I've never made an Actuator Arm adjustment. But, having numerous Turbos apart for cleaning, it is obvious there is an internal maximum that the Vane "sweep" can be adjusted in either direction. Internally, it will bottom out against a guide pin/roller (for a lack of a better term). The external set screw stop, is typically set where that internally nothing bottoms out.. very slightly off bottoming. For all those with paint on that screw, I've never found one that was set that allowed it to bottom internally (almost fully closed Vanes).
So, a maximum adjustment in either direction will not fully close or fully open the Vanes as a result of internal design. Also, as I understand the mechanism, considering there is no by-pass for the expanding gases, it's the directional angle of the Vanes that provides boost. Sort of like holding your finger over the end of a garden hose. When you shut down the normal flow of water to a small stream it comes out blasting............ with the optimum angle, it can apply some strong force.
I know this has been a subject of debate in several Threads. When the engine is started, the Actuator pulls the arm fully down to the set-screw, which results in almost closing the Vanes. That angle of the Vanes doesn't shoot the expanding gases onto the Turbine blades at the best angle to provide any significant boost, thus, virtually zero boost at idle (based on my boost gauge, at idle RPMs, there is a slight vacuum). As the RPMs are increased (with load), the Actuator moves the Vanes to the optimum angle increasing boost as necessary. Simply put, as demand (load on the engine) is increased, the Actuator moves the Vanes to increase boost and as load drops or maximum RPMs are reached (or before), boost will drop off. This scenario is very obvious with a dash mounted Boost Gauge.
So, it would seem to me that an over boost is not totally related to how far the Actuator Arm is adjusted up or down, but more with the free sweep of the Vanes in their normal range of operation. Thus, if there is only a momentary stop/hang/sticky of the Vanes in the scheme of things (acceleration), there can be an over-boost followed by an under-boost or visa versa.
If you have a Boost Gauge to observed, you would be amazed to see the instant response of just simply tapping the accelerator. It's really amazing. So, obviously, if there's one thing in the scheme of things not working properly, the ECU is going to catch it.
Also, as I think I previously stated, a faulty MAP sensor can play hell with boost. I put up with it for 6 months before I finally installed a new one.
Just how I see it.
