Bus Stop Classic: GM PD-4104 – The Most Dominant and Influential Bus Ever

(first posted 2/25/2017. Original pictures and some text by Paul N.)   Over the past several months, we’ve reviewed several different motor coach manufacturers that went toe-to-toe with General Motors in the intercity bus market of the 1950’s – ACF Brill, Beck, Fitzjohn, Aerocoach.  All fought the good fight, but ended up on the canvas. OK, enough boxing metaphors – let’s look at the bus that vanquished all of these challengers, and set the template for all buses to come. All hail the GM PD-4104 Highway Traveler.

Paul’s post on the 4104’s predecessor, the GM 3751 and 4151 “Silversides” coaches detail how GM first became a powerhouse in the coach field, thanks to innovative alloy semi-monocoque construction and GM’s new light but powerful 2-stroke diesel engine. That engine was so eminently well suited for buses with its transverse location at the very rear and angle drive to the rear wheels.

These and their predecessors were radically new before the war, and were given minor updates when introduced in the post-war market. They were excellent buses; well built and profitable to operate with the GM/DD 6-71 engine. But they weren’t the sweeping, all-new bus that Greyhound, GM’s largest customer, desired for the post-war era. It took a few more years for that model to be introduced – but when it was in 1953, it shook the intercity bus market to its core.

What made the 4104 so special, and how did it come to utterly dominate the market? Several factors, but first and foremost, it was just plainly superior to any other bus being offered at the time. Let’s look at the details.

Weight. Rather than use a separate body and chassis, 4104’s were fully monocoque-bodied (similar to the Gar Wood coaches we looked at in our Aerocoach post). High strength aluminum panels over the monocoque frame made for an extremely light yet rigid coach, easily able to stand-up to the constant over-the-road pounding an intercity bus endures.

Power and mileage. The relatively light weight 2-stroke 210 hp Detroit Diesel 6-71 engine, which was then an exclusive to GM coaches, combined with the light weight of the coach, made the 4104 North America’s most fuel efficient bus, and most likely one of its faster ones. A 4104 could typically get 8-10 miles per gallon of cheap diesel fuel compared to 2-3 mpg for the ACF Brill using the Hall-Scott gas engine, or 7-8 mpg for the Fitzjohn and Aerocoach which used the four stroke Cummins diesel. And it had a cruising/maximum speed of 65 mph.

The Roots-type blower on the side of the engine is necessary for a two-stroke diesel to scavenge the cylinders quickly as well as fill the cylinders with fresh air for the intake stroke.

A two stroke diesel operates rather differently than a gas-oil fed two stroke engine, as the diesel has two or four exhaust valves in the cylinder head. This shows its operation.

The Spicer unsynchronized four speed manual transmission was also more efficient than later automatics, but it was not exactly “slick-shifting”

The long gear shift lever was some 30 feet away from the transmission, so it required a deft hand. Upshifts were relatively easy, and the better drivers managed it without a clutch. Downshifts required double clutching and revving up the engine to the required speed in order to match the rotation speed of the gears on the output shaft. Pulling off a downshift without any hint of gear grinding was the sign of a superior driver.

Ride. While other coaches were experimenting with air suspension systems, GM had theirs ready for use on both its urban transit and intercity models – it provided an extremely smooth ride, no matter how many passengers were on-board, and just as importantly was very reliable.

These reinforced rubber bellow-type springs are utterly ubiquitous now on buses and a high percentage of trucks and trailers, but in 1953, this was a big deal. The improvement in ride quality was very significant, as steel-spring buses had to use quite stiff springs in order to cope with a full load. The air springs rode the same now matter how many  passengers.  I can remember riding on Marmon-Herrington trolley coaches in the early ’60s in my hometown of Columbus Ohio – these were steel spring buses and when lightly loaded, there was a cacophony of “Bam, Boom, Rattle” with lots of bouncing in the seats.

Storage. With no longitudinal frame, the 4104 had significantly more underfloor storage capacity than any other coach – more room for luggage and freight, in those pre-FEDEX days.

Appearance. Though somewhat hard to envision now, the 4104 was a revolutionary design when launched – more modern looking than any other bus (or car) on the road. It created as much buzz as the ’63 Stingray or ’66 Toronado. The full aluminum-encapsulated body and forward canted large windows became iconic design elements on buses for the next 30 years or more. Everyone rushed to imitate the 4104.


Quality. The folks who worked the line at the Pontiac Michigan Assembly Plant knew how to build a bus. The key word here is “solid” – GM buses of the 1950’s and 60’s had just a much more solid feel to them than any other coach. I can remember riding Greyhound 4104s in the ‘60s and you could tell they had lots of miles on them. Yet, they still had this “all-of-a-piece” feel.

It explains why the 4104s were still snapped up from Greyhound by smaller operators, and why they became the most favored bus for motorhome conversions for decades. They’re still popular in that role, and it’s relatively easy to keep one on the road.

Put all this together and you get a bus that gave passengers a significantly elevated travel experience while at the same time put more money on operators balance sheets – a win-win…

Their durability and cost efficient attributes were not lost on operators outside North America either.

Testament to the goodness of their overall design, the 4104 was superseded by the 4106 in 1961, which was essentially a 4104 with larger windows, an updated HVAC system, dual headlights, and the more powerful 8V-71 engine.

And the 4104’s advanced lightweight design, construction methods and even the slanted windows were all key aspects to GM’s “New Look” transit buses in 1959.

The 4104 laso had a big brother, the iconic greyhound PD-4105 Scenicruiser. They were essentially co-developed, with the Scenicruiser arriving in 1954, one year after the 4104. Unfortunately, the Scenicruiser did not enjoy the same sterling reputation as the 4104; it quickly became rather problematic, with body cracking issues and problems with the twin engines. Paul’s detailed write-up is here.

I routinely peruse various bus and transportation websites and you find nothing but praise for the 4104; “the perfect bus”, “the best bus ever built”…it still has lots of fans out there.

Over 5000 were built from 1953 to 60 – and their solid construction means many still remain on the road today.

And some have even been restored to their former all-original glory; now that’s what I like to see.



Curbside Classic: GM PD-4104 – My Dream Bus Comes To Visit, With A Formula Ford Racing Car In Its Belly

1947 GM PD-3751 “Silversides”: The First Modern Bus    PN

GMC “New Look” Transit Buses   PN

GMC PD-4105 Greyhound Scenicruiser   PN