Detroit Diesel 12V71

1. 11990sJarrett Bay

On record

Miss MagnoliaJarrett Bay hull 18 · 1996

General Motors introduced the two-cycle Series 71 in 1938. In the name, the first number gives the cylinders and the second the displacement of each in cubic inches.^1,3

The family was modular. Two V-6s bolted together made a V-12.¹

A two-stroke fires each cylinder once every revolution; a four-stroke fires once every two. That cycle gave the Detroit significantly greater power density than four-stroke diesels.¹

Even the engines badged for natural aspiration carried one or two superchargers to force fresh air into the cylinders.²

Horsepower was the point. Viking's engine distributor customized two-cycle Detroits for marine use, the only way to reach the performance the Healeys wanted.⁴

Hatteras offered its 60 Convertible with 650- and 825-hp GM 12-71s. Brokers call the 12-71 a very long-lasting motor with parts still readily available, and say the old Detroits are meant to be rebuilt.⁵

The two-stroke cycle let unburned fuel out the exhaust port before it closed, and its complex seals often failed and leaked oil. Emissions ended it: the cycle proved too dirty and inefficient to meet new laws.¹

Yachting calls boats with older two-cycle Detroits ideal to repower with smaller, lighter, more fuel-efficient engines.⁶