A.P. 129 RAF Flying Training Manual, Part I - Landplanes

[41Sqn_Banks]

Originally Posted by Crumpp

Exactly. It was used to test the engine at altitude during service maintenance.

As said: There is no "override" for Merlin II/III engines. There is no device for extra rich mixture for take-off. There is only a cut-out to disable the boost control. But this was not used for take-off, as maximum take-off boost is the regular rated boost of +6.25.

THAT IS WHY YOU HAVE PILOT ANECDOTES OF FLYING AT +16 LBS ON 87 OCTANE!

Great, show us the anecdote.

If they tested the engine to 10.5lbs then a low engineering safety factor of 1.6 yields 16.8lbs.

Once again you forgot atmospheric pressure in your calculation.
10.5lbs (+ 15.0) is 100%
16.0lbs (+ 15.0) is 121,56%
This is only a factor of 1.2; a factor of 1.6 would be +25.8lbs (+ 15.0).

Exactly. Only in emergency or special operational conditions the aircraft is flow at the "maximum rpm/boost for level flight" (later called "all out") limits and not above.