 It was quickly evident the Mustang's performance, although exceptional up to (the supercharger's critical altitude rating), was markedly reduced at higher altitudes.This was because the single-speed single-stage supercharger fitted to the V-1710 had been designed to produce maximum power at low altitude; above that, power dropped off rapidly.Prior to the Mustang project, the USAAC had Allison concentrate primarily on turbochargers in concert with General Electric; the turbochargers proved to be reliable and capable of providing significant power increases in the P-38 Lightning and other high-altitude aircraft, in particular in the Air Corps' four-engine bombers.