
May 2002
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Re: BOV's & Wastegates
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Tue, 04 June 2002 07:48
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| Quote: |
MAP senses the "Manifold Absolute Pressure" to work out how much air the motor is getting, the higher the negitive pressure the more air the engine is getting (sucking in), hence the more fuel it should be given to achive the correct mixture.
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Actually they work the other way round, which makes sense when you think about it.
When an engine is idling the engine is sucking & using air, but because the throttle is closed, that's when vacuum is highest. As the throttle opens, the pressure inside the plenum (on the engine side of the throttle) slowly equalises with the outside air pressure (atmosphere) until at wide open throttle the manifolds absolute pressure is zero, zero vacuum and zero boost. Unless you have a turbo engine, where the reading goes from vacuum at idle to pressurised at wide open throttle (WOT).
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Location:
Adelaide
Registered:
May 2002
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Re: BOV's & Wastegates
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Wed, 05 June 2002 02:47
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Just a question, I dont actually know this for sure, only a thought...anyway, Because the air velocity through the plenum is very high, wouldn't the pressure be lower still than the outside pressure even when the throttle is wide open? (you know, fluid dynamics and all that sort of thing? high velocity=low pressure) so It would, in effect not be atmospheric but less?
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Toymods member
I supported Toymods
Location:
Rydalmere, NSW
Registered:
May 2002
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Re: BOV's & Wastegates
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Wed, 12 June 2002 14:25
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| RobertoX wrote on Mon, 03 June 2002 4:33 PM |
Turbosmart have released a semi-plumb back BOV, (dont know the official name) that vents a small amount to the atmosphere and plumbs back most charged air.
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Well, I don't quite know how to say this, but apparently this is a load of crap. I received this email reply today from Turbosmart...
Quote: "Turbosmart does not offer a BOV like this. If you want to vent to atmosphere a Turbosmart BOV will operate 100% fine on your car. If you want to plumback a Turbosmart BOV will operate 100% fine on your car. There is no need with a correctly designed and manufactured BOV to combine the both."
written by David Marriott, Sales Co-ordinator for TurboSmart (dm@turbosmart.com.au).
I checked out the website and there's nothing on it there, so I put in a query on the 'Contact Us' link on the website, and the above reply came back.
Might I ask where you got your 'mis'-information from?
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I supported Toymods
Location:
Perth
Registered:
May 2002
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Re: BOV's & Wastegates
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Thu, 13 June 2002 00:24
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I don't ever recall Turbosmart making such a BOV. Go Fast Bits were the ones whou made the Hybrid one. I dont get how he says that running either one will be fine, every AFM car I've seen thats had a BOV puffs a little smoke on gear changes because it runs too rich.
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Location:
Adelaide
Registered:
May 2002
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Re: BOV's & Wastegates
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Thu, 13 June 2002 01:07
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Sorry, I thought that it was turbosmart... I saw one advertised in a magazine a while ago but maybe I wrongly recalled the manufacturer  What I am quite sure of though is that one does definately exist 
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Location:
South Australia
Registered:
May 2002
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Re: BOV's & Wastegates
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Thu, 13 June 2002 01:53
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So can anyone tell me if I should use a BOV on a suck through carby turbo system. I can't vent it to atmo coz it already has fuel mixed in. Could return it back in between carby and turbo?? Would this work... I think it might screw up my mixtures
Cheers
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