The story of the Merlin V12 is really the story of E.J. Houdry, a Frenchman who came to the US in 1930 and, with plenty of help at Standard oil, perfected fluid catalytic cracking of hydrocarbons.

FCC gave the US AVgas with octane as high as 150 by the end of the war. This allowed much higher engine manifold pressures than the Germans before detonation, who had mostly 87 octane fuel throughout the war. Engine power is roughly proportional to manifold pressure.

Source: a long rabbit trail of trying to understand why the Daimler Benz DB 605, while seemingly far more advanced on paper, was never better than parity with the smaller Merlin during WW2. Houdry being in occupied France at the wars' start would make an interesting alternate history.