The UK quattro Network
The UK's mobile Audi 5-cylinder mechanic - 07833 654800, 24x7
Specialist in turbos, quattros, electrics and non-starts
Of course, "schrapnel-knobben" is not a real German term. But it accurately reflects the potential of the variable wastegate spring preload screw, if it's screwed down too tightly.
In use, the nut is slackened off and the Allen recess in the top of the threaded rod is used to apply more preload to the wastegate spring - this makes the wastegate open later and increases boost. Adjustments are best carried out on a cool day. If the wastegate spring seat is wound in too far, overboost can occur and the overboost limiter in the ECU will cut the fuel pump.
Fine unless the overboost limiter has been disabled, e.g., by jumpering the fuel pump or the Zener diode mod. Or the wastegate control hose splits, or the wastegate diaphragm ruptures. In those cases, the schrapnel-knobben can live up to its name.