(This is the chronological first chapter of the eventual CC Complete GM Coach Chronicles “CCCGMCC”. The GM Transit Coach TDH-5105 “Old Look” is here)
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, light-weight semi-monocoque construction utilizing aluminum, high floor and underfloor luggage compartments. Only one of its features was not replicated: a four-on-the-tree shifter and its mechanical linkage over thirty feet back to the non-synchronized gear box; something had to be left to improve. Bigger windows, perhaps?
First, let’s consider the setting into which it appeared: inter-city bus service once played a much more significant role than today, with numerous companies competing. And until the late thirties, buses were generally built like this; a Yellow Coach on heavy truck-type frames with the gasoline engine up front. Luggage rode on a roof-top rack.
That all changed with the 1936 Yellow Coach (owned by GM) Model 719 Super Coach, a groundbreaking design. It was conceived during that very creative mid-late thirties period, when traditional approaches to cars, buses trains and airplanes were all being tossed overboard. Yellow, encouraged and partly financed by Greyhound, decided to reinvent the bus.
Dwight Austin, who had designed the ambitious unsuccessful double-decker Pickwick Nitecoach, was hired by Yellow/GM to head up the effort.
Austin brought with him his patented angle-drive system, which allowed the engine to sit transversely at the very rear of the bus for maximum space efficiency and accessibility. The new Model 719 also featured a semi-monococque (self-supporting) construction using aluminum to save weight, and large underfloor luggage compartments.
The engine was a gasoline GMC 707 CID six, as GM’s new diesel engine wasn’t quite ready yet. But by 1938 it was, and in 1939 GM restyled the 719 with the then fashionable fluted polished aluminum “silverside” cladding.
The also groundbreaking Pioneer Zephyr of 1934 introduced the stainless steel fluted cladding, which came to typify streamlined trains and modern buses until just the last decade or two. Needless to say, it was also a mighty durable exterior finish. And the Zephyr also pioneered GM’s Elelectro-Motive Division two-stroke locomotive engine, which went on to revolutionize the train world.
Let’s swing open these beautiful aluminum louvered doors on the back, to expose that famous Detroit Diesel 6-71 engine.
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.
The DD arrived just before WW2, and it quickly proved itself in the grueling conditions it was subjected too.
Conveniently exposed here to show one of its two overlapping lobed rotors, the 6-71 “Jimmy” Roots-type blowers soon found a new role as superchargers on dragsters and hot rods. In that application, they were overdriven to provide large amounts of boost; in the DD engine, they provide just enough of an increase above atmospheric pressure to evacuate and fill the cylinder with fresh air. Later versions also had turbochargers, which fed through the blower and increased power output. The blower couldn’t be eliminated though, because its boost is needed to start the engine and at idle.
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 bus’ 6-71 has 426 CID. Two (shown here), 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 even a V24 (shown here). 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 1990s, and millions of these engines are still at work in all manner of vehicles, boats, pumps, gensets, and other equipment. The same basic two stroke diesel design was also scaled up and used in submarines and diesel locomotives (CC here), where GM enjoyed a near monopoly for decades.
What finally put it out of production was its slightly lower efficiency than the four stroke diesel. This was a small price worth paying in exchange for its compact dimensions and light weight. Higher fuel prices in recent years finally sealed its fate, but tightening emission standards would have likely been impossible to meet as well. These 6-71 powered coaches were able to get up to twelve miles per gallon. With a full load, it was probably one of the all-time most efficient people movers (mpg/passengers) to ever roll down the highway.
Because of cheap fuel prices, diesel engines caught on slowly in the US. Initially, there were really just two common diesel engines for automotive (truck, etc) use in the US: the DD and the larger and heavier Cummins four stroke. And into the seventies and eighties, the two of them along with the Mack four stroke duked it out in the truck sector. But the DD was always instantly recognizable by its distinct exhaust howl, which sounded like it was revving twice as fast or more than the grumbling four strokes. But then all two strokes sound like they’re running twice as fast, obviously because they have twice as many exhaust strokes at any given engine speed. If you’ve ever heard a DD without a muffler, its scream will haunt you forever.
If you haven’t, here’s a video of the DD V12 in the hot rod Peterbilt above. As the owner points out, it may sound like it’s running at 6000 rpm, but its actual redline is 2500 rpm.
The next trick was to get the 190 hp or so that the early 6-71 made to the wheels. It’s torque being substantially more brutal than the gasoline engines it replaced, initially there were experiments with a diesel-electric drive, and a torque converter. Complications and efficiency losses with both led to a mechanical drive, an unsynchronized four speed, shifted by what has to be the biggest and gnarliest column shifter ever.
This picture is spoiled by the heavy tinting on the window and the light coming in the other side, but there it is, the black knob on the end of the shifter. Just try to imagine the mechanical linkage going back thirty-five feet to the transmission. Because of the challenges of the linkage, reverse gear was engaged by a solenoid, which can be clearly seen on top of the transmission along with the ends of the linkage (below). And check out that awesome art-deco driver’s seat.
You’re looking at the output end of the transmission and the angle drive that now sends the power forward to the rear axle. The working ends of shift linkage is visible, as well as a solenoid that engages reverse gear.I drove big transit buses, but never had the pleasure of trying to shift one of these stick versions. But I assure you, if you rode in these old buses, the shifts were very slow in coming, and if the driver hadn’t mastered the art of double clutching, down shifts never did happen, which could be deadly on a long steep downgrade in the mountains.
Since I’m snapping away, might as well get the output end of the angle drive and the short drive shaft to the rear axle, which had an offset differential. Also very visible are the big leaf springs, which was the one old-tech artifact. The next generation of GM buses, the almost equally revolutionary 1953 PD-4104, pioneered air suspension. The difference is huge; we had one old leaf spring transit bus, and it rode like a cart compared to the floating air-rides.
These 35′ coaches came in 37 or 41 passenger configuration, depending on how much leg room was desired. This obviously is the deluxe service version. Even a few 2 + 1 seating luxury versions were made. On-board lavatories were still a couple of years away, but air conditioning was available, which required its own engine, a Continental flat head four cylinder gas engine, to run the compressor.
This series of buses was made exclusively for Greyhound, but GM also made variations of the theme for all the other operators who clamored for them. If you look carefully at our featured bus, the outline of the greyhound logo is still visible. GM came to have an 80+% share of the bus market in the fifties, and was at risk of being broken up by the government, and GM made their engines and the later Allison automatic transmission available to the competition. That power train combination totally dominated the bus market until fairly recently.
Numerous problems with the Scenicruiser and other frustrations of dealing with a near-monopoly drove Greyhound to buy the Canadian bus builder MCI, and never looked back. Other builders eventually found their footing, and as GM’s market share plummeted in the seventies, they lost interest and pulled the plug in 1980.
This particular bus, like so many other old GM coaches, has been converted into a motorhome. It lost water heading back from Nevada, and damaged its engine, which is in the final stages of being replaced. With a freshly rebuilt 6-71, it should be good for another million miles or so.
A minor nit, but the fluting on the Burlington Zephyrs was not cladding, but an integral part of the monocoque stainless steel carbody. Edward Budd invented fluting to give sheetmetal added strength. I was fascinated by the process of creating fluting at the Budd Company which I went to fairly frequently in the mid-’70s. A coil of flat stainless sheet was fed into a progressive rolling die by one guy. This operator would advance the sheet inches at a time, all the while tapping in shims to add “camber” to the piece. In effect, this created a three dimensional piece with surface development in two different planes. It’s no coincidence that the Citroen H van looks as though it has fluting as Andre Citroen and Edward Budd had a close commercial association. The Citroen’s panels, however, were stamped sheetmetal.
I loved the sound of the Detroit Diesel two-strokes. Back in the mid-’60s it was common for semis to run without mufflers. After attending my first Indy 500 in 1964 and hearing the Ford four-cammers, it was deja vu all over again. I had been hearing this wail on Rte 22 in western Arkansas where my high school was located. For me this was the siren call of the open road.
I beleve I’m accurate in saying that Pullman-Standard used the fluted cladding, as Budd had patented the shotweld condtruction. Pullman could only copy the look.
Either way, the look was and still is a winner.
Memory fails me somewhat, but I never cared much for buses, but I would like to think I got to ride in a classic Scenicruiser in late 1969 after arriving at Beale AFB on one of my numerous trips between Marysville and Sacramento, CA. If I’m wrong, it was the next generation cruiser or something…
In an effort to mimic the look of the Budd streamliners in the late 1930s, Pullman-Standard added stainless fluting on the flanks of the cars it built. It was purely cosmetic as the stainless panels were not stressed. But the stainless behaved like stainless and did not corrode. It remained bright and shiny while trapped moisture in the sidewalls ate the carbon steel structure away. In less than five years the cars were totally rotted.
Quite right. I knew that, but glossed over the details. Quite a revolutionary process.
does anybody know the RAL color code from Greyhound Blue?
The Army used unmuffled Detroit Diesel 8V71T engines in their M107 guns, M110 howitzers, and M578 light recovery vehicles. The exhaust from the turbo was a short elbow about 6 inches in diameter. Hearing anyone of these running at full throttle was an experience not to be forgotten.
The attached photo is of an M110A2 self-propelled eight inch howitzer. The round holes are ports through the exhaust exits the hull.
There’s some additional background on the engines…I can’t add; but I’d love to know. The Electro-Motive Division, the locomotive builder, started as a separate company in the 1930s. Both it and the Winton Engine Company, which built diesels for EMC, were purchased by GM. And there, I understand, came the roots of the GM two-stroke industrial diesels.
There were many differences from truck diesels: first, the block was of welded construction. Second, two-stroke with Roots blowers. Third, they were never designed for ethylene-glycol antifreeze…at first because it wasn’t needed, as diesel locomotives never were shut down, given their service demands; then, they were kept indoors or idling out of custom and because there was no pressure to redesign the bearing surfaces to withstand exposure to antifreeze compounds. The problem exists to this day; and only new or recently rebuilt locomotive engines can use ethylene glycol coolant.
Where does the smaller Detroit Diesel engines fit in here? Were they copied, partly or completely, from the Winton designs? Obviously the two-stroke layout was.
Which came first, and who copied who?
Just off the top of my head, I think the Detroits were originally designed by a marine engine company that GM purchased. Gray (or Grey?) Marine if I recall.
Late ’30s time frame. When I was in trade school I recall we worked on very old 6-71s with a “battle” position on the governor, we were told they were surplus landing craft powerplants.
My uncle’s Chris Craft cabin cruiser had twin Gray Marine sixes. He told me that they were sourced from Chrysler. Lots of great memories floating on the Mississippi just below Lock and Dam 17. Suckin beer, watching the sun set. Damn, life was good!
No; designed by Charles Kettering at the Dayton-GM labs (his Dayton Engineering Labs, sold to GM in 1916). Winton was building distillate (kerosene) engines, and Kettering and his crew developed the big GM two-stroke diesel engine, for marine, submarine and then train use. And then they downsized it for the 71 family. Asked if developing the famous two-stroke diesels represented any unusual challenges, he said “I don’t recall having any trouble with the dipstick”
I remember seeing Piss’d Off Pete in an issue of Popular Science a few years back.
The Flxible Clipper buses seem to have been developed around the same time as these, but I get the impression they have a longitudinal engine rather than the transverse with angled driveshaft like these. Another difference is the luggage is in a rear compartment rather than underneath. I saw a group of Clippers a few weeks ago, as there is a very strong club for these buses out here – not so sure about the GM buses.
Regarding the ‘template’ for buses, there is certainly a divide between these long-haul coaches and the urban mass-transit type. There is an issue though that the passenger compartment floor of the former type, which most general charter bus companies run for flexibility, seems to keep rising as they chase more luggage capacity and bringing with it steeper steps which makes access more difficult. My grandmother skips a lot of bus outings because she finds it so difficult to climb on and off the bus half a dozen times a day.
Finally, the 2-stroke diesels have such a unique sound. In a bus it would be less of an issue, but it must get a bit wearing in a normal truck.
Yes, the Clippers were smaller, had longitudinal gas engines (Buick straight eights, among others), and the luggage was in the back. They were specifically designed for routes with lower passenger loads, one step down from the GM coaches.
There’s a superb Clipper in town, that is driven around, but damn it, I keep either not having the camera ready, or am going the other way. One of these days I’ll catch it.
There was a bus outfit around here – Cherry Stages – that ran a lot of shipyard worker buses in the late 60’s (and probably also earlier, but that’s when I became acquainted with them.) Most of their rigs were Flxibles, and I remember people saying it was impossible to find any Buick straight 8 engine in a wrecking yard around here because Cherry bought them all.
Paul what town or area have you seen the Flxible?
In Eugene. Some twenty years ago, when we first moved here, it was being used as a charter bus. In fact, my daughter’s class took a trip in it, and I got a look at it at her school.
They stopped doing charters, for one reason or another, and either restored or at least repainted it on the outside. It’s gorgeous now. And I’ve seen it a couple of times in town on the go, but have not caught up with it. One of these days.
Our US Coast Guard 40 foot steel utility boats were powered by twin DD 6-71s. Straight pipes with no mufflers right out of the transom. These boats were phased out in the mid to late 70’s. A few are still floating around, some restored in their original Coast Guard paint jobs.
One 40 footer in particular, the pilot boat Ranger, worked for the Delaware Bay pilots association in Lewes, Delaware. In the mid 1980’s while stationed at the Lewes Roosevelt Inlet Coast Guard Station, I would often sit outside on a hot summer night as the Ranger passed the station, heading deep into the Delaware Bay to drop off a pilot or make a delivery to an anchored tanker. It was amazing to hear those engines so far up into the Delaware Bay, on the blackest night that the bay could offer. 5-10-15 miles up until the music faded away. Music to my ears indeed! Something comforting too, listening to an old CG boat, still on duty after all those years.
Paul, I just want to again say what a pleasure it is to come here everyday to read and learn about these wonderful vehicles, the engines and the people behind them. Reading about the GM buses and the DD engines brought back a few memories of my CG time spent around DD’s……….for example (and someone please correct me if my memory has this backwards:, never adjust the fuel rack to the No-Fuel position. It can be done and our merry bunch of young CG mechanics just stared in bewildered horror as the port engine on one of those 40 footers immediately went to full fuel upon start up. And stayed there as a runaway diesel. Nothing more frightening to be around than a runaway diesel engine! I recall I dove into the forward cabin, not before watching some of my cohorts vault over the side of the drydocked boat. A senior MK had the sense to grab the CO2 extinguisher from the grasp of a panicked young MK and doused the air intake until she starved of air and went quiet. The final tally was the rear shaft strut completely ripped away from the steel hull as somehow the marine transmission was in gear while all the commotion was going on. And a bunch of young kids scared silly!
My nephew is in the CG, and about ten years ago, his first posting was at Cape Disappointment, on the mouth of the Columbia River. He showed us around, including a rather older steel boat, a 40 or 42 (quite likely the same as the one you described), used mainly just for towing stranded boats, and it too had a couple of 6-71s in the engine room.
Thirty feet of treeshift linkage OMG crash box or not that mustve been a mutha to drive glad I aint got a passenger licence
I just took a bus trip to the Falls with a group of our students. Our bus was an `88 MCI with a 6-92. It was a great trip having that DD as a soundtrack.
Great article. I did not know that the DD was two stroke and that there was an angle drive configuration. I grew up in a very small town which contracted with a guy to bus us to the high school about 5 miles away. He would buy up these old buses to do the job so we would climb aboard and relax in the reclining seats. What a treat that was!
Fully loaded to the max legal weight the OO-92 Detroit Diesel equipped tractor pulling a 40-foot ocean-going shipping container behind it waltzed up the steep on-ramp onto the San Francisco-Oakland Bay Bridge, ensuring the timely arrival of an enormous number of Anchor Steam Brewery beer bottles.
My free case sat in the sleeper for later imbibing.
The amount of raw power was intoxicating.
I estimate that around 15 megatons of torque applied to those 8 wheels may have led to the eventual Loma Prieta earthquake a few years later and either slowing or increasing the planets rotational speed.
That interior shot up above is just amazing. There was some real style there compared to my last Charter Bus experience (in a 90s MCI).
And doggoneit! The passengers aren’t dressed like “Larry the Cable Guy”.
I went back to visit the Parents Sunday. Walking around town I swear I could hear the ghost Growl of all those DD powered trucks and ICG “Piggyback” Locos that I grew up listening to day and night.
One correction – I just acquired one of these & the air conditioning units were Carrier and powered by a Continental 4 cylinder flathead gas engine, not a diesel. At first glance it looks a lot like motors I’ve seen in compressors, airport tugs and forklifts. Interesting setup – the a/c takes up the better part of two baggage compartments on the left side. The compressor is a big, beefy v-setup, either 2 or 3 cylinder.The condenser is on a hinge and swings out so you can get to the motor. From that point you can easily get to the plugs, distributor and so forth. Any major disassembly of the motor though, looks like a nightmare as there’s almost no clearance above it.
Thanks for the correction. That actually makes sense. I wonder if the later PD 4104 used a small diesel for its A/C, as I seem to remember hearing it running while the bus was stopped.
The Budd Rail Diesel Car or RDC was the railroad equivalent of diesel bus. Powered by 2 671’s and having stainless steel cladding they were a bus on rails. I rode in one in 2003 from Portland to Astoria as they were running for the 200th anniversary of the Lewis and Clark Expedition. My wife and my children are related to Lewis which made it especially memorable. Me. I am just a commoner with no such illustrious ancestors.
I used to go to sea from time to time on a 95-foot workboat that was powered by twin V12-71s. They were right behind the bulkhead of one of the staterooms, and I got used to falling asleep when they were droning away for hours on end, but would wake up when they stopped. The chief engineer, in those days before required hearing protection answered anything anyone said to him with “What??” And as recently as 10 years ago, before transit buses went to propane-fueled 4-cycle engines, I could always hear them pulling away from a stop sign 5 blocks away.
Great article and wonderful comments, gentlemen! I am perusing the web site this evening. Please ntoe that Alexander Winton, a Scottish immigrant, lost interest in building his fine Winton automobiles in 1913 which is the year that Rudolf Diesel came from France to promote his invention (invented in the late 1800’s). By 1923 or 1924, the last Winton automobiles were built. However, Winton was producing the two-cycle Diesels that bore his name. He died in 1932. The company became known as Cleveland Diesel. The engine that powered the first Diesel locomotive (as seen above) had the Cleveland Diesel name on it, not Detroit Diesel. It left Denver on its maiden run and traveled to Chicago. Total fuel cost was $13.65. thus, Alexander Winton, even postmortem, played a major role in the demise of steam locomotion. GM later bought Cleveland Diesel and changed the name to what we now know – Detroit Diesel.
There is a small paperback book on the life of Alexander Winton that was written by a man who bought one of the Winton manufacturing plants in Cleveland. He took an interest in Mr. Winton after viewing artifacts in the building that he purchased.
Workin on my Dad’s old 1947 GM PD-3751, the engine runs fine but we tried to pull it out of the barn the other day and there is a slight incline on the road in front of the barn, we shifted it into gear and started to drive it out of the barn and up the incline and despite all of our attempts it wouldn’t roll any farther forward as soon as it got to the incline. Any thoughts what is going on? It was already in gear and there was no grinding even as we revved the engine to try to get it to go up the very small hill. It would only drive forward and reverse on flat surfaces.
Any success with your bus since you posted? I can only suggest one source of mechanical,advice from a traveling Detroit Diesel bus mechanic. He has a YouTube channel and a Patreon page, and he goes by the name “Bus Grease Monkey”. You can contact him for advice and all manner of bus repairs.