The 6-71 Gray Marine Diesel Engine is a marinized version of the General Motors Detroit Diesel 6-71 engine produced by the Gray Marine Motor Company. It was used in landing craft during World War II and is used today in private boats and training facilities.
The 6-71 is an inline six cylinder diesel engine. The 71 refers to the displacement in cubic inches of each cylinder. The firing order of the engine is 1-5-3-6-2-4. The engine's compression ratio is 18.7:1 with a 4.250 inch bore and a 5.00 inch stroke. The engine weighs 2,185 lb (991 kg) and is 54 inches long, 29 inches wide and 41 inches tall. At 2,100 revolutions per minute the engine is capable of producing 230 horse power (172 kilowatts). V-type versions of the 71 series were developed in 1957.
The 6-71 is a two stroke engine. As the engine will not naturally aspirate, air is provided via a roots-type blower. However, on the 6-71T models, a turbocharger and a supercharger are utilized. Fuel is provided by unit injectors, one per cylinder. The amount of fuel injected into the engine is controlled by the engine's governor. The engine cooling is via liquid in a water jacket. In a boat, cool external water is pumped into the engine and passed through a heat exchanger mounted on or nearby in a "double loop" configuration.
The Gray Marine 6-71 Marine Diesel was developed from the General Motors 6-71 diesel engine that was in use for on-road applications, farm machinery, and other off-road applications. Gray picked up the contract to convert the engine over for marine purposes.
The 6-71 was used for Higgins LCVP, a landing craft used in larger numbers during World War II. During the war years, about 100,000 (including the Gray-Marine variant) GMC 6-71 were built, serviced, and operated.