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Old 02-04-2001, 01:52 PM   #1
MattC
Scooby Specialist
 
Member#: 158
Join Date: Aug 1999
Chapter/Region: NESIC
Location: CT
Vehicle:
2011 STi
PBP 1999 2.5RS RBP

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Edwin,

Let me see if I can take a stab at this.

I believe that the turbo has to spin quite a lot more at 5psi@6000rpms than 5psi@3000rpms, so how tight the bov has to be has as much to do with rpms as boost. I also believe that the pressure around the bov when the TB is open is much less than when it is closed, because when it is closed the air has no place to go. That is why if the bov is tight enough it won't ever open, unless the TB is at least partially closed. Also, the faster the turbo is spinning when the TB is closed the longer and farther the bov has to open.

To set my bov I first made sure it wasn't letting out any air at idle by taking a piece of tissue paper and holding it in front of it. Then it was a simple trial and error. I made sure the bov would open at low boost+low rpms w/ the TB close, but wasn't opening at high boost+high rpms w/ the TB open. I believe there is a great amount of error, so don't stress over it too much

As with anything posted on the internet, take it with a grain of salt. For all you know I could be typing with my ass


Cheers,

MattC
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Old 02-04-2001, 06:49 PM   #2
wac
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Join Date: Dec 1999
Chapter/Region: NESIC
Location: Lebanon, NH, USA
Vehicle:
2005 Outback 2.5 XT
Champagne Gold Opal

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Quote:
...under boost I'm wondering - what keeps the BOV from opening and letting all my boost out prematurely?
The BOV only operates when it senses a sufficiently large pressure differential or drop between the turbo outlet and the intake header. The throttle valve (and intercooler, if applicable) is located between them. Some people also refer to this condition as "vacuum".

Under boost, the pressure drop is less than that at idle. Although airflow and throttle valve position both affect the pressure drop, I think it's just a happy coincidence that it's lower while under throttle than at idle. Otherwise, someone would have invented another yet control input (TPS, perhaps?) for the BOV.

If you adjust the BOV just past the point where it seals at idle, it allows the quickest response when you go from boost to lifting the throttle (closing the throttle valve). You can certainly turn the BOV up some more, at the expense of a little more reponse delay. You want to minimize the delay, so that the trapped air does not slow the turbo down for your next upshift.

(I think I got that right?)

-WaC
Wayne

Edit: UBB code, and "inlet" changed to "outlet".

[This message has been edited by wac (edited February 05, 2001).]
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Old 02-05-2001, 12:50 AM   #3
efoo
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Member#: 80
Join Date: Jul 1999
Question BOV settings

How does one set their BOV? I've been trying to reason out how it works in my mind, but I'm getting lost. Someone told me to tighten the spring up so that it doesn't open at idle, but not much past that. But then, under boost I'm wondering - what keeps the BOV from opening and letting all my boost out prematurely? Should I really be setting the BOV not to open until my max psi is reached, like my wastegate? But then it won't open when I let off the gas unless I was previously at max boost, if I am thinking correctly.

-Edwin
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