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Bypass Valve function...
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Posted by REM in Seattle (more from REM in Seattle) on Wed, 2 Nov 2005 21:05:49 Share Post by Email
In Reply to: BPV Question, Viggowner, Wed, 2 Nov 2005 17:40:00
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The object of a bypass valve is to relieve the excess boost pressure that builds up between the turbo and throttle body when you let off the gas to shift or decelerate.

If you are operating at full throttle of say 15psi boost, and then closed the butterfly valve on the throttle body by removing your foot from the accelerator to shift or slow down, the boost pressure bounces off the closed butterfly valve and sends a pressure wave back toward the turbo compressor wheel. This does 3 things...

First...it stalls the turbo compressor wheel so that when you open the throttle back up it has to spin up all over again, increasing turbo lag and time to accelerate.

Second...It puts a terrible load on the bearing and compressor wheel of the turbo which can cause premature wear and failure.

Third...It subjects all the intake plumbing and connections from the throttle body back through the intercooler to the turbo compressor wheel to excessive pressure wave stresses.

When the engine is under boost, the bypass valve is in the shut position by virtue of a compression spring. This insures no boost leaks. When the butterfly valve on the throttle body is closed, the engine produces a vacuum at the base of the throttle body. A vacuum line runs from the base of the throttle body to the bypass valve and is used overcome the compression spring force and open the bypass valve. The excess boost pressure vents to the intake system side of the turbo providing some added intake pressure and allowing the turbo compressor wheel to keep spinning.

REM in Seattle
2005 Saab 9-3 Aero Convertible; 5-speed; 6000 miles
1998 Saab 900SET; 5-door; Automatic; 80,000 miles
1984 Saab 900T; 3-door; 5-speed; 242,000 miles (RIP)
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