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Roden 816

PD-3751 'Silversides' - an intercity bus created in the mid-1930s by Yellow Coach for one of the largest coach companies, Greyhound Lines. The bus was a real revolution in automobile construction; corrugated aluminum and plastic materials were widely used in its structure, and special attention was paid to maximum comfort for passengers because after the construction of a wide network of high-quality highways between all the cities of the country during the Great Depression, rail passenger traffic almost stopped due to its inability to compete with high-speed road transport. Mass production has been going on since 1939 at the Pontiac West Assembly plant in Pontiac, Michigan, where General Motors used to build pickups and trucks. With the entry of the United States into World War II, the mass transportation of recruits at assembly points, and by the military in all countries, presented a major challenge, and to solve this problem the US decided to use the Greyhound Lines buses. As early as 1942, they carried more than 130 million passengers, both military and civilian, more than just an impressive figure, as it accounted for almost half of the country's population. After the war, Silversides continued to conduct passenger traffic within the country, and some of them were resold to third countries, mainly in Latin America. This bus is one of the symbols of the United States, along with Ford cars and Boeing aircraft, and embodies the significant technological progress that made this country great in the first half of the twentieth century.

This GM bus revolutionized the industry, and set the template for all over-the road buses to come: forward control, rear transverse diesel engine, the famous fluted aluminum “Silversides” cladding, semi-monocoque construction, high floor and underfloor luggage compartments. But its wildest feature was not replicated: a four-on-the-tree shifter and its mechanical linkage back to the non-synchronized gear box; something had to be left to improve. Let’s check it out and delve into the history and workings of its legendary Detroit Diesel 6-71 engine, which first made its appearance here.

GM’s two-stroke Detroit Diesel (“DD”) engine is one of America’s engineering marvels of the twentieth century. Designed under the direction of Charles Kettering at the GM Advanced Labs, the two-stroke principle was used in part because of the desire to have a compact and light engine for use in GM’s coaches, which dominated the industry, or would very soon. Many of you already know its operating principles, but for the uninitiated, they’re worth repeating again.

The DD is different from the typical loop-scavenged two-stroke engine, which has no valves and relies strictly on ports (openings) in the cylinder for the intake and exhaust, as well as crankcase pressure to help keep the gases flowing (typically, but not always). The DD two-stroke “Uniflow” has port openings in the cylinders for the intake air, but has two or four exhaust valves in each cylinder’s head. In order to fill the cylinder with fresh air since there’s no intake stroke, a blower is essential for its operation. Here’s a short video showing the DD two stroke cycle.

The DD engine family was designed from scratch to be modular, to be built in many cylinder multiples. The “71” indicates the cubic inch displacement per cylinder; therefor this 6-71 has 426 CID. Two, three and four cylinder versions were offered from the beginning for powering everything from smaller trucks, gen-sets, pumps, tractors, marine use, and a host of other applications where its small size and durability put it to great advantage. Later, larger multiples were also built, including V8, V12, V16 and according to one source, even a V24. The smaller 53 family soon joined, and in more recent years, a 92 family largely replaced the 71 series. But the 71 family lasted into the 1990’s, and millions of these engines are still at work in all manner of vehicles, boats and equipment. The same basic two stroke diesel design was also scaled up and used in diesel locomotives, where GM enjoyed a near monopoly for decades.