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As KACEY has pointed out in a previous thread, and confirmed by Ricardo after the Great War in his tests on many of the Allied aviation fuels in use during this period, many of the aero engines of this time would have been suffering from mild detonation problems because of the variable quality and knock resistance of the fuels. If you build a replica engine today, or restore an original to its full working condition, and fill it with modern aviation fuel (or even fuel that meets the RAF's specifications of the early1930s) then this mild detonation will not be there and the engine will produce more power than expected as a result.
The British didn't have a wartime Type acceptance procedure (a Type Test) for engines until after the war. They allowed the engine manufacturers to specify average power output and test each engine, under the watchful (but sometimed not so watchful) eye of an AID Inspector. Spot checks were done on individual items (component parts as well as fully asembled engines), and an acceptance test was then done on the aircraft and its engines once they had been matched together (often, after orders had already been placed). This led to disasters such as the ABC Dragonfly, and the system was changed after the end of the war. Also, many wartime manufacturers sub-contracted work to other companies, and engines could be of variable quality (and even power output) depending on who built them - this was a particular problem where the whole engine was licence-built by a manufacturer other than the original company.
As Greybeard has pointed out in another thread, when you get to the rotary engines there is also the problem of 'gross' and 'net' values (with or without the windage effects). Manufacturers liked to quote the larger 'gross' value in their publications.
The Royal Aircraft Factory at Farnborough tested engines, both Alllied and Central Powers, and these tests are, I think, generally a better guide than manufacturers' figures (although the tests on German engines were often skewed, I think, by the difference in Allied and German fuels).
Bletchley
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