The problem is IMO too overexposed as 'If only it have had a Merlin Engine', wich clearly relates to the widespread succes of the Merlin in the hands of RAF - And the succesfull adaption at the P-51 (the late P-40 got a Merlin too, wich increased it's abilities, though without comparison with Spitfire and Mustang). However, the real 'what if' lies in the hands of American producers of aviation engines:
- What if American interwar car racing hadn't been restricted in a manner that kept supercharging out of the mindset of American automotive engineers? England and Germany raced with up to 650hp automotive engines at same time as the newly developed Allison V-1710 only(?) had 1000hp and as an aiviation engine undoubtly had challenges around decreasing power in the heights!
- What if American engineers had lurked at the development of European aviation engines?
- What if American aviation engineers had realised the advance in beating any possible enemy aircraft in ceiling?
- What if the Americans at an early point had taken notice of the RAF criticism of the american fighters lack of ceiling, instead of nationalistic(?) denial of the problems? It took until the american deployment of P-39's on North Africa to realise that, wich could have been fatal for USAAF and the Mediterranian campaign in all, if not it was possible to get a lot(!) of Spitfires almost instantly - And seemingly didn't the engine producers in the US increase research and development of supercharging after that smack in the face.
I mentioned the P-40: It did get a Merlin, however for some reason with reduced supercharging, wich clearly shows that it's all about supercharging and not just the British engine!
The USAAC had invested heavily in the turbocharger, and that restricted funds for 2 speed or 2 stage engine development for Allison.
The radial manufacturers, meanwhile, had more involvement with the Navy, which wanted little or nothing to do with the turbocharger, so had funding to develop 2 speed and 2 stage engines.
The P-39 was originally, as the XP-39, designed to use a turbocharger. The installation was, however, less than stellar.
The forerunner of the P-40, the P-37, was also fitted with the turbo. In an unusual arrangement, the intercooler and coolant radiator were fitted between the engine and the pilot, requiring the pilot to sit far back in the fuselage. Issues with the reliability of the turbo, and the impracticality of the layout led Don Berlin to request a non-turbo altitude rated V-1710 for the P-40.
The P-40 got the Merlin primarily because the initial Packard contract was for 6,000 Merlins for the British and 3,000 for the USAAF, the only suitable aircraft for the Merlin being the P-40.
The P-40 received a 20-series Merlin (the V-1650-1/Merlin 28) which was a two speed, single stage engine. The -1 was the first American Merlin produced. It would be 1943 by the time the 2 stage V-1650-3 (60 series Merlin) would become available in the required numbers, and by then the Mustang X had been tested with the Merlin 65 and the XP-51B was undergoing flight testing as well. The P-40 was not going to get the good engine with that competition - it had already been seen as obsolete as a fighter.
George Mead, of Pratt and Whitney, toured UK engine manufacturers in 1936 or 1937. When he returned to the US he designed the Pratt And Whitney X-1800 - a sleeve valve H-24 of 2240 cubic inch capacity. That kinda sounds familiar....
As for the Merlin in the P-39, a 20-series may have fitted, and certainly would have increased altitude performance, as it did the P-40F and L. A Merlin 45/46/47 would also have fitted, giving high altitude performance at the cost of low down performance. The Merlin 60-series 2 stage engines, however, would have been too long to fit in the P-39's engine bay. And the extra 200-300lb weight may have caused problems for the CoG.