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The wastegate - exactly. Maybe I was a bit overzealous when I posted this (late at night again). I was under the impression that the wastegate (which bypasses the compressor) was the same thing as the CBV (compressor bypass valve), but the wastegate controls exhaust flow and the CBV is a spring-loaded device similar to the wastegate that opens when the TB is closed. These sound like the same thing to me... but anyway.
Ok, so my understanding of the wastegate is that when there is manifold vacuum, the wastegate is open, allowing exhaust to bypass the impeller blades.
What I'd like to do is force the wastegate open with a steady amount of vacuum, and thereby disable the turbo, BUT without affecting any other component.
As I understand it, the spring on the wastegate works to close the wastegate (and force the exhaust through the exhaust side of the impeller, producing boost) and there is a diaphragm that keeps the wastegate open when vacuum exists. As the throttle plate opens more, vacuum production drops and the wastegate closes partially. That is the property I'd like to change about the wastegate setup.
By applying steady vacuum, the wastegate would be forced open (allowing gas to flow around instead of through the turbo), unless I turn off the vacuum pump. With the vacuum pump off, the car would drive as normal.
There's a problem with this... If I have a pump applying vacuum to the turbo, I'll be applying vacuum to everything else because the feed line for the turbo comes from the TB. I'd be creating a false vacuum to the fuel pressure regulator for example, and consequently starve the engine of fuel. ]
To counter this effect, I'd install a one-way vacuum valve - to allow air to flow towards the throttle body only and make a closed system for the pump to function. With this one-way valve, the manifold could apply vacuum to the turbo or the secondary pump could apply vacuum to the turbo, but the vacuum pump would not feed the manifold.
This sounds quite do-able to me, but if there is a change in behaviour for the wastegate (say the bellows inflates and closes the wastegate even more completely), then my one-way valve (which blocks air flow to the turbo from the TB and prevents the actuator from being pressurized) would cause a problem and a decrease in available boost.
The talk I did about a second vacuum pump was not something I care about, so please ignore it.
-Will
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1990 740 Turbo, on its way to stock specs, maybe beyond
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