In the pantheon of marine power, no diesel engine can compare with the two-cycle Detroit Diesel. Designed and developed by General Motors in 1938, the engine achieved almost mythic status because its two-stroke combustion cycle (one power stroke for each crankcase rpm) gave it significantly greater power density than diesels using the more common four-stroke combustion cycle (one power stroke for each two rpm).
The two-cycle Detroits were produced in different series, the first being the 71 Series. The original engine model was the 6-71, with the first number designating the number of cylinders and the second the displacement in cubic inches of each cylinder. There were 53, 71, 92, and 149 Series with anything from four to 16 cylinders. The engines were also modular, which meant that two V-6s could be bolted together to create a V-12 and two V-8s could be combined to make a V-16.
The principal disadvantage of the two-cycle Detroit was that its combustion configuration allowed unburned fuel to pass out the exhaust port before the port closed, resulting in excessive emissions. The engines’ design also required a complex system of seals that often failed over time, causing oil leaks. (This problem was largely overcome when Roger Penske purchased Detroit Diesel from General Motors in 1988.)
The engines’ high emissions finally proved their undoing. For a while, electronic controls were able to somewhat reduce emission levels to allow the engines to meet ever-stricter regulations, but eventually the two-stroke cycle proved to be just too dirty and inefficient to meet the new laws. In 1998, MTU, which had purchased Detroit Diesel from Penske in 2006, finally ceased production of all two-stroke Detroit Diesels. (The company continues to manufacture the four-cycle Series 60 Detroit Diesel.)
Today, there are still thousands of these engines powering boats of all types all over the world. Unfortunately, genuine replacement parts for them have been either difficult or impossible to come by, leading many owners with no choice but to repower. But recently, MTU announced that it will again begin selling both genuine replacement parts and fully rebuilt and waranteed engines under its Reliabilt brand. If you have one of these old war horses and need parts or a new engine, contact your local MTU distributor.
