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The GM XP-500 was a concept car built in 1956.

The XP-500 was the world’s first free piston automobile. It was an experimental vehicle built by the General Motors Research Laboratories that was completed and announced to the public in 1956, after 14 months of development.

As it was finished just prior to the GM Technical Center dedication ceremony, it was featured at the exhibition day and on the national television broadcast that covered the event.

The XP-500 was similar in style to the gas-turbine powered Firebird II, having a similar rounded all glass roof to the car. The doors and Gull wing style side windows opened for access to the interior.

The hood sloped down from the windshield to the low, horizontal grille opening and opened to reveal the engine compartment that housed the flat Hyprex engine. The rear fenders had a wing-like quality and arched back over the wheels toward the tail of the vehicle.

The XP-500 was powered by the GM 4-4 Hyprex engine. The engine consisted of two parallel cylinders, each containing a set of two horizontally opposed pistons. An air-fuel charge fired between the pistons and the compressed air that was created by this action was piped into a turbine that helped power the drive train. This engine had an output of 250 horsepower.

Because of technical shortcomings and more promising research in other areas of alternative propulsion, General Motors halted further development on the free piston automobile within three years of the XP-500’s completion.