Paul mentioned in his most recent PD 4104 post that we had yet to review the GM PD 4106; the 4104’s immediate successor. Indeed – let’s correct that oversight, and take a quick look at the coach that had the enviable task of replacing the “most dominant and influential bus ever built.
So, how do you replace an icon? GM took the smart approach – “go slow” and fix any noted weaknesses while improving upon its many strengths.
GM PD 4104 (top) and 4106 (bottom)
Strength – Styling. The 4104 was a major styling sensation when it was introduced in 1953 – and was still a styling leader seven years later. But some trends had moved on – for the 4106, GM added a pair of headlights in front, larger windows all around, and “squared up” the front and back of the coach. The rear-side emergency door on the 4104 was replaced by a single window, as the new windows could all be used as emergency exits. It was a subtle update – and I imagine many bus riders barely noticed.
Strength – Passenger Comfort. The 4104 used the revolutionary air-bellows suspension which regulated ride firmness based on load, while most other coaches continued to be suspended on heavy steel springs – and the 4106 retained this feature. But it did upgrade another key area – the HVAC system.
The DD 6-71 diesel engine, as we’ll highlight below, was adequate to power the 4104, but didn’t have the capacity to run any accessories. As such, the HVAC system was powered by a separate “pony motor” – a small gas engine that sat in the forward luggage bay – which was noisy and more complex. The larger 8V-71 engine in the 4106 could run the A/C compressor and solved that problem, and in turn, the HVAC system was more quiet, reliable, and efficient.
Weakness – Power and Speed. It’s hard to overstate the importance and significance of the GM Detroit Diesel two-cycle 71 series engine. If you’ve yet to read Paul’s PD 3751 post which has an in-depth feature on this engine, I encourage you to do so. The 6-71 engine as used in transit and highway buses was a game-changer. While it was more efficient compared to concurrent gas engines and lighter than four-cycle diesels, its power output was a bit marginal, especially as bus size and speeds increased.
As a result, the 4104 and the urban transit “Old Look” coach that also used this engine, were on the slow side by the end of the ’50s. Even with optimal gearing, a 4104 worked hard to reach sixty-five mph and to stay there. The larger 8V-71 engine in the 4106 addressed that power deficiency. The 6-71 made 210 hp and 611 ft lbs of torque, while the 8V-71 as used in the 4106 was rated at 272 hp @2000 rpm and 770 ft. lbs. @1200 rpm. But Greyhound and many operators had them governed to a maximum of 1650 rpm, at which speed they made 235 hp. It was common though to increase the governed speed.
In 1966, with operators looking for more underfloor storage space, GM essentially lifted seven-eighths of the 4106 body twelve inches and created the PD 4107 – the GM “Buffalo”
GM PD 4106
Flxible FlxLiner
MCI 5
In summary, the 4106 was an incremental improvement to an already superb bus – and was successful in its own right. With over 3200 built, it was not quite as dominant as the 4104 (over 5000 made), but it sold in superior numbers compared to its main competitors; the Flxible FlxLiner and MCI 5.
And the 4106, like its predecessor, is still a favorite in the motor home conversion market.
The primary bus photos appear to be taken at the Old Town museums in Sacramento. Was it part of a display, or was it in some sort of active service? The very nice condition and generic paint job, with lack of modern logos, suggest a museum piece. I also think I see a peek at another red vintage bus behind it.
The pictures are all from the web. There’s not many of original 4106s. Most have been modified or turned into motor homes.
172 is owned by the Pacific Bus Museum in Fremont CA. It is often used at events. The red bus in background is a Flxible Visicoach, also owned by the Pacific Bus Museum. I believe theses coaches were on display at Sacramento Rail Museum for a special event
http://www.pacbus.org
The GM two stroke diesels are the most difficult motors I have ever driven. The powerband is like 1000 RPM.
Just like some of the old school two stroke bikes I have ridden. That grip in your right hand is not a throttle; it is a on-off switch. I love these old GM buses, keep them coming!
The 4106 and 4107 buses were fun to drive as they had pretty decent power. Shifting was a bit of a challenge. The linkage wasn’t really a problem. The MC5 and MC7 also had yards of shift linkage. Greyhound did not switch over to automatics until the MC8, The shift linkage on the MCI coaches like the GM coaches was well supported. The MCI had the advantage of a simpler linkage because the transmission location is directly behind the diff rather than around the corner because of the angle drive. The GM coaches rode nicer or softer than the MCI coaches.
I found Greyhound adhering to the philosophy of specing “adequate horsepower”.
The 8V-71’s had undersized injectors and top gov RPM set to low. The top RPM was used to govern top speed which was a bad idea. Electronics was the solution. Set the engine to run in the sweet spot for fuel and power and the electronics to control max speed. Turbos also were the other issue the bus manufacturers were slow to pick up on. Turbos were especially effective at widening the powerband and getting some low end grunt going.
I really like the 4106 – classic styling that still looks good today. No wonder many of them have been converted.
These were really packaged tight from stem to stern. The spare tire was stowed behind the front bumper. Reach through the two round holes to unbolt the bumper and swing it down on hinges to access the spare.
At the other end the engine’s lower valve cover was so close to the tailgate that if the valve cover’s bus-specific hardware was substituted with more common truck engine thumb-screws the gate would rub the engine.
We had to use a forklift to get the spare tire out of that compartment behind the bumper on some units that had hit parking curbs and pushed the floor of the compartment up, then you crawled in there with some lumber and bottle jacks and tried to push the floor back down.
The 4014 bus is good looking, but the 4106 looks even better. It looks even good in today taste. I wonder what is the reason of three pieces of windows in the rear portion, which totally destroys the overall style theme — back then the design style people had a lot of influence. In my view GM often produced good bus, the last.one RTS is advanced in both looks and technology then and now.
I also noticed it had transverse mounted engine configuration, I assume GM already had developed T-drive in 1950s. Again this is another advanced design.
Detroit Diesel kept its two-stoke design until in 1990s it introduced four-stoke four-cylinder engine to compile with clean air standard.
The three side windows on the rear of the 4101 are there because the center on is actually an emergency exit. On the 4106, they made all the windows able to tilt out in an emergency, so more need of that.