A flat engine is an engine with cylinders that operate horizontally and parallel with the road's surface. In a typical vehicle, the engine's cylinders run straight up and down or at an angle of varying degrees in a "V" configuration. Automobiles, such as the original air-cooled Volkswagen rear-engine car, use a flat engine design. Porsche utilizes a flat engine configuration in many of its vehicle offerings. Space savings, better weight distribution properties and cooling efficiencies have made the flat engine design very popular in many off-road vehicles as well as in some of the most popular street-driven vehicles sold around the world.
Based primarily on the tremendous sales and popularity of the flat engine-equipped Volkswagen Beetle, General Motors designed and built its own version of a flat engine-equipped vehicle in the 1960s. The Corvair, built by General Motors, was a rear engine, air-cooled, flat engine design which was readily accepted by consumers. The vehicle was offered in several trim packages, which even included a turbocharged model. The vehicle was so well-received that aftermarket companies such as Yenko Chevrolet, a US East Coast Chevrolet dealer, even built and marketed specially-equipped Corvair's with powerful engines and enhanced suspension to be used in racing competition.
While used in street-driven vehicles with great success, the flat engine really found its niche in off-road buggies and race vehicles. By using the rear-engine design, chassis builders could create dune buggies and race vehicles that did not force hot engine heat onto the vehicle's occupants. The air-cooled design of the engines allowed the vehicles to operate without the aid of a cooling system and with no fear of a coolant leak. Operators could abuse the vehicle practically beyond reason and it would continue to perform flawlessly.
In a sports car racing package, the rear engine design created a well-balanced vehicle which could take turns effortlessly. The air-cooled feature of the engine prevented any danger of cooling system failure, and the vehicles were often used to bump competitors out of the way without fear of damaging a radiator. The only major concern in racing one of these engines fell into the area of cooling the engine oil. Huge oil coolers were placed in the air stream flowing over the roof of the vehicles.
Motorcycles from the BMW to the Honda Goldwing use the flat engine design to create a motorcycle with a low center of gravity. The design allows the engine to sit lower in the frame, making the machine more easily handled. The smooth, powerful torque curve of the engine design makes it a well-suited engine for use in touring motorcycles.