Identification
Units built for the Great Northern Railway were equipped with high short hoods and set up to run long-hood forward. Western Pacific received ten GP20s with high noses and dual control stands.
While the EMD SD24 with six axles was producing 2,400 horsepower (1,800 kW) with an engine of the same displacement, the four axle GP20 was limited to 2,000 horsepower (1,500 kW) by the capabilities of the traction motors and the direct current traction generators of the time. In appearance the locomotive was similar to a late version GP9 or GP18 (i.e., often with a low "front hood", but with the two exhaust stacks over the engine replaced by a single stack forward over the exit of the turbocharger, in common with later turbocharged EMD locomotives. An identification detail of the GP20 is the small radiator fan added ahead of the large aft fan.
De-turbo-ed GP20s, which employ 645 power assemblies, with Roots blowers replacing the EMD turbocharger without de-rating the horsepower of the locomotive, are somewhat common. As the reliability of the EMD turbocharger significantly improved, these de-turbo-ed examples became less common.
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