Best operating rpm?

[m35a2]

I notice during my commute to work (30 miles of moderate hills) that my truck seems to prefer to get down into the 1100 rpm range. I generally force it to a lower gear that puts me closer to 1500.

My experience with other diesels (my tractor and my deuce) has taught me that operating at low rpm is a bad thing. Fuel in the oil, crud buildup on cylinder walls, potentially even engine fires if things get bad enough.

So what's the recommended operating rpm for a Cummins?

 

[SnoKing]

The factory has this figured out. If you want to run in a lower gear and higher RPM push the Tow/Haul button. Or you can continue to do it manually like you stated.

 

[Newsa]

You are fine running in regular or tow haul mode. You never want to go easy on a diesel because light loading can cause more blow by and carbon build up on cylinder walls. I think that these engines actually like the 1100-1400 rpm range when not towing because it simulates a load equivalent to towing at a higher rpm. In both cases your rpm changes slower and the control unit gives you the precise and correct fuel injection amount.
These power-trains will take care of themselves. I have a manual transmission, and i probably load my engine similar to what the automatics do.

 

[spoon059]

The manual says to avoid driving in a manner that causes the engine to lug. I find that when I drive around town, the trans likes to shift to 6th pretty early and stays there, even at 35-40 mph. I prefer to put the truck in tow-haul mode whenever I hop in. That prevents shifting to 6th at such low speeds and it helps with more aggressive downshifts and exhaust brake when I need to scrub speed as I approach red lights or stop signs. I prefer to use exhaust brake rather than service brakes when possible.

 

[m35a2]

Thanks guys - I'll put it in tow/ haul mode and see how that does. Spoon059 described what I'm seeing to a T.

 

[sag2]

They would not program it to go into 6th at lower speeds and loads if it was going to hurt something. But drive it any way you want. Thats why they put the selector on the shifter.

 

[m35a2]

You are fine running in regular or tow haul mode. You never want to go easy on a diesel because light loading can cause more blow by and carbon build up on cylinder walls. I think that these engines actually like the 1100-1400 rpm range when not towing because it simulates a load equivalent to towing at a higher rpm. In both cases your rpm changes slower and the control unit gives you the precise and correct fuel injection amount.
These power-trains will take care of themselves. I have a manual transmission, and i probably load my engine similar to what the automatics do.

So it isn't necessarily low rpm that's bad, but more so lack of load?
I wonder where the fine line between "loaded" and "lugging" lies? Because in 6th at 35 approaching a hill, it sure feels like lugging. But could be that I'm just not used to the 6.7.

You guys have given me excellent advise already, continuance of the discussion is purely academic at this point. Although being an engineer myself, I would caution some of you not to be so trusting of engineers. Especially regarding a new product. LOL just ask any service guy!

 

[EDankievitch]

This 6.7 engine in my 2013 is a very diffrent engine than the 5.9 I had in my 05. I let the transmission do what it wants and find that it likes to keep the engine RPM's low. Only even with the RPM's low it just pulls, very rearly will it down shift when i encounter a hill. It just pulls the hill never loosing speed. In tow Haul mode it will delay the up shift and keepthe RPM's closer to 1400 but again she'l just pull that drag and not down shift. The low end tourque realy is impressive.

 

[RustyJC]

The 385/900 Cummins 6.7L in my truck likes to operate at a much lower RPM than the 350/650 Cummins in my previous early 2011 truck. Whereas the 350/650 Cummins would downshift and rev to 3000 RPM or so on grades when towing, the 385/900 seldom exceeds 2300 RPM. The higher torque Cummins like to operate at lower RPM - running 1100 or 1200 RPM at light loads and small "throttle" openings isn't unusual.

The 350/650 used horsepower (torque x rpm / 5252) to get the job done - the 385/900 uses its much fatter torque curve.

Rusty

 

[Bob4x4]

The definition of lugging a diesel is when the rpm is too low to be able to accelerate with out shifting.

 

[Newsa]

So it isn't necessarily low rpm that's bad, but more so lack of load?
I wonder where the fine line between "loaded" and "lugging" lies? Because in 6th at 35 approaching a hill, it sure feels like lugging. But could be that I'm just not used to the 6.7.

You guys have given me excellent advise already, continuance of the discussion is purely academic at this point. Although being an engineer myself, I would caution some of you not to be so trusting of engineers. Especially regarding a new product. LOL just ask any service guy!

http://www.proboat.com/2015/02/why-you-shouldn-t-go-easy-on-a-diesel/


That low pitch throaty hum you hear at 1100-1200 RPM isn't lugging. It's normal.

Extensive idling will make a huge mess of your cylinder walls and sooty oil.

 

[Cummins12V98]

The 385/900 Cummins 6.7L in my truck likes to operate at a much lower RPM than the 350/650 Cummins in my previous early 2011 truck. Whereas the 350/650 Cummins would downshift and rev to 3000 RPM or so on grades when towing, the 385/900 seldom exceeds 2300 RPM. The higher torque Cummins like to operate at lower RPM - running 1100 or 1200 RPM at light loads and small "throttle" openings isn't unusual.

The 350/650 used horsepower (torque x rpm / 5252) to get the job done - the 385/900 uses its much fatter torque curve.

Rusty

I agree! My 11 compared to my 15 is the same as yours. I have tried to manually downshift while pulling a grade lets say in the low 2,000's but it did not speed up at all.

 

[gsbrockman]

You are fine running in regular or tow haul mode. You never want to go easy on a diesel because light loading can cause more blow by and carbon build up on cylinder walls. I think that these engines actually like the 1100-1400 rpm range when not towing because it simulates a load equivalent to towing at a higher rpm. In both cases your rpm changes slower and the control unit gives you the precise and correct fuel injection amount.
These power-trains will take care of themselves. I have a manual transmission, and i probably load my engine similar to what the automatics do.