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Posted: Tue Jun 07, 2005 1:24 pm Post subject: Blow off valves?

I'm new to the turbo world. I was wondering if these help or hurt you? Which ones are good and which are bad.

Posted: Tue Jun 07, 2005 3:16 pm Post subject:

heres my knowledge, but take it with some caution as i'm no turbo expert. hopefully rob or someone else will be able to correct me if i'm mistaken on any details.

for a turbo-charged vehicle you have to have a blow-off valve of some sort to prevent the pressure building up behind the throttle body when you close the butterfly by letting off the throttle which can cause damage to the turbo. Your stock motor will come with one, its connected from just before the 3000 pipe back to the intake before the turbo. Its a small black cylinder with hoses on either end.

it can be clearly seen here, its the only hose coming off the tube before the throttle body and after the turbo, and you can see it connects back to the intake before the turbo. he substituted a custom pipe for the 3000 pipe.

http://www.webpages.uidaho.edu/~simm8647/Images/IntakePiping1.JPG

I've never understood where the nipple at the top of the BOV is supposed to connect, i have it setup just as shown in that picture and mine seems to work ok, i dont know where the vaccum line terminates at the end opposite the BOV, doesnt appear to be going to the intake mainfold Question

on upgraded turbos the stock valve is unable to hold high boost pressures and will leak boost, so to fix that you install an upgraded BOV. This is where i get a little lost as i've never asked. Upgraded BOVs vent to atmostphere as opposed to back into the intake (maybe not all aftermarket ones ? I do not know) this causes the metered air let go to be lost which the motor does not compensate for, causing extra fuel to be added leading to a rich condition, and i've heard, sometimes backfires or flames out the tailpipe on shifts. MAP sensor fuel systems instead of metered air systems can accomidate this problem, but are expensive.

Thats all i know, hopefully others can add more and further my knowledge as well Laughing

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94 4runner
7MGTE / SAS / 35in MTRs

Posted: Tue Jun 07, 2005 8:26 pm Post subject:

I know that they all have them. What I wondering about are the external ones. Thanks

Posted: Thu Jun 09, 2005 1:46 am Post subject:

Unless you plan on running more than 18 psi or so, a bosch BOV (like the one on my truck that jmg posted above) will do just fine. I have also seen people run two bosch BOV's in parallel to increase the capacity rather than going external. The external ones are easier on your turbo at high boost pressures, but they also require changing the mass air flow sensor setup so it all depends on what you want to do.

Oh and the small line on the BOV is just manifold pressure; vacuum == BOV open, pressure == BOV closed.
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Posted: Thu Jun 09, 2005 6:11 am Post subject:

so what initiates the BOV to open again when the throttle closes?
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94 4runner
7MGTE / SAS / 35in MTRs

Posted: Thu Jun 09, 2005 4:39 pm Post subject:

vacuum opens it. I don't think that I'm gonna boost 18 lbs right off the bat. Maybe when I blow it up.
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Posted: Tue Jun 14, 2005 3:18 pm Post subject:

the Bov senses the differential pressure across the throttle plate by being hooked up to both sides. when theres a large difference it opens releasing the air and limiting compressor surge. tons of ppl run open atmospher BOV's without any big effects if your really worried go with a MAFT setup and run it blow thru. i ran my SSQV open wiht the stock AFM for over a year and never had a problem. Now i run a MAFT and SSQV within inches of one another and have no problems. wiht my ddp i was hitting 17psi quite often. you can also get a recirculation kit for the ssqv to reroute the air back into your intake b4 the turbo
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