Why French piston engines looked underpowered after 1940

Why did French piston engines such as the Hispano-Suiza 12 and Gnome-Rhône 14 seem underpowered?

They were competitive when designed, but France's defeat denied them the supercharger, fuel, cooling, and wartime development gains enjoyed by later Allied engines.

The panel rejects the premise that the Hispano-Suiza 12 and Gnome-Rhône 14 families were inherently weak. Their performance was broadly comparable with contemporary British and American engines before piston-aircraft power began advancing especially quickly. Better superchargers, larger and better-cooled engines, and higher-octane fuel then delivered major gains. British access to higher-octane fuel is cited as one source of extra fighter performance around the Battle of Britain. France's defeat interrupted the funding and iterative wartime development that pushed Allied engines through successive improvements. Later comparisons therefore place a largely frozen French baseline beside competitors that received years of intensive refinement.