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Posted by ELaw [Email] (#699) [Profile/Gallery] (more from ELaw) on Fri, 27 Feb 2015 04:18:53

In Reply to: Premise for this, thetallguy [Profile/Gallery] , Wed, 25 Feb 2015 07:07:46
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I always had a problem with people's claims that a performance BPV was needed because stock ones "couldn't hold under high boost" - logic doesn't really support that claim.

The situation is actually better than you describe, because at WOT the entire side of the valve diaphragm connected to the intake manifold will see boost. But on the other side of the diaphragm, only about half of it is exposed to boost - the other half sees atmospheric pressure. So higher boost would tend to hold the valve closed harder - not the other way around. Of course at *really* high boost, you'd get to a point where the diaphragm could rupture or the plastic housing could explode, but you're talking awfully high boost levels for that to happen.

I also have to say I'm skeptical of the claims that a BPV keeps the turbo spooled when the throttle is closed.

Have you ever tried covering the inlet or outlet of a shop vac while it's running? When you do that (and the compressor in a shop vac is fairly similar to the one in a turbocharger) it doesn't slow down, it speeds up. IMHO the purpose of a BPV is to avoid compressor surge which is a sensible thing as surge can cause all sorts of problems. But I'm not convinced it actually keeps the turbo's speed up when the throttle is closed.

Oh and re the computer controlling the BPV, that is being done! On the 9-5 and maybe some of the other newer Saabs, there's a computer-controlled solenoid valve that controls whether the BPV sees manifold pressure or atmosphere. And I know many of the newer VAG (VW/Audi) cars with turbos have a BPV that's just a solenoid controlled by the computer.

->Posting last edited on Fri, 27 Feb 2015 04:23:03.

_______________________________________ Eric Law Current collection: '90 Audi 80 quattro (modded to death), '12 Audi A4 quattro (stock for now) Gone but not forgotten: '72 99, '77 99 (became a turbo, twice) '80 900T 5-door, '81 900T, '86 900S (became a turbo), '86 900 SPG '86 9000, '97 9000 Aero A bunch of Audis, '69 Firebird, '64 Toyota Corona (first car, cost $35 and worth every penny) Be alert... America needs more lerts!

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