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Originally Posted by Blackdog_kt
Because it's possible to change octane ratings as long as you keep to the relevant limits?
Again, cue the Blenheim Mk.IV pilot's notes where it's clearly stated that it was standard operating procedure to switch between 87 and 100 octane mid-flight: the only limitation was keeping boost within the appropriate limits for each fuel type, the engine won't suddenly explode if you just throttle back and switch to the 87 octane tanks, even though the engines were modified for 100 octane use.  |
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P/O Art Donahue's account of using +12 boost during his first combat of 5 August 1940, whilst flying Spitfires with No. 64 Squadron out of Kenley, is typical:
“There are bandits approaching from the north” In quick response to this information, our leader sang out a command: “All Tiger aircraft, full throttle! Full Throttle!” That meant to use the emergency throttle that gave extra power to our engines. I was flying in our leader’s section, on his left. As he gave the command “Full throttle”, his plane started to draw ahead, away from me. I pushed in my emergency throttle in response to the command, the first time I had ever used it, and my engine fairly screamed with new power. I felt my plane speeding up like a high spirited horse that has been spurred. http://www.spitfireperformance.com/spit1vrs109e.html
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Kinda hard to keep formation if some aircraft are using 87 octane and some 100 octane...
In a Blenheim flying a long range recon or ferry mission (which is the only time they could use the auxiliary tanks) it is quite reasonable that there will be long periods where the expectation of enemy encounters are low, and thus mixing octane types is a reasonable risk. The problem is that it will take many seconds before the change back to 100octane can be made, and during that time overboost will not be available and damage to the engine may result if overboost is applied too soon.