Ah but...the Napier Sabre was not an opposed piston engine. A 24 cylinder 'H' configuration, one horizontally opposed flat 12 sleeve valve engine on top of another, but no opposing pistons.
true, but it was another bit of Napier innovation, which yielded the first piston engine to achieve 2,000+hp while weighing (initially) <2 tons; in 1938! Later, of course, they achieved even more (but also weighed a bit more). The sleeve valves (also used by the Bristol Centaurus) were also an innovation (but not a home-grown Napier one).
In US, these sorts of powers and power/weight ratios were achieved in production engines only with later, multi-row, radials; although there were some (unsuccessful) Lycoming trials of the H layout.
</nerdish intrusion on an 'entertainment' thread
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