Quote:
Originally Posted by Robo.
It was possible (but not common) to fill Spitfire Mk.II's Merlin XII with 87 octane fuel, hence the brackets.
That is not a speculation but a known and well documented fact. I suggest you get more information (other that those available to anyone online) before you offer your services to 1c.  No offence ment, just a frank observation. You are, of course, totally right about assuming that higher octanes allowed higher boosts, but you happen to be wrong here as for Merlin development history and few important details that give a bigger picture. I found it quite amusing as you, an expert in BoB fighter performance, came over to a2a with that post.  You surely don't lack confidence.
Of course, 100 octane fuel was necessary to achieve the +12lbs. boost in BCC-O setting on both Merlin III and Merlin XII, but the actual nominal rating has not changed to +9lbs on Merlin III. That was purely later mark Merlin (XII) with different coolant and various modifications (or improvements over Merlin III) that allowed higher boost, not the higher octane fuel itself.
Early Merlins as such were able producing much higher MFPs, but the nominal ratings were considered safe by the manufacturer and MoD and they were certainly +6.25lbs for Merlin III no matter what fuel you poured in it. The document you present confused you because there was certain time when the Merlin III has been further developed (as it was always the case with RR) and only later became Merlin XII. |
I have the Merlin III engine manual (AP 1590B) updated for the use of 100 octane fuel.
Amendment List No. 4 to AIR PUBLICATION 1590B includes a completely new page with "Performance" and "Limiting operational conditions" that is dated November, 1940 and gives nominal ratings with +6.25 lb. per sq. in.
I'm confident that these new pages from November 1940 wouldn't state outdated values. So I'm convinced that there was no rated power of +9 lb. per sq. in. on a regular Merlin III engine (without modifications to bring it to Merlin XII standard).
The infamous +9 boost document is useless without knowing the source. It is Page 40 of a larger document. At least we need to have the other pages to bring it into the correct context. It even could be a typo (III instead of XII).