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I dont think the Packard production of the Merlin engine can be used as a model for re-engineering an engine.
I am pretty sure RR would have provided drawings / specifications / templates and support from the parent factories to make this happen as smootly as possible.
Gnome-Rhône 14N - Wikipedia, the free encyclopedia
620 kg. Dry weight.
38.67 Liter displacement.
Carburetor. 87 octane fuel.
1,164 HP. Take off.
This engine might have potential for further development and power German aircraft such as Ar-196, Fw-200, Ju-252, He-111 etc. Acquiring a license to produce the French engine should not be difficult after June 1940.
So was P&W R1830 yet we produced 173,000 of them.
Uber engines such as R2800 get most publicity but smaller engines are important too.