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The AM-42 started out in life as the AM-34, designed in Italy, and morphed into the AM-35, then the AM-38 and, finally, the AM-42. It had some Soviet input in that time, naturally, but was a larger-displacement engine than the MNerlin or the Allison ... alrgely due to les sophisticated fuel used in development and operation. It had the very unique characteristic of having a longer stroke on one side than the other due to articulated connecting rods on one side!
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AM-34 was designed in Italy, while the AM-42 have had only 'some Soviet input'? You are unbelievable, Greg.
BTW, the standard Soviet fuel was 95 oct, suspiciously close to the German of 96 oct C3 fuel, and not that far away from Allied 100 oct.