The EMD 645 is a family of two-stroke diesel engines that was designed and manufactured by the Electro-Motive Division of General Motors.

These same considerations apply to the 645 and 710, as these engines were a logical extension of the 567C, by applying a cylinder bore increase, 645, and a cylinder bore increase and a stroke increase, 710, to achieve a greater power output, without changing the external size of the engines, or their weight, thereby achieving significant improvements in power per unit volume and power per unit weight.

The turbocharger can revert to acting as a supercharger during demands for large increases in engine output power.

While more expensive to maintain than Roots blowers, EMD claims that this design allows "significantly" reduced fuel consumption and emissions, improved high-altitude performance, and even up to a 50 percent increase in maximum rated output power over Roots-blown engines for the same engine displacement.

Output power for naturally aspirated engines (including Roots-blown two-stroke engines) is usually derated 2.5 percent per 1,000 feet (300 m) above mean sea level, a tremendous penalty at the 10,000 feet (3,000 m) or greater elevations which several Western U.S. and Canada railroads operate, and this can amount to a 25 percent power loss.

When used solely for traction purposes, the engine speed varies depending on the throttle position.

In December 1965 and January 1966, EMD built three SD45 demonstrators (numbers 4351 through 4353) to field test the 20-645E3 engine.

All of these locomotive models extensively share common components and subsystems, thereby significantly reducing cost and increasing interchangeability.

The engine block is made from flat, formed and rolled structural steel members and steel forgings welded into a single structure (a "weldment"), so it can easily be repaired using conventional shop tools.