Placing the Bell P39 Aircobra.

[Graham Boak]

You are stepping outside the point of the original comment. Both the Hercules and the Twin Wasp were in the 1300-1500hp class in 1941: that the Hercules was later developed beyond that is a measure of its quality rather than relevant to a discussion on entry-into-service dates. In 1943, both the Double Wasp and the Centaurus were in the 2000hp class (as indeed was the Sabre). Postwar power outputs are irrelevant here.

That the US chose to make bigger engines rather than developing their earlier designs to the utmost is an interesting engineering option: it does not make the long-delayed Centaurus an equivalent to the 3350.

I stand by my initial premise: that British engine production policy in WW2 was biased towards the in-line engine because of a failure of British radial manufacturers to produce competitive engines in the requisite timescale, and that this was because the sleeve-valve approach ran into problems that extended development times.