While I definitely like the idea of an Allison-engined D.520, why not choose the Merlin? It's much closer in proximity which eases logistics, and the Merlin is a much better engine IMO. Also doesn't put the French aircraft into the low altitude trap the early Allison aircraft fell into.
You have to decide in the Spring of 1940 at the latest. The logistics problem is minor. France is going to have to import machine tools from the US in any case. Packard signed the deal in Sept 1940 but talks had been going on since July aft the Ford talks stopped. Packard was looking at the sample engine and drawings for weeks before the deal was signed. Packard was supposed to build 800 engines per month. They built 42 engines total in 1941 and didn't hit the 800 engines a month goal until July 1942. They hit 702 in June and 602 in May. That is sort of the time line, Packard and the US and England were all lucky in that the 2 speed Merlin was the engine selected. A few months earlier and it might have been the single speed Merlin XII?
French do get domestic production of Allisons for all the P-40s and P-322 Lightnings they ordered
The Low altitude Allison was offering 1090hp at 4000meters.
The H-S Y-45 was offering 920hp at 4200 meters and that is with P-S supercharger.
The Troublesome H-S Y-51 was offering 1000hp at 3250 meters.
Yes the Allison was heavier, but at least it worked.
Problem the early Allison had was the 7000lb plus engines they stuffed into. An empty P-40B weighs within a couple hundred pounds of a loaded D.520.
Shouldn't the theoretical Allison-engined D.520's be roughly comparable to the Spit IX?
Why? Did the British even know how the Merlin 60 was going to turn out?
French need to plan for something like the Spitfire II (Merlin XII) or Spitfire V (Merlin 45).
We cannot expect magic. A Merlin 60/61 in a D.520 is putting 1000 liters in a 500liter bottle. You need not only the supercharger, you need bigger radiators, you need the intercooler/s and you need a bigger prop.
The basic D.520 was equal to the Bf 109 E-4 and Spitfire Mk I, and the 520Z showed solid performance that would likely get even better with a superior engine. Britain also probably has their hands full trying to make enough IX's for themselves around this point, so not having to export those would ease their heavy burden.
The D.520 was equal or close to the 109E-4, the Spitfire I not so much and in July/Aug the standard (thing to measure against) was the Spitfire II with Merlin XII engine.
It doesn't matter what the 520Z was promised to do if the engine would not deliver (stay running).
Merlin is a superior engine in all aspects - timing, being debugged, and making better power.
I'm not sure how come the R-2600 became a fighter engine after 10-15 years of convincing us that it is bad in that role.
You are right, the Merlin was better. Now is France going to get the "deal" or is the US given the deals France is already getting from the US and fact that France needs to buy the tooling from the US anyway. Britain was buying a lot of the machine tools from the US for British shadow factories anyway. The whole P-322 fiasco was the French and British wanting interchangeable engines with the P-40 Tomahawk.
The R-2600 was not a good fighter engine, it was what France had a chance of buying/making.
Mediocre engine beats bad engine (G-R) or expensive, hard to make engine (Hercules) or engine that is not quite there yet (R-2800). France has to get through the last half of 1940 and all of 1941 before worrying about 1942-43.
The R-2600 has a chance of fitting in a MB-157 airframe. A two stage R-2800 in a MB-157 airframe, has two chances and slim was on the noon train to Dover.
Yes France can come up with new airframes. Service use in 1943? FW 190 first flew in June 1939. Maybe the French don't make some of the mistake that Fw did?