General Motors Once Built The Holy Grail Of Buses, And It Was A Twin-Engine Diesel Beast That Changed Bus Transportation

For decades, bus fleets all around America entrusted one name to get their passengers to their destinations on time and in comfort. The GMC Truck and Coach Division once dominated the bus industry, and one of the buses that would become one of the most iconic in all of American history was the PD-4501 Scenicruiser. Built in the 1950s, these coaches, which were originally so big and so heavy that they required two diesel engines, would change highway bus travel. Now, the very first one has come up for sale.

There was once a time when many Americans didn’t board planes to ride between cities. Back in the 1930s, the popularity of passenger trains waned as steam power seemed antiquated and cars were becoming more accessible to the average American. The federal government had been funding road projects from the 1910s and had been building a national highway network since the 1920s.

But as the National Archives notes, many of these roads, especially in rural areas, weren’t paved, had rickety bridges, had no services, and permitted only single-digit average speeds. The Army in particular didn’t like this, as these roads limited its ability to mobilize quickly. Soon came a national debate about a national, standardized superhighway network. By 1940, the Pennsylvania Turnpike opened and would become the prototype for the modern superhighway.

President Dwight D. Eisenhower was a champion of the idea for a national superhighway network. He witnessed how terrible America’s roads were as an Army officer, and as the Allies marched towards victory in World War II, Eisenhower appreciated the quality and speeds of Germany’s Autobahn. Finally, after decades of research, debate, lawmaking, and planning, the Federal Aid Highway Act of 1956 was signed by President Eisenhower. Among other things, the Act authorized a 41,000-mile superhighway network.

The Interstate Highway System would go on to become one of America’s greatest projects, and with the standardized highways came an entire support system. Gas stations, motels, diners, entertainment, mobile home parks, and so many more enterprises were built alongside the Interstates. These new highways weren’t just for car owners, truckers, and the military. The Interstate Highway System primed the intercity bus for its golden era, and leading the way was a coach that General Motors and Greyhound spent seven years cooking up.

Greyhound’s Builds Up

The intercity bus exploded in popularity after World War II. In 1947, Look magazine reported that bus travel was the fastest-growing form of transportation in America. Flying was too expensive, trains had lost their luster, and many Americans still didn’t own cars. It was imperative for bus lines and bus builders to meet this demand with new, comfortable buses that reflected the optimism of the era.

Greyhound Lines was a powerful name in America back in those days. It was the first name in intercity bus travel, and had so much pull that Greyhound could reach out to bus manufacturers and get them to design an all-new bus just for the line. In 1944, Greyhound announced that it was working on a bus that carried 50 people in the same footprint as its current buses. That was an extra nine passengers, and Greyhound’s insistence that it put those people into a bus that was no longer than before wasn’t a random choice. There were state-imposed length restrictions, and Greyhound’s buses were already maxed out on legal length in most states.

Greyhound’s solution was to build up, and it convinced Consolidated Vultee Aircraft Corporation (Convair) and General Motors to build prototypes penned by famed industrial designer Raymond Loewy. Convair and GM would eventually leave the project, forcing Greyhound to do it alone. The GX-1 “Highway Traveler,” as it was called, was only 35 feet long, but carried 50 people on two decks. This all-metal bus was hilariously heavy, and there wasn’t a single diesel engine on the market that was powerful enough for the job. So, Greyhound Chief Engineer Milo Dean designed the bus to use two air-cooled V6s.

Ultimately, this design went nowhere, and Greyhound went back to the drawing board, modifying a 35-foot PD-4151 that it got from GM. Its new idea was a bus with a stepped deck design. The 1947 Greyhound GX-2 carried 37 people on the upper deck and 13 people on the lower deck.

By stepping up the deck, the bus had ample space for baggage or potentially even freight, which could help make the bus more profitable. Unfortunately, the GX-2 was 40 feet long, which made it illegal in most states, and weighed 34,485 pounds when loaded. The 200 HP 7-liter Detroit Diesel 6-71 straight-six under the metal struggled to move all of the weight. Even deleting the engine’s governor wasn’t enough.

The Bus That Changed The Industry

So, Greyhound went back to the drawing board again, and this time, it wasn’t just going to modify an existing bus. General Motors and Greyhound would join forces to not just create the bus of the future, but lobby to change the laws that forced buses to be so short. The project would take a total of seven years to reach production.

My retrospective continues on the creation of the Scenicruiser:

The bus, called the Scenicruiser, was punted to GM to finalize and its engineers came up with the PD-4501. The Scenicruiser borrowed a lot of its technology from the PD-4104, which came a year before. The new bus had a smooth air suspension, aluminum panels over a monocoque construction, large windows, a fresh design, and Detroit Diesel power. Its upper deck allowed most passengers a wide view of the open road ahead while the massive baggage compartment promised additional fortunes. However, unlike the PD-4104, the PD-4501 got really weird in the engine bay. See, a PD-4501 weighs about 29,000 pounds empty, towers 11 feet high, and these buses were built in 1954 before the advent of the relatively compact and strong diesels that buses have today. How did GM get around that? The company took two of its 4.7-liter Detroit Diesel 4-71 straight-fours and plopped them into the engine bay.

These engines were mounted side by side longitudinally and connected via a fluid coupling that was controlled by a solenoid-operated clutch. The driver shifted a three-speed transmission with a two-speed auxiliary transmission, giving a total of six gears. Greyhound says these engines power the rearmost axle and together pump out 300 HP. Torque should have been somewhere in the ballpark of 800 lb-ft. Starting was interesting as just one of the engines had a starter. Once one engine started, the other would start through the fluid coupling.

Greyhound touted the double-engine setup as not only a great source of power, but also redundancy. The bus was engineered to run on just one of the engines, so if one failed it wasn’t the end of the world. A Scenicruiser would just carry the dead engine to the next shop, get a new engine, and get back on the road. Greyhound made sure to stock engines along Scenicruiser routes just for that purpose. A special dolly was created with the idea of allowing Greyhound shops to quickly swap engines and accessories quickly. Quick-disconnect couplings were deployed all over the engine bay to make these swaps faster.

The first production Scenicruiser, No. 001, rolled out of the factory in 1954. Greyhound ordered 1,000 Scenicruisers, which were built by 1956. One more Scenicruiser was added to the fleet after the prototype, EXP-331, was converted to PD-4501 spec and given serial number 1001.

The bus of the future wowed the public, but quickly became a thorn in Greyhound’s backside. The novel powertrain system was hilariously complex and unreliable. According to the book Scenicruising: The Greyhound Scenicruiser Story, all of the first 570 Scenicruisers needed repairs due to transmission or clutch failures. Apparently, the same system that allowed the bus to run one of the engines independently was its undoing.

As Bus Conversion Magazine notes, the left engine drove the power steering system, so if that engine or its transmission coupling failed, the driver was in command of a 29,000-pound box with only 160 HP and no power steering. Losing the right engine or its transmission coupling wasn’t any better, as that one ran the air-conditioner. If an engine failed and the driver didn’t close off its pipes, it was possible for the roots blower of the surviving engine to suck in debris from the bad engine, killing it and leaving the bus entirely dead.

That wasn’t all, as the aluminum monocoque of early Scenicruisers was prone to cracking, an issue that GM would struggle with in future bus designs, too. Many Scenicruisers ended up completely sidelined, which crushed the Scenicruiser’s availability rate. Greyhound would get so desperate for a fix that it shipped buses to Europe in hopes that Mercedes-Benz had a solution.

Greyhound was so fed up that, in 1956, it publicly demanded compensation from General Motors because of the unreliable buses. Three months later, the two companies came to a settlement.

Greyhound later went all-in on Motor Coach Industries (MCI) buses. But while it still had GM coaches on hand, Greyhound invested in upgrades. In 1959, Detroit Diesel launched the 8V-71, a 9.3-liter V8 diesel that made 318 HP right out of the box, or more power than the Scenicruisers had from two 4-71s. Greyhound passed $10 million to Marmon-Harrington to upgrade the Scenicruiser fleet. The twin-engine complicated drivetrain was thrown away. Each bus was repowered with a Detroit 8V-71 (below) and a more conventional four-speed manual transmission, solving the transmission issues once and for all.

The bus would go on to become beloved by America’s traveling public. The Scenicruiser became so popular that Greyhound’s rivals got in bed with GM’s competition to make buses that looked and performed like the Scenicruiser.

Greyhound would begin to phase the Scenicruiser out in the early 1970s. The bus line didn’t like the idea of selling America’s favorite bus to its competition, even if it was old by that point. Instead, Greyhound hoped to export Scenicruisers out of America. Greyhound found few interested parties, so most Scenicruisers were scrapped. Some were sold off to charter companies, and others were sold to bus enthusiasts, who converted them into motorhomes. By 1978, Greyhound had only a few examples left in its fleet.

This 1954 Scenicruiser

This example, production unit 001 and fleet number 1954, is the first production Scenicruiser. Technically, it’s not the first-ever Scenicruiser, as that distinction goes to the EXP-331 prototype, but that bus wasn’t converted to production spec until after Scenicruiser production ended in 1956.

Greyhound held onto No. 001 for decades, and has kept it in pristine shape. The bus has even gone through at least one restoration to keep it perfect. In January 2023, the owner of Greyhound, FlixBus, decided to thin the Greyhound historical fleet, and No. 001 was put up for sale. The bus went unsold for nearly two years before being listed on Facebook Marketplace for $650,000. I wrote about this bus when it appeared for sale in 2024. Apparently, it sold for around $495,000.

Now, it’s for sale again.

From the looks of things, No. 001 looks the same as it did in its 2024 listing. But what’s so beautiful is that, this time, thanks to Mecum’s photographer, the bus comes with a set of stunning imagery. Just bask in the ethereal presence of this machine:

This bus is a time capsule in every sense of the term, from the glorious red Detroit Diesel 8V-71 in the rear to the color-matched wheels and white-letter tires. Something I’ve long appreciated about this bus is that it was not converted into a motorhome or a party bus. It looks just as it did when it was in service as a people carrier.

The Mecum auction also gives us more details. The air conditioner has been converted to R-134A, and there’s a warning in two places on the bus that the air conditioner must not be started unless the engine is running. The instrument cluster is also interesting, given its lack of a tachometer and no fuel gauge. The engine was rebuilt in 1973.

Given the provenance of this bus, I would not be surprised if it fetches around a half-million dollars. It is the first production Scenicruiser, and very likely the cleanest completely stock one left in existence. Dare I say, if I were obscenely rich, I think I’d rather have this than any Ferrari or other classic car of similar value. I mean, how many people can say they own the holy grail of coach buses?

If that sounds like something you want to do, this Scenicruiser rolls across the block on November 14 in Las Vegas. You bet I’m going to watch this auction.

Whoever gets this bus will get a real piece of history. The Scenicruiser was so influential to bus design that it essentially set the benchmark. Countless intercity buses from that point forward, and still today, have features pioneered by the Scenicruiser, like large windows, ample baggage holds, and large passenger capacities. Just the fact that today’s intercity buses can be long can trace its roots back to the Scenicruiser.

This is why I’m so happy that some regular vehicles are stuffed away and preserved. It’s so awesome to be able to see the ancestors of the vehicles we sometimes take for granted today. That boring Greyhound bus you last rode? It can probably trace its roots back to a Scenicruiser, if not even further back. I hope whoever buys this one will keep this history alive and show it off. Sure, a bus may not turn heads like a classic Porsche, but it’s just as important that this rig sticks around.