Quote:
Originally Posted by 41Sqn_Stormcrow
If however I well understood, the boost cut out was put because this high pressure was damaging to the engine in the long run and therefore its use was limited to real emergency cases.
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ABC was introduced in order to prevent pilots from breaking engines by overboosting them below FTH.
I suspect that the cutout was originally provided in case the ABC failed, so that the pilot would still have access to direct throttle control.
The cutout modification was basically a hack which allowed the cutout to function as a combat power detent without extensive modification to the rest of the cockpit.
Later Spitfires didn't have a boost control cutout, but were instead fitted with a gated throttle which allowed the independent setting of the various engine ratings, including the combat rating. They also had double-acting ABC instead of single acting ABC, which meant that the throttle behaviour was a bit different. This is explained in the relevant AP from the period (I forget the number now, but it's been linked to in quite extensively in this forum).