jess33
Lawton, Oklahoma
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Joined: 11/27/2005
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Had the same question and was afraid to ask, now I'm glad I didn't, because I still do not know the answer. So guess I will just keep on runing with the mode switch on like the book says.
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dons2346
Sioux Falls, SD, formerly of So. CA
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Joined: 11/21/2001
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jess33 wrote:Had the same question and was afraid to ask, now I'm glad I didn't, because I still do not know the answer. So guess I will just keep on runing with the mode switch on like the book says.
Yep.
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Triker33
Homestead, FL
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Joined: 11/30/2002
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Cummins Best Gear & RPM to Use
Larry Full Time Since 99
1999 34Q Discovery DP ISB 275HP 6 Speed Allison
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stevelv
FullTimers at Last!
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On a diesel engine, at WOT your fuel consumption is a direct relationship to RPM - not exactly 1:1 but pretty darn close.
At 2200rpm you are using almost twice the fuel than at 1100rpm
The most fuel efficient way of climbing a hill is at WOT and lowest RPM - that may be 35mph even though by dropping a gear you might do it at 55mph, but you will use more fuel in doing so. That's a direct effect of the higher engine RPM and ignores the extra wind resistance that 55mph creates.
Diesel engines can handle being lugged - unlike gas engines.
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slobote
Fort Myers, Florida, Long beach Ca.
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In steep hill climb territory I turn mode off and manually shift to keep RPM/horsepower up to keep engine from lugging.
Bob and Mardel
2001 Beaver Monterey Trinidad DP
One well behaved cat (CC)
Fort Myers, Florida
Long Beach,California
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JUrban
Delaware
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Cool site Triker33. Says it all on what Cummins recommends.
John
2008 Tiffin Allegro Bus 40' QSP
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stevelv
FullTimers at Last!
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slobote wrote:In steep hill climb territory I turn mode off and manually shift to keep RPM/horsepower up to keep engine from lugging.
In which case you are doing it wrong - that is the correct procedure for a GAS engine, not a diesel.
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Deen
Vancouver, WA
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Joined: 12/07/2000
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stevelv wrote:On a diesel engine, at WOT your fuel consumption is a direct relationship to RPM - not exactly 1:1 but pretty darn close.
At 2200rpm you are using almost twice the fuel than at 1100rpm
The most fuel efficient way of climbing a hill is at WOT and lowest RPM - that may be 35mph even though by dropping a gear you might do it at 55mph, but you will use more fuel in doing so. That's a direct effect of the higher engine RPM and ignores the extra wind resistance that 55mph creates.
Diesel engines can handle being lugged - unlike gas engines. 1100 rpm is below the rpm that is considered safe by Cummins. If you go to their web site and look at the charts of HP and torque most will start @ 1200 rpm and that's considered to be the lowest you should fully load the engine. However the Allison should take care of that for you.
The instructions I've seen say to climb the hill in the highest gear that will allow you to accelerate if needed and NOT at WOT. Admittedly that is for manual trans trucks with many more gears than RV's have. Wish I could get an automatic with 13 speeds although the Banks kit goes a long ways to making it seem that I have more gears.
Deen - Vancouver, WA
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Oday
Madison, WI
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When the Mode button is pushed it activates a transmission shifting phase dictated by a computer chip that has "learned" your driving habits in terrain where transmission shifting is required to maintain optimum engine RPM. On flat terrain it has little or no effect on shifting and therefore no effect on fuel economy.
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slobote
Fort Myers, Florida, Long beach Ca.
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stevelv wrote:slobote wrote:In steep hill climb territory I turn mode off and manually shift to keep RPM/horsepower up to keep engine from lugging.
In which case you are doing it wrong - that is the correct procedure for a GAS engine, not a diesel.
Next time you are are going up a long steep grade push the throttle to the floor, and if you do not accelerate you need to downshift. I do not wait for Mr Allison to do this for me, I do it myself to keep from losing momentum. I keep my rpm at 70 to 80% of max RPM to keep my horsepower rating high while climbing. That is why truckers run hills at high RPM and NEVER get close to lugging range.
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