The only American liquid-cooled engine to see service in World War II was the Allison V-1710.

Allison was an Indianapolis firm that had done well in a small way with Liberty modifications and with reduction gears for others' engines. The Allison V-1710 turned out to be the sturdy and reliable powerplant that its designers had striven for. The only thing that stood between the Allison and real greatness was its inability to deliver its power at sufficiently high altitudes. This was not the fault of its builders. It resulted from an early Army decision to rely on turbo supercharging to obtain adequate power at combat heights. Even this decision was not a technical error. A turbo supercharged Allison was as good a high-altitude engine as most. The trouble was that the wartime shortage of alloying materials, especially tungsten, made it impossible to make turbo superchargers for any but a small proportion of Allisons. Bomber engines got the priority. 

The few turbosupercharged Allisons that were made were allocated to P-38s, making the high-altitude performance of that plane its best feature. All 14,000 P-40s got gear-driven superchargers and, as a result, were never first-class fighter planes. Donaldson R. Berlin, the P-40's designer, has said that P-40s experimentally equipped with turbo-superchargers outperformed Spitfires and Messerschmitts and that if it had been given the engine it was designed for, the P-40 would have been the greatest fighter of its era. This may be to some extent the bias of a proud parent, but there is no doubt that the deletion of the turbo supercharger ruined the P-39. Had Allison's engineers been able to put the effort into gear-driven superchargers that Pratt and Whitney and Rolls-Royce did, it might have been a different story. As it was, there can be little doubt that the V-1710 had more potential than was actually exploited. 

Specifications:  
Allison V-1710-G6  
Date:  1941  
Cylinders:  12  
Configuration:  Inline V, Liquid cooled  
Horsepower:  1,250 hp (932 kw)  
R.P.M.:  3,200  
Bore and Stroke:  5.5 in. (140 mm) x 6 in. (152 mm)  
Displacement:  1,710 cu. in. (28 liters)  
Weight:  1,595 lbs. (707 kg)  

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