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Re: 2 blow off valve Q's (Long, read on...)
Posted by Talladega900 (more from Talladega900) on Sun, 29 Jul 2001 22:02:06
In Reply to: 2 blow off valve Q's, Quinn S. Kurz, Sun, 29 Jul 2001 20:49:18
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In a matter of speaking they're the same, or more specifically, very similar. A blow-off valve vents to atmosphere, a ByPass Valve vents back to the intake side of the turbocharger.
In all cases, the valve is used to vent boost pressure when the intake is in a WOT state, then suddenly slams shut, as when you shift. When slamming shut the pressure wave has no where to go, and quite possibly can rebound off the throttle plate and impact the compressor, potentially causing severe damage.
By venting, the boost pressure has somewhere to go. Two theories apply to how to vent -- to atmosphere, or recirculate. A BOV vents to atmosphere, the boost pressue essentially wasted. A BPV has a return hose to the intake side of the turbo, thus that boost energy is contained within the system. (By now you can guess which theory I subscribe to...)
Virtually all Saabs have a BPV, made by Bosch. When increasing boost performance, however, the Bosch unit's spring is so weak that boost pressure simply blows open the valve, thus when the throttle is still wide open, boost pressure is still escaping through the BPV/BOV. All the performance increases are thus for naught as the valve spring is just too weak to stay closed. The solution is an upgraded BPV (or if you *need* to hear the "hoot", a BOV).
Does it increase hp?
Not really, but it retains that hp you've increased through upgrades to boost pressure.
Hope that helps! Good luck and happy Saabing!
Jim M.
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