Series 50 Postscript

[Mike in GA]

It has been enjoyable to read the reactions to my original post about several Southeast Bus Nuts' migration to the four cylinder DD Series 50. If you missed it, click on the original story:

http://www.sebusnuts.org/Chapter%20Newsletters/Newsletter%202015%20Spring.pdf

     As a non-engineer I had presumed that the main reason for improved fuel efficiency was four cylinders and fewer cubic inches versus the six and eight cylinder two-strokes they were replacing.
     But there's more to it. My original article did not have room for the following data, which in my (non-engineer) viewpoint is also a big factor in improved MPG:  lower RPMs at cruising speed. My 8v92TA with Allison 750 hits 62 MPH at 2,100 RPMs in top gear. All the Series 50s get to 65 MPH with slower revolutions. Probably a major contributor to better fuel economy.
Bus                Trans                 Rear end                Cruising speed             RPM
H Best             8 sp RR               .333                       65                            1600
J Campbell       B-500                 .410                        65                            1380
L Snyder          750                    .373                        65                            1700

     I get about 5.9 MPG with my 8v92. These guys get between 7.3  and 8.5
     Of course it would be folly to make the transplant with the notion that fuel saving will amortise the expense of the newer engine. One of our pioneers estimated that it would take about 90,000 miles at today's fuel prices to recover the initial expense of the swap.
     No pun intended, but YMMV.
Mike in GA

[luvrbus]

You need to change your rear gear to a 3:36 where you have a better cruising speed it is not hard to get 7.5 from a 8v92 I done it for years with a 740 you run the series 50 at 2100 rpm all day I doubt you get 5 mpg.
Jerry uses a double overdrive B500 for his low rpm numbers you get the 8v92 down to around 1700 rpm at 65 mph you will be surprised at the difference in fuel mileage I wouldn't use the 3:73 with a series 50 either JMW 

[lostagain]

I have the 3.36 rear end ratio. My 6V92/HT740 spins at about 1800 rpm at 65 mph. I get about 7.2 miles/US gal. So it is not so much the type of engine, but more the gearing that makes a difference.

JC

[Dave5Cs]

I really like the article where it says the series 50  has power up a hill. I wonder what that is like, ummmm
Would my 644 fit on the back of it and how far would I have to move the engine mounts?

[TomC]

Dave-I don't think the MT644 would like either the vibration nor the low end torque of the Series 50.

Whether you have a Series 50 or 60, you want to gear the engine to be turning 1400-1600 at 65mph for best cruise. As compared to the new high economy engines-Cummins ISX 450hp @ 1600rpm with 1750 torque @ 1000rpm, and the Detroit DD15 400hp @ 1600rpm with 1750 torque @ 975rpm. The engine manufacturers want these engines geared for 1100-1200 rpm at 65mph. Now with common rail fuel injection, these low rpm engines are running smoothly without rumble. They are finding out, slower the better. And with full torque at 975rpm, clutch engagement is with nearly full torque. Hence, Spicer has come out with a 35 series Ujoint to combat this ultra low torque. Good Luck, TomC

[Oonrahnjay]

Dave-I don't think the MT644 would like either the vibration nor the low end torque of the Series 50.

Whether you have a Series 50 or 60, you want to gear the engine to be turning 1400-1600 at 65mph for best cruise. As compared to the new high economy engines-Cummins ISX 450hp @ 1600rpm with 1750 torque @ 1000rpm, and the Detroit DD15 400hp @ 1600rpm with 1750 torque @ 975rpm. The engine manufacturers want these engines geared for 1100-1200 rpm at 65mph. Now with common rail fuel injection, these low rpm engines are running smoothly without rumble. They are finding out, slower the better. And with full torque at 975rpm, clutch engagement is with nearly full torque. Hence, Spicer has come out with a 35 series Ujoint to combat this ultra low torque. Good Luck, TomC 

     You're talking big engines here, of course, Tom!  I found a surplus bus with a Cummins ISC 8.3 (2003) engine (approx 300K miles) and a B500R (put in approx. 1 year before the bus was retired) for $4200.   I had the transmission "refreshed", it was in pretty good shape but needed a new torque converter and some new seals so I'm glad I did that while it was out.  The transmission and engine are sitting on the floor of the shop right now while we're running wires and drawing chalk marks on the floor.

     It's a "little" engine like a S50 but it will take me from 150 Hp with 4 speeds to 325 Hp with 6 speeds.  The Gillig/Meritor rear end with the original rear gearing is going in my bus, too.

     My original transmission lost reverse and the engine was very old technology; it hasn't been practical to get parts for at least 20 years (unless you found someone with NOS halfway around the world).  I feel like I *had* to do something.  Will I ever make up for the cost in fuel savings?   No, but I'm glad I'm doing this.

     Tw

[gg04]

We started to use a series 50 when we re-powered, but Bob Sheaves talked
me out of it. He really didn't have much faith in them for long term low maintenance.  90,000 miles later ..Glad we went with 6l71ta DDEC love the sound of 6" straight exhaust at 2300rpm..rdw

[TomC]

Bruce-that is a smart choice. 325hp from a 8.3 is not too much. You can get 350hp and 1000lb/ft torque out of them. The 8.3 is not being used in trucks anymore. The 8.9liter ISL has taken its place, with it being detuned when lower horsepower is wanted. Good Luck, TomC

[Oonrahnjay]

Bruce-that is a smart choice. 325hp from a 8.3 is not too much. You can get 350hp and 1000lb/ft torque out of them. The 8.3 is not being used in trucks anymore. The 8.9liter ISL has taken its place, with it being detuned when lower horsepower is wanted. Good Luck, TomC 

    Thanks, Tom.  It fits really well (based on our drawings on the floor so far) and I'm amazed at how short ("un-tall") the engine and transmission are.  I was afraid we were going to have to cut out a big hump at the rear firewall lower section for the transmission -- it will have to be cut out but not much at all.  The small cutout will make for easy bracing.  I'm really looking forward to that 6-speed!