[Graham Boak]

I can't think of a source that directly quotes the performance change: it would show in terms of inability to achieve higher boost pressures. I have a feel it is of the order of 10% maximum power, but that's not to be taken as definitive. The other effect was of decreased reliability, roughness of running and loss of power due to damaged valve stems and the like. I have read something of this in RR publications, dealing with support and repair of Merlins in Russia - probably in one of the RR Historical Trust books, but I'd have to look it up.

The Russians found the Hurricane less agile than their own designs, and didn't like the Browning machine guns, though these were often replaced with their own superior weapons. However, such a Hurricane in decent condition would not be significantly slower than an average LaGG 3 or Yak 1 with half the firepower, even though being three years older in design and having an engine only 2/3rd the size. However, by 1942 the Hurricane was certainly outclassed by the fighters it had to face, and had no development potential left. The RAF was able to make use of it as a fighter-bomber and FR aircraft, but this didn't fit the Russian doctrine of slow armoured assault aircraft.

The Russians put particular effort in maintaining a supply of 100 octane fuel for their Airacobras. With only limited amounts of the additives, it is understandable that it was restricted to the later, more popular, type.

I'll just make one comment on the wider issue: soaking up 75% of the German Army must have been moderately helpful to the Western Allies.