Thanks, I had no idea, so just now looked it up.
See now where there were more than a dozen variations of the Merlin used in nearly two dozen different aircraft, both British & US, including Spitfires, Lancaster & Mustangs.
You could even say that engine, in it's power & adaptability, was almost, ahem, magical.
;-)
IIRC other than the P-51 the only other US aircraft to use the Merlin, in a production aircraft, was the P-40 in the F & L variants.
https://en.wikipedia.org/wiki/Curtiss_P-40_Warhawk#Variants_and_development_stages
Also recall that the P-51 started out with an Allison engine. The Mustang I or P-51A / A-36 was your basic dog over 15,000ft but excelled at the lower to medium altitudes. Both North American Aviation and the British has started to look at installing a Merlin to see what would happen to the Mustang. And the rest is history, eh?
As a side note what made the Merlin special was that it had a 2 stage mechanical supercharger. The US Allison engine did not. With no supercharger the P-40s and P-39 were dogs above 15,000 ft or so. The P-38 used Allison engines but they had turbo-superchargers. Hence the P-38s stellar performance at altitude.
The US aero engine that would be the equal of the Merlin in importance IMHO would have been the Pratt and Whitney R-2800 engine. Among notable aircraft powered by the R-2800 were the P-47 Thunderbolt,F6F Hellcat, F4U Corsair, A-26 Invader, P-61 Black Widow and C-46 Commando. The Grumman F7F and F8F also used the R-2800 but did not see service in WW-II.
FWIW :-)
Regards
alfa6 ;>}