See
this post for the relevant pages of AP1565A Vol. I that cover +12 Boost and 100 Octane.
http://forum.1cpublishing.eu/attachm...9&d=1332111633
http://forum.1cpublishing.eu/attachm...0&d=1332111638
http://forum.1cpublishing.eu/attachm...1&d=1332111649
http://forum.1cpublishing.eu/attachm...2&d=1332111659
http://forum.1cpublishing.eu/attachm...3&d=1332111666
Note Sec. 2 Para XII where the increase of boost to +12 is mentioned. Note Sec. 8 Para 7 the use is "authorized" for short period and when 100 octane is used. Also note the "List of Content" of Sec. 8, it shows Sec. 8 was issued with A.L. 6 in July 1940.
Quote:
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Originally Posted by Crumpp
In order to use +12lbs on the Merlin engine, you had to replace the heads with a new design with a increased spigot depth of .020. There were two authorized heads being manufactured to convert engines. One could use the existing rings while the other required a specific set of rings to be installed. Once that was done, the fuel metering system had to be modified. You can figure they would have had a 4 to 1 stock of heads before they started modifying aircraft. They would maintain that ratio even if it limited the size of the force they could convert.
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Note that Sec. 8 Para 7 also refers to AP1590B Vol. I (manual of Merlin II and III) for the required modifications. IIRC in this manual only Mod 154 (modification that limits the boost to +12) is mentioned as required and the other modifications are recommended. I will look these pages up later. EDIT: The manual states that both mods are required.
Quote:
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Originally Posted by Crumpp
The July of 1940 Spitfire Mk II manual clears the Merlin III engine for use of 100 Octane fuel and notes +12lbs may be used.
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Please correct this statement. It's unlikely that the Spitfire II manual (which has a Merlin XII engine) clears anything for the the Merlin III engine.