You are correct, at rest vanes are closed down to the set screw limit. As boost increases the vanes are opened to reduce boost and redirect the exhaust gasses away from the turbine blades.
The balancing act for a well setup vnt is to maintain boost without overly restricting the exhaust gas flow.
which is why they are setup in a flow rig at the factory.
If you remove the actuator don’t touch the locked nut on the actuator rod.
This position is critical to the VNT operation as is the set screw.
Once the position is lost it is virtually impossible to reset it without the above mentioned flow rig.
Cheers,
Paul.
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