What made the Superchargers of the Rolls-Royce Merlin engines better than the Superchargers on the Allison engines of the same era, around 1940 when the competing US fighters didn't have the high altitude performance. Did the British Superchargers have multiple stages compared to the Allison?
This thread might help. https://ww2aircraft.net/forum/threads/what-was-the-problem-with-the-allison-engine.29243/
So they had a differently designed supercharger than the Merlin. It looked like neither the Spitfire nor the P40/P39 had a turbo, but the Spitfire's Merlin had a better Supercharger system?
It's a shame, the P-39 had a solid attributes, and when tested with the high altitude turbo it was right up with its contemporaries.