Truck History: 1939 GMC – First Year For The Legendary “Jimmy” Detroit Diesel — It’s Still Being Built 85 Years Later

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– Posted on January 3, 2022

The classic Detroit Diesel has become not just legendary, but iconic. Thanks to its two-stroke design, it always sounds like its revving twice as fast as it really is, and that along with its blower resulted in its nickname “Screaming Jimmy”. The -71 family—named after the number of cubic inches per cylinder—firstfound its way into the 1938 GM/Yellow Model 719 coach and in 1939 was regularly available on mid and heavy-weight GMC trucks, which also got a handsome restyle that year too.

The Detroit Diesels were exclusive to GMC for its trucks and buses for almost two decades, giving them a competitive advantage. Although the two-stroke DD engines are no longer available for trucks and highway equipment, they are still being built new 85 years later by Detroit Diesel/MTU for military applications. Factory rebuilt units, parts and service are also widely available, so expect to hear the unmistakable scream of the Jimmy well into its second century.

These two engines are the Adam and Eve of all Detroit Diesels. They were the first two-stroke diesels built by GM’s Winton Engine subsidiary, for the 1933 Chicago Century of Progress Fair, where they powered GM’s large exhibit. They were the result of a lengthy effort by Charles Kettering, director GM’s Research Laboratory, to develop a lighter, faster-running diesel engine with an eye to railroad and marine use.

There were a number of obstacles, including the requisite high-pressure unit injector and adapting a suitable type of blower (Roots), which was necessary for the two-stroke diesel cycle to blow in fresh intake air and force the exhaust out through the valves all in within just about half or less of one stroke. The result was the 600 hp eight cylinder engines as used at the 1933 exhibit, which were essentially test units as there were still considerable reliability issues to resolve.

Burlington Railroad President Ralph Budd, who was in the early stages of developing a radical new lightweight streamlined train, had heard about their development. Despite GM’s reluctance, he convinced them to sell him one of the first of these engines, which powered his 1934 Zephyr to unheard of levels of efficiency and speed. The GM/Winton 201 engine would soon be powering a number of locomotives, and its successor engines gave GM’s EMD division practically a monopoly on diesel electric locomotives for decades to come. Here’s a more detailed look at the development of GM’s first generation of these large diesels.

Step two was to create a family of significantly smaller engines utilizing the same basic operating principles, for transport use as well as many other applications. The development of what became the -71 family began in 1934, and by 1937, it was ready. It offered unparalleled power-to-weight ratios compared to the larger, heavier and slower-running four stroke diesel engines of the time.

Some 400 units were built in 1938, with GM’s very advanced Yellow Coach Model 719 being the primary recipient. Its rear-mounted 6-71 DD made 165 hp—also available in GM’s transit buses—making it the first truly competitive diesel coach engine against the popular Hall-Scott gas powered buses. In terms of fuel economy there was no comparison. The DD-powered GM coaches quickly came to utterly dominate a market that had been very fragmented, with dozens of medium and smaller bus builders. By the late ’50s, GM highway and transit coaches practically owned the market.

In 1939, the Jimmys were available in a wide range of GMC trucks, from 3.5 to 8 ton ratings. There were two versions each of the 3-71 and 4-71, with 83.5 and 110 hp respectively, as well as economy versions with smaller injectors, rated at 65 and 86 hp respectively. The 165 hp 6-71 with a very stout 525 ft.lb. torque rating was also available on special order, and then standard on all 900 series trucks starting in 1940. All of them made their maximum rated hp at 2,000 rpm.

It’s helpful to keep in mind that over-the-road speeds of trucks back then were typically in the 30-40 mph range, but with the advent of the higher-powered diesels like the 6-71, speeds pushed up into the 45-plus mph territory.

Here’s some period ads extolling the virtues of the new Detroit Diesel.

Diesels were just starting to make some headway in highway trucks in the 1930s, although the difficult economic situation of the extend periods of recession put a damper in the investment of new trucks. By 1939, the time was right, in more ways than one.

The compelling story of the greater efficiency and much lower fuel costs of diesel fuel was increasingly harder to ignore. This trucker swapped out the gas engines in his existing GMC trucks. This was not uncommon at the time.

GM’s modular family of -71 diesels allowed them to cover a wide range of power needs, on the road or off, in applications from pumps, generators, off-highway equipment, etc.; anything as long as it wasn’t in direct competition with a GM division’s products.

That led to a lot of complaints, given GM’s dominant market share in so many segments. A federal consent decree in 1956 finally forced GM to sell the DD engine to competitors, including truck and bus manufacturers. The DD quickly became almost universal in transit and highway buses, and steadily increased its market share in trucks, like this 1958 Kenworth. I don’t have ready market share stats available, but in the sixties and seventies, the DD was very popular, duking it out with Cummins for leadership in the heavy duty truck engine market. That directly lead to a number of smaller independent diesel engine makers bowing out, including Hercules, Buda, P&H and Waukesha.

Here’s a couple more of these early pre-war GMC trucks. This one, like the one at the top, has the 4-71 version, the tell-tale being their regular-sized hoods. 110 hp doesn’t sound like much, but it was about the same as GMC’s mid-sized six cylinder gas engine of the time, the 361, which was rated at 118 hp but had considerably less torque than the 4-71 (284 vs. 350).

This GMC, from 1941-1942, has the 6-71, evidenced by its extended hood in the front to accommodate it. GMC ‘s largest gas engine at the time (1941-1942), the 477 six, was rated at 152 hp and 385 lb.ft. of torque, both less than the 165 hp and 525 lb.ft. of the 6-71.

The GMC COE (Cab Over Engine) line also was available with the diesels, although I couldn’t readily find a vintage shot of one, as this one is missing a vertical exhaust stack.

Total maximum GCVW (Gross Combined Vehicle Weight) of the largest of these GMC trucks was 90,000 lbs, which is more than the current baseline US federal maximum of 80,000 lbs (a number of states have higher exceptions to that).

The Detroit Diesels were quickly conscripted for service during WW2, which required a significant expansion of their production facilities. There was of course an almost insatiable need for power plants of all sorts during the war, and the DD quickly found itself in a number of applications.

By utilizing extra large 90 mm injectors, it was possible to increase the normal 165 hp of a military marine 6-71 to 225 hp in “Battle” mode for a (hopefully) short duration.

Its compact size made it suitable to be packaged in multiples, most common in this twin setup used in a variety of marine and tracked vehicles.

This was of course long before the 12V-71 was developed, which simplified high power tasks.

There was even this quad power package.

It was a foreshadowing of the ultimate member of the -71 engine, the 24V71T, this one sporting four turbochargers.

Turbos and four valve heads became available in 1957, and the 6-71’s power rating went up to 190 hp (non-turbo) and 236 hp with turbo. Later versions with larger injectors and higher maximum engine speed upped that, to 238 hp (without turbo) and 270 hp with turbo. In later years, marine versions like this 6-71 TIB (Turbo Intercooled Bypass blower) outfitted by one of the marine suppliers was rated at up to 485 hp, at close to 2600 rpm. Keeping one healthy required careful attention to the cooling system, as excess heat was the enemy of longevity.

The Roots blower on all Jimmys did not make them “supercharged”, as they only created the necessary “draft” to properly scavenge the cylinder, not an increase in actual atmospheric pressure therin. Given the limited duration of the compression cycle on these, even turbos did not add all that much actual boost; by blowing into the blower’s inlet, they mostly allowed the blower to “freewheel”, since it did not have to work to create the necessary pressure. That alone added much of the additional power of the turbo versions, as these blowers easily required twenty or more hp.

The DD diesel family was soon expanded in both directions. The larger (110 Series) was essentially a scaled-up 71 series. In 1950, the smaller 51 series arrived, and it made do without any exhaust valves at all, operating on the loop-charge principle—not unlike most two-stroke gas engines—except that its fuel was of course injected directly into the cylinder.

Here’s a cross section of a 4-51 that shows both the intake and exhaust ports (right side) and the pushrod-actuated unit injector at the top.

This system limited their use to non-automotive applications, as they did not operate as successfully over the wider range of engine speeds required in that type of use. They were used as pumps, generators, other stationary applications and in marine use where engine speed varied little.

In 1957, the -51 series was replaced by the -53 series, which was essentially a scaled-down -71 series. Like the -71 series, it came in a large variety of cylinder multiples, including this 8V-53. Apparently there were some 25 12V-53 engines built, but few have survived. Given the overlap in power with the various -71 engines, it’s hard to imagine what the point was; most likely it was a military installation that had very specific space requirements, since the -53 was a more compact engine.

The 3-53 and 4-53 were widely used in medium sized trucks, mostly GMC. Studebaker used these two when it decided to pioneer diesels in the low-medium truck range, offering the 3-53 even on one ton trucks, the smallest at the time.The -53 series was very widely used in marine applications as well as all sorts of industrial ones. Like the -71 series, they were compact, powerful in relation to their weight, durable, and relatively easy to fix if something did go wrong, given that all these DD engines used cylinder liners. That allowed for quick in-frame replacement of worn liners, pistons and rings.

With ever increasing demands for more power and higher truck and bus speeds, the Series 92 arrived in 1974, in V6, V8, V12 and V16 forms. They were essentially Series 71 engines with larger bores, sharing the same basic architecture and physical size. These engines soon replaced the -71 as the engines of choice in buses, where they were still highly dominant, as well as trucks and the usual wide range of other applications.

As with the 71 Series, power outputs varied depended on the size of the injector nozzle as well as whether it was turbocharged. The 6V92 came in 270, 307, 322 and 335 hp versions; the 8V92 with 360, 365, 430 and 435 hp.The 6V92T was marketed as a more fuel efficient alternative to the then-popular 8V-71, offering similar power with a 10-15% improvement in efficiency.

In 1987, DD introduced the Series 60 four cycle inline six engine with full electronic controls. It was the beginning of the end for the two-stroke engines for on-highway use, and by 1995, the last “Screaming Jimmys” were sold for that purpose. There were two main reasons: ultimately, the four stroke diesels were more efficient, thanks to their longer intake and power strokes. Also, two stroke diesels are more challenging for emission controls, essentially for the same reason. And four stroke diesels can be turbocharged to very high boost levels, allowing the higher outputs of modern diesel engines. All of the early advantages of the DD two-stroke diesels ended up being negated or surpassed by modern four stroke diesels.

The only place were two stroke diesels are still unbeatable is in the largest marine applications, like this Wärtsilä-Sulzer RTA96-C, the world’s largest diesel engine. But there are a number of unique features that have optimized its efficiency that are not feasible in transport-sized engines, including using each piston’s down stroke to compress the intake air for the neighboring piston, as well as an extremely long stroke in relation to its bore.

Even then, Wärtsilä’s smaller four stroke marine engine, the 31, is more efficient by a small margin. Both of them can exceed 50% efficiency.

GM exited the on-road market via a joint venture with Penske, who eventually sold out to Daimler. The two-stroke diesels, by then only built for off-highway use, ended up with MTU, and sold to the Swedish firm Tognum in 2005. Tognum was bought back by Daimler and Roll-Royce Holdings, who own it jointly. MTU continues to build a limited number of new two-cycle DD engines for military applications as well as offering a full line of remanufactured engines, parts and support for classic Detroit Diesel engines. It would not surprise me to see it still being built on its 100th birthday, a feat unlike any other engine, to the best of my knowledge.

I’ve been a bit obsessed with Jimmys ever since I first heard one as a kid, which left an indelible aural memory. And as a young adult, I drove a Jimmy-powered city bus for almost  year. It’s not just their ability to “scream”, it’s also the distinctive throb idle and the way they sound as they’re revved. My obsession has recently been (self)ignited by some videos by Bus Grease Monkey, who goes around the country in his awesome 1940s 6-71 Silversides bus, helping folks to get old DD hulks running again. This video is pretty typical, inasmuch as it usually doesn’t take all that much.

These diesels are very simple devices, with no ignition or carburation systems, and of course no electronic control systems, although the later DD’s did have those, to the consternation of old-school mechanics. There’s no doubt in my mind that folks will still be waking up long-dormant Jimmys and getting them to scream again well into their second century.

 

Postscript:

Although the 1939 trucks were the first regular production GMC trucks with DD diesels, there appear to have been at least two earlier applications of a diesel in a GMC truck. I ran across this photo at a forum, of a 1934 vintage T-61 fitted with a Hercules four cylinder four-stroke diesel. There is nothing on it in my “GMC Heavy Duty Trucks 1927-1987” book, and it was most likely either a test bed or a customer that requested it.

In that book I did find this photo that claims that this older T-95 was fitted with a “GM Model 546 diesel engine” as a test bed. Since there never was a Model 546, I assume that caption is meant to say “567”, which was the EMD locomotive engine that replaced the original Winton 201 in 1937. But these were enormous engines, with 567 cubic inches per cylinder. The engine is apparently a four cylinder, and without a doubt, not a version of the 567 family, which wouldn’t begin to fit in this engine bay, even a one cylinder version.

It might have been a competitor’s engine used for bench-marking or possibly it might have been an early prototype  of a DD 4-71, but it looks different then they ended up being. By 1937, the -71 Series engine was finalized and going into pre-production. Given that it’s in a working truck, the caption might well be wrong, and that it’s got a diesel engine from another source.

 

Related reading:

The Birth of the GM Two-Stroke Diesel – Excerpts from “My Years With GM” by Alfred Sloan

Curbside Classic: 1947 GM PD3751 “Silversides” Coach  – The First Modern Diesel Bus

Trackside Classic: 1955 Union Pacific EMD E9 – The Last Of The Classic Diesel Streamliners

1959-1961 GMC DLR/DFR 8000 “Crackerbox” – GM’s Deadly Sin #32