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Ok, so if you were running a spitfire with 100 oct would you have to change the plugs drain the tanks, clean out the fuel pipes etc before putting 87 in it?
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I don't know the plugs authorized for 87 Octane so I can't tell you. It is a possibility. I can tell you if you could do it, it would noted in the operating instructions or the maintenance manuals.
I can also say, you could not use +12lbs boost if you contaminated the system with significant amounts of 87 Octane.
I think you are beginning to see the maintenance nightmare airplanes can be even with something as simple as putting new gas into them!!
If I wanted to convert and it was not possible to convert the entire force, the first thing I would do is get as much 100 Octane gas to the airfields as I could before anyone converted. Then my conversion pool has a supply of gas.
In theory, the operating limits of the engines were not raised no matter which avgas you put in it. As long as you did not use +12lbs with 87 octane gas, you sould be ok with an engine modified for 100 Octane.
There is a thing called a ferry certificate that covers things like this in aviation. Depending on the technical issue, it can be complicated or very simple to get one. I am sure in the RAF, something like this was a phone call to the Maintenance officer who did the paperwork and approved it in order to ferry a plane back under special conditions.
Typically you can always go higher in octane but never lower in piston engines but airplanes are not typical. In airplanes each installation even of the same engine type is different. You generally can't tell much about the Merlin in a Hurricane by looking at the Spitfire's instructions for example. That is why the Air Ministry tested both types.
Your 87 Octane engines in theory could run without incidence on 100 Octane. It would be specified in the Pilots Notes and the fuel tank placarded for all fuel types authorized for the aircraft.