Is There an Optimal RPM for a 400hp Cummins Diesel. - RV-Dreams Community Forum

LifeBeganInaWinnebago · RV-Dreams Family Member

Is There an Optimal RPM for a 400hp Cummins Diesel.

MarkS · RV-Dreams Family Member

I'm not sure about the medium duty motors. The target RPM for the big ones is about 1200. The big ones have a fan clutch that kicks in at about 210 degrees. When climbing they will heat up, it's what they do. When running flat they run about 180 degrees. Again, I don't know about the little ones but the big ones give a check engine light when things aren't right and shut down when the engine is in danger of damage.

Barbaraok · RV-Dreams Family Member

What does your manual say? Which motor model do you have? What temperature does it climb too? What temperature is it at when cruising along the highway. Why are you doing 70?

bjoyce · RV-Dreams Family Member

As Barb says, a little more information on your models and year, both motorhome and engine, would be helpful.

I have seen torque/horse power/RPM charts for various Cummins engines and most have an RPM range where the torque and hp are near their highs. 400 hp is probably an ISL and each model year they tweak these engines. Our 370 hp ISL likes to be around 1400. With a 6-speed transmission, you can't keep it optimal.

The Mode button sets economy mode, which adjusts the shift points to help fuel economy. But it is only useful for slower roads (under 50) with gentle hills, so most ignore it.

I haven't seen it in action, but the "trim" might be a Blue Ox TruCenter, http://blueox.com/chassis-performance/trucenter-steering-control/, or something similar.

LifeBeganInaWinnebago · RV-Dreams Family Member

Thanks Mark and Bill, that is what I was looking for, just some comparison to see what was normal... Barb/Dave, from the manual I gather about 200° would be the normal temp. It talks about the cooling fan running as full speed at 200°. Bill is correct it is the ISL 400 with the 6 speed allison transmission. The temperature climbs to about 210° on the sandhills here in Kansas when driving 60 to 65mph and about 200° on the flat. But at 70 it stays about 200° all the time. Bill, I pulled up the web site for the trucenter steering control and I think you are correct. I was afraid to turn the thing on not knowing what it was... Barb/Dave, As for why I was doing 70. I really don't have a good answer for that. lol

Thanks,
Mark

TRAILERKING · RV-Dreams Family Member

LifeBeganInaWinnebago wrote:
As for why I was doing 70. I really don't have a good answer for that. lol

Thanks,
Mark

I was doin' 70-75 with mine on the some of the Interstates......................because it can. Plus we were on a "No Doddling" time line. Had to make the 2800+ mile trip in less than a week plus deal with any issues at the Canada/U.S. border when bringing back the new rig.

ticat900 · RV-Dreams Family Member

LifeBeganInaWinnebago wrote:
We have only had this RV a couple weeks. We drove it home about 700 miles and just drove about 400 miles this weekend. I have noticed that it runs warm when going up hills at 60mph but at 70mph it does better. Is there a good RPM to run at and what temperature should it run at on the highway? What is the "mode" button on the button pad for the transmission? One last question, there is what looks to be an add-on, mounted next to the shifter that has what looks like a pressure guage, an on off switch and a dial that says "trim" I think the gauge does say something about power steering. I have never seen anything like this on any of the other motorhomes when we were shopping. Anyone one have one of these or know what it is?

Thanks,
Mark

regarding temperature on my 425isl it runs 210 on hard pulls and about 195 flat driving and 170ish going down hill . 70MPH is just fine on the freeway and don't worry about it one little bit

I have found 1650 is about right for decent mileage that's about 63MPH someone suggesting 1200 and 1400 is not correct at 1400 you will be going 50MPH or less

the mode deal is the converter lock up system and delayed shift down/up speeds etc I have never found any notable difference between on and off so I leave it off

LifeBeganInaWinnebago · RV-Dreams Family Member

ticat900 wrote:
LifeBeganInaWinnebago wrote:
We have only had this RV a couple weeks. We drove it home about 700 miles and just drove about 400 miles this weekend. I have noticed that it runs warm when going up hills at 60mph but at 70mph it does better. Is there a good RPM to run at and what temperature should it run at on the highway? What is the "mode" button on the button pad for the transmission? One last question, there is what looks to be an add-on, mounted next to the shifter that has what looks like a pressure guage, an on off switch and a dial that says "trim" I think the gauge does say something about power steering. I have never seen anything like this on any of the other motorhomes when we were shopping. Anyone one have one of these or know what it is?

Thanks,
Mark
regarding temperature on my 425isl it runs 210 on hard pulls and about 195 flat driving and 170ish going down hill . 70MPH is just fine on the freeway and don't worry about it one little bit
I have found 1650 is about right for decent mileage that's about 63MPH someone suggesting 1200 and 1400 is not correct at 1400 you will be going 50MPH or less
the mode deal is the converter lock up system and delayed shift down/up speeds etc I have never found any notable difference between on and off so I leave it off

Those are about the same temperatures I am getting. We just made a round trip of about 2500 miles from Salina Ks to Rochester NY to attend a wedding. The RV ran great with no problems. left for Rochester on a Wednesday evening attended the wedding ( and was the photographer). On the trip home we left Rochester at 4:30AM on Sunday and rolled in to Salina just before midnight. Not bad for a four day weekend. I am sure the FTers would think I was crazy for making that drive... And they are probably correct lol.

I am really looking forward to some down time to enjoy the RV.

Thanks,

Mark

-- Edited by LifeBeganInaWinnebago on Friday 16th of October 2015 04:20:23 PM