In the mid Thirties, General Motors wanted to come up with a compact and powerful diesel design, as most of the diesel engines of the time that produced much power were large and cumbersome. In 1939, they finalized development of a 2 stroke diesel engine with six cylinders (among others)of 71 cubic inches each (totaling 426 cubic inches, coincidentally). It was called a “6-71”. These 2 strokes did not have a separate intake stroke, so a forced induction system was required, and the Roots blower design was used for the purpose, resulting in the GMC 6-71 blower. There were inline engines of 1 cylinder to 6 cylinders, with resulting appropriately sized blowers. In the application, the blowers did not provide significant air compression, just air movement, and the engines were considered naturally aspirated. Turbochargers are added when boost is required. There are numerous GMC engine models with a different number of cylinders and corresponding blower case length, as in the chart below. Some of the models were built by joining two existing models together, i.e. two 6V-71 joined to make a 12V-71.
The earlier Roots blowers used on hotrods and race cars were usually taken off a GMC diesel and modified for such use. The blowers used currently in the top blown classes of racing are derivatives of these units, usually using the same nomenclature like 10-71, 12-71, 14-71, as different blower manufacturers added case and rotor length as air flow requirements increased.
There are also 53 and 92 series GMC engines with blowers, but for some reason they were not adapted for performance use. They also were designated as a model of so many cylinders of 53 or 92 cubic inches.