Quote:
Originally Posted by Crumpp
Without the specific mention of using +12, an anecdote that makes any reference to boost override, pulling the tit, boost cut out, or anything other term related to the system is irrelevant to the use of 100 Octane.
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The RAF training manual quote only mentions the use of boost control cut-out to obtain maximum permissible take-off boost. In case of the Merlin II/III this boost is +6.25 which is obtainable without cut-out.
The maximum permissible boost with 87 octane is documented since 1938 to be +6.25 and since January 1939 it is documented that in excess of this boost 100 octane must be used. The later documents are perfectly in line with the regulations.
This leaves the following explanations for the mentioning of use of boost control cut-out:
a) there was a failure in boost control and the cut-out is used to maintain a boost below +6.25, this doesn't provide any information about the fuel used as it could happen with 87 octane or 100 octane.
b) a boost in excess of +6.25 was used, either for take-off or combat/emergency, this is a proof for the use of 100 octane fuel or the proof for violating the maximum permissible boost.
c) a boost below +6.25 was used and the boost control was disabled without any logical reason and this boost was kept manually by the pilot below +6.25. Again no indicator for the fuel used.
Reason c) is highly unlikely as this would be a violation of the handling guidelines outlined in RAF Flying Training Manual ("This must only be used in emergency and not, in any circumstances, for ordinary flying, ... even if the correct boost is not exceeded") without any benefit.
In most cases reason a) can be ruled out by the context, in case of engine control failure the pilot would try to return to the airfield and avoid any combat and most certainly mention the engine troubles in the report.
So in the remaining cases the reported use of the cut-out is:
- proof for use of 100 octane fuel
- in the other case where 87 octane fuel was used the proof for overstepping of the regular maximum permissible boost. It shows that this was physically possible (or a breakdown of the engine or evidences for detonation would be reported in context) and in addition it was either permitted or tolerated by authority (or it would be reported as a violation in the context, e.g. I'd expect this to be mentioned as one possible reason here
http://www.spitfireperformance.com/dowding.pdf).
In both cases it is a proof that the performance of the engine was increase. If this is true, this renders the whole 100 octane debate obsolete as pilots would simply use the increased power if necessary independent of the used fuel.
My understanding is that this is not true.