Having repaired the 620 ti the performance wasn't what you would expect from a 200 bhp saloon. It was somewhat flat, and lacking the urgency that you get with high performance turbo charged motors.
To investigate I fitted a boost gauge, upon opening the throttle boost built up slowly to a max. of 7psi. At first this would point to the electronic boost controller. A device that 'bleeds' boost away from the actuator increasing boost levels above the spring pressure of 7psi.
However this vehicle is fitted with a manual boost controller, so it isn't a problem with the electronic control. Increasing the pressure setting of the MBC didn't increase boost levels.
This points to a problem with the actuator, often with older cars the spring tension weakens, and more pre-load is required to restore correct operation. However too much pre-load results in potentially damaging boost spikes and boost creep at higher rpms.
The correct pre-load can be tested with a simple pump and gauge, increase pre-load to the desired pressure. This is done by adjusting the length of the actuator rod.
After adjustment test drive the vehicle checking boost levels under differing driving loads and speeds.
I had to reduce the settings on the MBC resulting in 12psi of boost (max safe level with std engine) for much of the WOT speed range.
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