Inaccurate performance data for BOB fighters in COD comparing to RL data

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Originally Posted by 41Sqn_Banks

Exactly. For example Operational Notes for Pilots on Merlin II and III. 2nd Edition January 1939. It doesn't mention +12 boost, it mentions that a unspecific higher boost than +6 1/4 can be used for take-off by operating the boost-control cut-out.
It doesn't mention any modification, only that 100 octane fuel must be used. (Note that the 4th Edition from April 1940 mentions that "sparking plugs approved to withstand this high boost must be used", so if there was any modification required in January 1939 it would have been mentioned.)
It does however state that this higher boost setting has to be determined on the ground before it is used by listening if there is detonation.

The 4th Edition from April 1940 now gives +12 boost and as already said mentions that specific sparking plugs must be used and that the boost-control cut-out has to be modified to limit boost to +12 boost.
Looks like in April the cylinder head modification was no longer required (maybe because all engines had been modified).

My copy confirms your statement:

Emergency +12 lbs./sq. in. Boost Operation: Pilot's Notes, Merlin II, III and IV, 4th Edition, April 1940, page 6.