Bus Stop Classics: 1961-1965 GM PD 4106 Intercity Coach – How Do You Replace an Icon?

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– Posted on December 19, 2020

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

GM PD 4106

 

Flxible FlxLiner

Flxible FlxLiner

 

MCI 5

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.