Mpg. Question when towing

[AClayton]

I have not had my CTD long and consequently am unsure of a few things when towing my trailer. I have years of towing experience and have driven MH's pulling trailers and cars and the like ..... but my experience has been with gas power and not diesel power. With ever increasing fuel costs,I have slowed down on the freeways like many have posted here to boost mpg. My truck has been modified somewhat and seems to pull very hard and strong..... at 65-70mph. At 55mph,however, it seems sluggish and unresponsive pulling the same grades. I have a lock-up controller that helped considerably for my truck, but at 55 and locked my RPM's go down to 1300. So what I do to compensate for it is to turn off OD and RPM's go to 2000. The EGT gauge likes this much better but the question is will my fuel economy go up or down with the increase in RPM? Any speed 65 or above I shift into OD and things are business as usual. What RPM should I be towing at for near perfection mpg. I know the 3rd Gens have 3. 73 gears and can tow a bit more... . should I opt for a lower gear-set or continue doing the OD cancel. Your thoughts please!!!!





Alan

 

[mberry]

I definitely would not be towing for any length of time at 1300RPM. You're lugging the engine with almost any load at that RPM and EGT's get hot as you noted.

As for what to do, your engine will run relatively efficiently at 2000RPM at that speed, but any MPG increase may be offset from slowing down and running out of OD. If it were me, I would opt for the 4. 10 gears and run in OD if my RPM's were 1800-2000 in OD at 65MPH.

 

[Grizzly]

Tow with OD off. Optimum RPM is 2000 or slightly less, which should help your mileage. The most significant factor on fuel economy is wind drag and speed. What's the length and height of your 5er. If you have an 11 to 12' tall 5er, don't expect great mileage. My 27' Arctic Fox 5er is nearly 12' tall and I get a little better then 10 mpg. If I'm reading your signature correctly, you have a 3. 54 gears. You can adjust your final gears by changing rear tire size.

 

[Gary - K7GLD]

YUP, I have the 3. 55's, and also run 285 tires on the rear. Our 24 ft. 5er is a low-profile model, so minimum wind drag. I run in O/D in the flats at about 1600 RPM (about 60 MPH) - as low as I will go with this load. When we hit the grades, I drop into 5th (direct) gear, and RPM runs about 2000 - throttle response is excellent, and boost and EGT are low.



1300 RPM under any significant load is lugging the engine down, and hard on it, regardless of how it SEEMS to be pulling...

 

[DonTX]

I beg to differ with some of the comments, at the risk of offending anyone which is not my intent. . Maximum torque for our engine is attained at 1500, or a little less. Increasing RPM not only uses more fuel, but DECREASES torque! At the RPM some seem to recommend, your torque is down from 440 (stock) to around 340. Torque is what twists the driveshaft, we spent our money for a torquey engine, so why not use it?

Most MOPAR engines eating GASOLINE reach max torque at around 3500 rpm, and I think that gasoline backgrounds have tainted our fine diesels. I rarely ever get the rpms over 2000, yet there are those that cannot understand my good mileage figures.

I use my OD, I have an automatic, and Dodge has spent a lot of time and money arranging the shift points where they belong, much better shift points that one can manage on his own. Why reinvent the wheel?

 

[Jumbo Jet]

I tow in OD. 3. 73 gears. At about 1950 RPM, there are not many hills that could slow the Dodge Cummins diesel down. Around 1700 RPM, RPMs would drop off.

BTW - the Torque curve for the Cummins is relatively FLAT from 1500 to 2600 RPM.

 

[DonTX]

Sorry, you must be looking at something else. The torque curve falls off SHARPLY after 1500 rpm, in the 5. 9 that is.

I tow in OD. 3. 73 gears. At about 1950 RPM, there are not many hills that could slow the Dodge Cummins diesel down. Around 1700 RPM, RPMs would drop off.



BTW - the Torque curve for the Cummins is relatively FLAT from 1500 to 2600 RPM.

 

[HBarlow]

Alan,



Study the torque curve of your engine to understand how to operate it.



The torque peak of all Cummins ISB engines beginning with the 24 valve has been 1600 rpm and the torque curve is flat from 1600 to 2700 rpm. It does not drop off above 1600 rpm but does drop below 1600 rpm.



The torque peak of an earlier 12 valve may be the same but I know the torque band did not extend to 2700 rpm. I don't know about 12 valve engines because I didn't own one.



It is acceptable to run the ISB at 1600 rpm loaded, even at full throttle, but not below. You should not use heavy throttle when loaded below 1600 rpm.



You should not tow at road speeds which will allow the tach to drop below 1600 rpm. If the speed limit requires slower speeds you should downshift.



I guess you have learned that clutch engagement torque, in other words, the torque at idle, is between 350 and 375 lbs. ft. on a Cummins ISB. What this means is it is unnecessary and not advised to rev the engine above idle when launching even heavily loaded. You simply select first gear and ease out the clutch then bring the engine speed up after the clutch is engaged. This method saves clutch wear.

 

[DonTX]

I have no idea about other than 12v Cummins, but the torque peak on them is at just under 1500 and drops off drastically after that. I have trouble believing that the 24v has a "flat" torque curve from 1600 to 2700! That don't sound correct at all, are you sure you are not looking at HP curve instead of torque, comments anyone? IF THAT IS TRUE I think I will buy one this week or sooner!

Alan,



Study the torque curve of your engine to understand how to operate it.



The torque peak of all Cummins ISB engines beginning with the 24 valve has been 1600 rpm and the torque curve is flat from 1600 to 2700 rpm. It does not drop off above 1600 rpm but does drop below 1600 rpm.



The torque peak of an earlier 12 valve may be the same but I know the torque band did not extend to 2700 rpm. I don't know about 12 valve engines because I didn't own one.

 

[DonTX]

My sincere apologies HBarlow! I looked it up, and you are quite correct on your 24v torque curve, damn that is an impressive output! I had been told the 24v was just an emissions requirement, had no idea of the difference in torque curves.

Obviously when we are discussing trucks, we very much need to define if we are talking about 12v or 24v engines.

Here is a link to the curves, very impressive:

1998 - 2000 ISB engine specs

 

[RVTRKN]

Well I don't know about the MPG of towing grades, but you can prematurely wear your CTD when towing as described. My preference is around 2200-2300 RPM when climbing grades heavy. Two reasons for this is more air to fuel mixture for lower EGT and both of my CTD engine's seems to respond better for performance. The fuel economy is not a priority on the grades for me, and I don't think it is that much of a loss in the overall picture towing long distances.



The CTD can run all day long at the governed RPM and not hurt it, but your MPG will drop fast at 3200RPM. My speed is at 60 MPH at 2200 RPM with my setup (look at my sig) towing the grades in 5th gear and I never tow grades in 6th even though it can.

 

[AClayton]

My truck has injectors and a B-1 turbo so the final HP and torque curves may be somewhat altered from stock but for the most part it has power or torque above 1600RPM. Below that it seems I'm wasting the engines potential. My mods are towing specific and the turbo and injectors were modestly selected from reading others' posts when I 1st bought the truck,so egt was never a problem until I started slowing down to 55mph and locked. Like Gary, i also run 285's so my overall ratio is taller with the 3. 54 gears. Everyone seems to agree that 1800 to 2000 is a good towing rpm so I will just lock OD out to maintain that cruise level vs. changing gear-sets. I kind of like the cruising rpm down low when not loaded.



Grizzly-I do not tow a 5th wheel... . it is a 24' enclosed trailer with all my tools and equipment for my side-business repairing RV's. I had to buy this truck last year because the weight increased in the trailer so much that it exceeded the GCWR of my old truck..... a gasser.



Thanks for all the help !!!!!



Alan

 

[HBarlow]

I put 325,000 miles on my '01 HO/6 speed w/ 3. 54 gears before selling it to my son-in-law and daughter. It now has 347,000 miles on it and I still drive it occasionally. Most of those miles were towing miles.



For its entire life I ran it loaded at highway speeds at or just over 1600 rpm with the cruise control set. When I started ascending a grade I allowed it to pull at full throttle mandated by the cruise control. If the mighty ISB could maintain engine speed at or above 1600 rpm fully loaded at full throttle I let it pull. If the tach began to droop below 1600 rpm I shifted down. To this day the engine runs strong and uses very little oil between changes. Below is my reasoning for ignoring TDR expert advice and running it that way.



Right after I purchased it I joined TDR and was quickly confused because all the "TDR experts" frequently posted their opinions that the engine needed to be revved to pull hills. I couldn't understand why an engine that was designed to develop maximum torque at 1600 rpm had to be revved to 2200 to pull a grade.



I purchased a handy little book from Cummins, Inc. titled "Operation and Maintenance Manual ISB Engine" to learn what Cummins had to say about using the engine.



Here are a couple of quotes from the manual:



"Section 1 - Operating Instructions" in the Cummins manual offers the following on Page 10 under the heading:



"Engine Operating Range"



"Do not operate the engine at full throttle operation below peak torque rpm (refer to engine dataplate for peak torque rpm) for more than 30 seconds. Operating the engine at full throttle below peak torque will shorten engine life to overhaul, can cause serious engine damage, and is considered driver abuse. "



"Cummins engines are designed to operate successfully at full throttle under transient conditions down to peak torque engine speed. This is consistent with recommended operating practices. "



"Operating the engine below peak torque rpm can occur during gear shifting due to the difference of ratios between transmission gears, but engine operation must not be sustained more than 30 seconds at full throttle below peak torque rpm. "



And this, on Page 1-79 under the heading:



"Driving Techniques"



"ISB engines produce maximum power at an rpm less than governed engine speed. Placement of maximum power has been changed on ISB engines to encourage operation in the most fuel efficient engine speed range. "



"To obtain optimum engine performance on a grade, allow the engine speed to load down to near peak torque before shifting. This will result in an engine operating speed in the maximum power zone after the shift is completed. "



"Refer to the engine dataplate for peak torque rpm and governed speed rpm. "



Surprised that the advice in the Cummins manual contradicted the TDR experts I sent an e-mail to Cummins, Inc. to clarify. I provided the same quotes from the Cummins manual provided above and explained that I routinely allowed my Dodge to pull at full throttle at 1600 (peak torque) rpm. I explained that I downshifted if the engine could not hold 1600 rpm. I asked if I was correctly understanding the instructions contained in the Cummins manual or if my driving practice would cause shortened engine life or damage. I posted my message and the reply from Cummins back in '01.



The reply from Cummins joked about the fact that I was quoting their manual and assured me that I understood their manual and was not shortening engine life or causing damage to my engine.



A lot of misinformation circulates on the TDR website and at truckstops and is passed on as gospel. Most of it is simply not true.



The Cummins ISB develops full torque at 1600 rpm and it WILL NOT suffer harm or shortened life if it is run indefinitely at 1600 rpm. If the driver chooses to operate the engine at higher rpms up to redline it will not cause damage but higher rpm operation in a lower gear means the engine is turning more revolutions per mile traveled and that means shortened life.

 

[AClayton]

HBarlow-I find this interesting because I have noticed also that my truck seems to maintain the speed better at 16 0r 1700rpm but the egt climbs a bit higher also. I'm sure most of it comes from added fuel from injectors and electronics but it never goes over 1200deg. when loaded that way. I have also learned that accelerating gently up hills and letting the turbo build boost keeps egt's from spiking yet strangely maintains the same speed. The other thing I needed to get used to when I switched from gas to diesel was that when the trans upshifted to the next gear,I would quickly press the throttle down to compensate for the lower predicted power. It didn't take long to realize that you don't need to push the throttle down more when this truck upshifts..... the upshift places the Cummins right back in its powerband and subsequently lunges the truck forward!



RVTRKN-I guess my 19K doesn't require a 2500rpm boost... . my truck rarely needs over 2000 if I drive it sparingly. I can see that necessary if my GCW were to increase though. My truck feels like it can do much more if need be and with some of the GCW's I see advertised on these threads, I am certainly an underdog.

 

[HBarlow]

My sincere apologies HBarlow! I looked it up, and you are quite correct on your 24v torque curve, damn that is an impressive output! I had been told the 24v was just an emissions requirement, had no idea of the difference in torque curves.

Obviously when we are discussing trucks, we very much need to define if we are talking about 12v or 24v engines.

Here is a link to the curves, very impressive:

1998 - 2000 ISB engine specs

Don,



No need to apologize but I'll accept the apology in the spirit it was offered.

I am not offended when someone disagrees with me or challenges an assertion I make.



You apparently doubted your own doubts and did a little research and discovered something new and posted it. It was an honorable thing to admit you were wrong. Discussions, differences of opinions, even arguments conducted here benefit all of us including many silent readers who we often don't even know are there. They learn as much as the participants in the discussions often do.

 

[Grizzly]

Don,



Discussions, differences of opinions, even arguments conducted here benefit all of us including many silent readers who we often don't even know are there. They learn as much as the participants in the discussions often do.

You have had some very good posts here Harvey and this has been a great thread. I'm one of those silent readers who has learned a great deal from this thread. With my hat in hand, thank you.

 

[HBarlow]

Grizzly,



Well, I'm humbled. Thank you, my friend, for a kind word.

 

[DonTX]

Well I had plenty of crow to eat on that one, diesel engines ( and I have had quite a few) NEVER have a flat torque curve for that length! Yeah, right. I can't get over the specs on the 24v Cummins.

Apparently that is a common problem judging from many of the posts, and for sure anytime information is offered by someone, you about have to know what engine he is running or referring to. I love my ol truck and had planned to have it forever, but I sure do like the figures on the 24v.

By the way, I really enjoyed your post on RPM useage, I hope everyone read and heeded those wise words, and most of all think about what you said and their driving habits. I only have a little more than a million miles on diesels, and still learn something everyday!

.

Don,



No need to apologize but I'll accept the apology in the spirit it was offered.

I am not offended when someone disagrees with me or challenges an assertion I make.



You apparently doubted your own doubts and did a little research and discovered something new and posted it. It was an honorable thing to admit you were wrong. Discussions, differences of opinions, even arguments conducted here benefit all of us including many silent readers who we often don't even know are there. They learn as much as the participants in the discussions often do.

 

[RVTRKN]

HBarlow-I find this interesting because I have noticed also that my truck seems to maintain the speed better at 16 0r 1700rpm but the egt climbs a bit higher also. I'm sure most of it comes from added fuel from injectors and electronics but it never goes over 1200deg. when loaded that way. I have also learned that accelerating gently up hills and letting the turbo build boost keeps egt's from spiking yet strangely maintains the same speed. The other thing I needed to get used to when I switched from gas to diesel was that when the trans upshifted to the next gear,I would quickly press the throttle down to compensate for the lower predicted power. It didn't take long to realize that you don't need to push the throttle down more when this truck upshifts..... the upshift places the Cummins right back in its powerband and subsequently lunges the truck forward!



RVTRKN-I guess my 19K doesn't require a 2500rpm boost... . my truck rarely needs over 2000 if I drive it sparingly. I can see that necessary if my GCW were to increase though. My truck feels like it can do much more if need be and with some of the GCW's I see advertised on these threads, I am certainly an underdog.

With my setup, 2200-2300 is a good RPM (speed) and pulling power and I'm still below peak torque curve as some say. I've had this conversation before with HBarlow and we agreed to disagree. I'm not saying he is wrong but it is the way I like to drive the grades. I have asked many techs this and they have all come back with "you can drive at governed speed all day long". I just can't stand it when I have all this torque and power but I am told I need to drive at 50 MPH over the grades. This is due to 6th gear/3:73 too tall pulling heavy and 1700-2100 RPM too slow. I respect HBarlow and he has a good point about what Cummins has written about, but it also is within operating range that I drive it. I think I will lug my CTD next pull and see if it is just a feel or it really is lugging, because that is what it feels like to me.

 

[HBarlow]

With my setup, 2200-2300 is a good RPM (speed) and pulling power and I'm still below peak torque curve as some say. I've had this conversation before with HBarlow and we agreed to disagree. I'm not saying he is wrong but it is the way I like to drive the grades. I have asked many techs this and they have all come back with "you can drive at governed speed all day long". I just can't stand it when I have all this torque and power but I am told I need to drive at 50 MPH over the grades. This is due to 6th gear/3:73 too tall pulling heavy and 1700-2100 RPM too slow. I respect HBarlow and he has a good point about what Cummins has written about, but it also is within operating range that I drive it. I think I will lug my CTD next pull and see if it is just a feel or it really is lugging, because that is what it feels like to me.

You seem to be a little confused. The torque peak on your engine starts at 1600 rpm and remains essentially flat at around 610 ft. lbs. to 2700 rpm. You can operate it at full throttle under full load anywhere in the range between 1600 to governed engine of 3200 rpm but torque drops off after 2700 rpm and peak horsepower occurs at 2900 rpm and drops off above that engine speed.



If you prefer revving it to 2200 - 2300 rpm to pull grades and don't mind the additional fuel used or additional wear on the engine it is your choice to do. I certainly didn't tell you that peak torque occurs below 2200-2300 rpm or that you should pull hills at 50 mph. I said you can pull hills at 1600 rpm without being forced to shift down.