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les_garten les_garten is offline
Macht Schnell
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Join Date: Jun 2003
Location: PSL, FL
Posts: 861
I think it would be instructive for you to get a better understanding from your mechanic on what the Shuttle valve, Bypass valve, recirculating valve, or blow-off valve does. It is called all these things by various wrenches. It's purpose is not exactly as you stated.

I no longer have one like yours but they all do similar duties. When you take your foot off the throttle when in boost, this valve receives a pressure signal from below the throttle blade that goes from + Manifold pressure to a rapid change to a relatively negative manifold pressure (ie vacuum). This causes the recirculating valve to open and allow the turbo to spin against little resistance by recirculating the intake charge from the Intercooler around in a circle within the intake circuit. If this were not in place the Turbo would spin against the closed throttle blade and encounter very high manifold pressure. This would cause the Turbo to spin down rapidly and possibly shear the Turbo shaft in half. You should ask your wrench to trace the path for you while you are standing next to the car. It makes a lot more sense that way. Typically those O-rings fail and you make boost but not as much boost, or you hear a clicking sound when you snap the throttle open and closed.
---Les Garten---85 930, Andial IC, GHL Headers, Fabspeed Dual, K-27 HF2, 3.4 JE Full Finned 7.5:1 CR, 964 CAM'd, Carerra 3.2 Manifold Cut/Flipped, Tec3r, Siemans 55#, GSF Fuel Rails, Clewett Crank Trigger, Clewett Cam Trigger,Dual Plugged, ARP Head Studs/Rod Bolts, Clewett Wires.Tial 46mm WG, Tial 50mm BOV, WEVO Shifter,934 Boost Gauge, Wideband EGO Sensor/Gauge, C2T Head Gaskets, '88 MB 300TE,BMW R100RT
Old 06-11-2008, 11:14 AM
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