1994-2002 [Subscribe to Daily Digest] |
You have the BPV right.
You have the BPC right.
The third hose is the vent for the bleed orifice in the BPC valve. It connects back into the filtered air stream. Because the BPC valve is really an eletro-modulated bleeder valve, the air flow through the bleed orifice can reverse under some situations and one needs to ensure that unfiltered air cannot come back in through the orifice. So you connect it to the intake downstream of the filter. The Dawes and other ball-spring advanced MBC's are essentially check valves, the Dawes is acutally make from a check valve. So reverse flow is not possible, so the bleed orifice just vents to atmosphere. There is a slight chance that some dust could get in thru that orifice and into the wastegate actuator, but its a rolling rubber diaphragm and who cares.
Much of the hosing is driven by past practice and configurations. One can hook up the pressure side of a bleeder valve to the thottle body, upstream of the throttle plate. That provides for some interesting benefits, and sometimes side effects. But then there folks who will hook a bleeder type valve to the manifold. The the bleeder orifice reverse flow could be motivated by a manfold vacuum! And if hooked up above the throttle plate, there can be pressure drops through the pipes, intercooler, turbo and air filter before the boost comes on. Then there is a greater but still slight negative pressure than with the stock configuration. Actually, if you pressurize the stock BPC valve from above the throttle plate, the boost pressures are not increased in any way, but the turbo is a bit more reponsive, but Saab chose to save 2.5 feet of rubber hose.
You never want to apply manifold vacuum to the wastegate actuator, because that will distort the diaphragm in a fashion that could lead to failure. So best not done. But with a diesel engine, the boost controller can be hooked up to the manifold because there is no throttle, no spoon either.
As for installing the MBC, you have it figured out. But PBC stays connected electrically to keep the electrical connectors sanitary. You want to have the end of the hose from the actuator hose barb looped back up to close off the PBC hose barb exposed when you removed the hose from the turbo. That keeps the PBC sealed from dust and mosture contamination.
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