Pulling grades RPMs

[TDEvans]

Hi All, Previous truck was a 05 Dually, New truck is a 12 Dually with High Output motor,old truck would pull hills at 2500 rpms with 5. 9 motor,what rpm range should I be using with this motor,pulled some hard grades this last week,truck pulled great but rpms wanted to stay around 2000 rpms,is THAT high enough or should I downshift it to get the rpms higher. Love the truck and the exhaust brake is AWSOME. Thanks Terry.

 

[HBarlow]

Your engine, like all Cummins B motors in Dodge Rams, develops maximum torque at 1500 or 1600 depending on programming. Any rpm including max torque is fine but 2000 rpm is better because road speed and engine rpm can fall off a little without dropping out of the power band.

Our engines develop more horsepower as rpm increases up to 2700-2900 rpm, again depending on programming, so it will feel better and hold road speed a little better between 2000 and 2500.

 

[RustyJC]

Take a look at the torque curve at the bottom left of page 2 HERE. Peak torque is down at 1600 RPM. You can rev the engine to 2500 if you want, but torque is falling off as RPM increases above 1600. By holding the engine at 2000 RPM, the computers are giving you the benefit of torque rise if engine speed drops further - the more it drops toward 1600 RPM, the more torque you have available.



Rusty

 

[AH64ID]

If it wants to hold 2K rpms I would let it. As your weight or the grade increases you may find that it wants more rpms.

Peak torque is 1500 rpms, but try pulling a grade at 1500 rpms and it just won't happen.

Remember that torque gets a load moving and hp maintains it. That is why you can't hold 1500 rpms up a 6% grade at 23K lbs, but it will do it at 2500 rpms, despite less torque. Torque may start to fall off after 1500 rpms, but hp increases all the way to rated rpms so pulling at more rpms is not a hindrance.

There would be nothing wrong with dropping a gear manually and going up in rpms from 2000 to 2500 or so. You would run lower EGT's and coolant temp, and fuel burn would probably be similar. It's all up to you, but he OEM programming (assuming all stock) will not fail you.

 

[RustyJC]

The following explanation of torque rise might be worth a read:

Torque and Torque Rise



Horsepower is a rate of doing work. Torque is "the rotary force in a mechanism" according to a dictionary definition. The two are related (Torque lb-ft = H. P. X 5252 / RPM) but torque is often misunderstood. Since there is a fixed relationship between HP (or kWm) and torque, two engines having the same horsepower at the same RPM will have the same torque. However, at work, the two engines may perform very differently. The reason for this is they may have very different torque rises. Therefore they respond differently to the demands of the load.



The length of piston stroke, number of cylinders, rotating mass and other factors, affects torque rise. Newer electronically controlled engines are able to produce torque characteristics that could not be achieved with mechanical fuel controls.



Engine horsepower curves often show the torque curve or "pull down torque" as well. This curve illustrates the amount of torque available from the engine as a load is applied that exceeds the engine's rated torque at the operating RPM. The difference in the torque at the rated RPM and the maximum or peak torque is the "torque rise". It is usually expressed as a percentage. (Peak Torque - Rated Torque / Rated Torque = Torque Rise X 100)

In this case the rated torque is 477 lb. -ft. and the peak torque is 657 lb. -ft. @ 1200 RPM. The torque rise is: 657 - 477 divided by 477 = 38%.



Note that although there are two horsepower curves, continuous and intermittent, only the intermittent torque curve is shown. The assumption is that if the engine is "pulled down" (reduced in RPM by the load), then the engine will be performing on its intermittent rating curve. Torque curves are usually available for any published horsepower rating.



What all this means in actual usage is that, in many applications, the engine with the greater torque rise will do its work more quickly. It will seem more powerful and responsive. This difference will be very evident in applications where the engine is routinely pulled down from its rated speed by the load. Examples of this are a drilling rig lifting the rod string, a grinder processing a stump or a loader digging into a rocky bank. Even applications not generally thought of as sensitive to torque rise such as generator sets and marine engines can, under some conditions, benefit from good torque rise characteristics. (Pulling a heavy trawl net, bucking a current or starting a motor load, for example. )



Torque and torque rise are very important considerations in many applications particularly those where the engine is routinely pulled down from its rated speed owing to the effects of the load. Greater torque rise allows the engine to run at a higher r. p. m. under load and thereby accomplish its work more quickly. In extreme conditions, inadequate torque rise will prevent the engine from accepting the load and it will stall.



There is a lot to add on the importance of torque. "Droop" and "isochronous" governing, electronic or mechanical controls and other factors enter into the aspects to consider. Again, if the intended usage is clear, the best option will usually be apparent.

Whether you want to start the pull at 2000, 2500 or 3000 RPM is up to you. The main factor is that you're operating above the torque peak so that the bonus of additional torque is available as the load pulls engine speed down.



Rusty

 

[Jim W]

I agree with the other three posters but I also have gages on my truck. When I am pulling my 5er on a 6% grade or a steeper grade I tend to watch my EGT'S and engine water temperature gages as I go up hill. I try to keep the EGT'S around 850F to 1000F and the water temp under 200F if I see them creep up, I will manual down shift the auto trans to 5 gear to keep the RPM's up to cool the EGT's and water temps down.
If your truck is stock and you want to keep it that way, I would at least add gages to your truck. I think this is the number one thing missing from all of our trucks. If the marketing people were to ask me what I want and would I pay a little extra for gages I would answer YES.
Jim W.

 

[AH64ID]

The problem with "pull-down" torque is that is technically lugging the engine. Lugging is defined as anytime more fuel can't produce more rpms (when trying to accelerate, not maintain). In addition the automatic transmission won't allow for that, it will downshift long before you get to pull down torque.

But pull down torque is how the ISB is rated, you will likely not ever be able to make 800 ft/lbs at 1500 rpms in a pickup.

To pull a grade you need hp, plain and simple. To accelerate you need torque. At 1600 rpms at 800 ft/lbs you only have 240 hp, but at 2200 rpms you have about 330, so 90 hp more, even thou the torque is over 100 ft/lbs less you will pull the hill easier.

I agree with the other three posters but I also have gages on my truck. When I am pulling my 5er on a 6% grade or a steeper grade I tend to watch my EGT'S and engine water temperature gages as I go up hill. I try to keep the EGT'S around 850F to 1000F and the water temp under 200F if I see them creep up, I will manual down shift the auto trans to 5 gear to keep the RPM's up to cool the EGT's and water temps down.
If your truck is stock and you want to keep it that way, I would at least add gages to your truck. I think this is the number one thing missing from all of our trucks. If the marketing people were to ask me what I want and would I pay a little extra for gages I would answer YES.
Jim W.

Have you changed to a cooler thermostat that stock?

Why so cool on the EGT's? Your leaving a lot of power unused.

 

[RustyJC]

I'm not going to argue with you. If you want to drive it like a gasser with the RPM bumping against the rev limiter, have at it. I'm merely attempting to explain why the engineers have programmed the computers so that the 6. 7L with the 68RFE (1. ) will try to pull a grade under load at 2000 RPM vs 2500 or 3000 RPM and (2. ) why it's not automatically downshifting until RPM pulls down to a predetermined level (you haven't towed a load with a 68RFE, right? If you had, you'd know that the transmission doesn't rush to downshift under full throttle. Instead, it will keep the torque converter locked up and allow the RPM to drop until the computers command a downshift. ) The engineers have programmed the computers to take advantage of the torque rise characteristics of the engine. If you want to override the programming manually, the +/- switch gives you that capability.



Rusty

 

[AH64ID]

I didn't say to ignore what the truck is doing, just stating the additional rpms won't hurt anything and can actually help certain parameters. But if the the setup is stock then whatever it wants to run is fine. 2000 rpms is plenty of air/coolant flow. I am speaking in more general terms than this specific application. With a CR and electronic injection hp increases as rpms do, all the way to rated rpm. The days of only having a narrow band of peak hp and a torque curve spike went away with 100% mechanical injection.

The programming will downshift long before you get into the torque rise from a pull down. Downshifting is based on rpms and load. Torque rise takes full fueling to be effected. Directly from the quote you have to have rpms reduced by load, and without a manual trans you won't be able to do that.

 

[RustyJC]

The programming will downshift long before you get into the torque rise from a pull down. Downshifting is based on rpms and load. Torque rise takes full fueling to be effected. Directly from the quote you have to have rpms reduced by load, and without a manual trans you won't be able to do that.

With all respect, that's not the way the 68RFE functions. You can pull a grade in 6th gear at full throttle with the torque converter locked up, and it will NOT downshift until RPM drops significantly. It will hold a gear and ride the torque curve down until a downshift is required - just like a fully loaded big rig pulling a grade (and, no, I don't have any large car chauffeur aspirations here. ) With 4. 10 gears, my truck will downshift from 6th to 5th under full throttle only after the speed has fallen to ~58 MPH (~1600 RPM).



Rusty

 

[RonD]

Hi All, Previous truck was a 05 Dually, New truck is a 12 Dually with High Output motor,old truck would pull hills at 2500 rpms with 5. 9 motor,what rpm range should I be using with this motor,pulled some hard grades this last week,truck pulled great but rpms wanted to stay around 2000 rpms,is THAT high enough or should I downshift it to get the rpms higher. Love the truck and the exhaust brake is AWSOME. Thanks Terry.

At a lower speed 2000 RPM's will be fine, pulling say a 6, or 7 % grade loaded at 55 MPH I like 5th gear, sometimes 4th, and 2200-2500 RPMS, it just feels better, want to go 65 up that grade you will have some RPM's for sure, there is not set number, drive it like your old one.

 

[HBarlow]

Rusty,

How about translating the engineer speak to a brief, elementary comparion of our Cummins ISB engines compared to V8 diesels based on torque rise characteristics. I KNOW that our little ISB in line six cylinder engines develop more idle rpm or clutch engagement torque and pull stronger at lower engine speeds than V8 diesels but don't speak torque rise.

 

[HBarlow]

Rusty,

I've posted many times in previous years that when I owned my faithful old '01 Ram HO/six speed w/3. 54 gears I always allowed it to pull down to 1600 rpm when pulling a trailer up steep grades. I simply left the cruise control engaged or put my boot to the floor and kept on trucking with one eye on my egt gauge. If egt climbed beyond 1325* I would back out of it slightly but if it held at 1300* and no lower than 1600 rpm I kept my boot to the floor and let it pull.

Dozens of TDR experts over the years told me repeatedly what a fool I was and how it was going to self-destruct any day but I operated it just like that for 325k miles without any problems. The man who was my mechanic and maintained that truck owns that old Dodge now and it has somewhere between 350 and 400k miles on it now, still without engine problems.

Bill Stockard told me my method was stressing sixth gear in the transmission and he was probably proven correct because when the NV-5600 was rebuilt by Standard at about 302k the rebuilder showed me that sixth gear was indeed worn and had to be replaced.

As an engineer speaking of diesel engine pulling characteristics, please explain. Was I unknowingly allowing the torque rise to work for me or was I simply using the peak torque?

 

[RustyJC]

Harvey,



From the source I quoted above: "In extreme conditions, inadequate torque rise will prevent the engine from accepting the load and it will stall. " In layman's terms, an engine with more torque rise will feel like it's putting its shoulders under the load as it keeps pulling with even more torque as the RPM drops until (if the load is not excessive for the engine) a point of equilibrium is reached - the engine output will match the load, and RPM will stop dropping. An engine with low torque rise will, as stated above, feel like it's falling on its face as the load pulls the RPM down.



Rusty

 

[RustyJC]

Rusty,



I've posted many times in previous years that when I owned my faithful old '01 Ram HO/six speed w/3. 54 gears I always allowed it to pull down to 1600 rpm when pulling a trailer up steep grades. Was I unknowingly allowing the torque rise to work for me or was I simply using the peak torque?

The 2001-2002 stock 24V HO is sort of a special case since the darned thing was programmed with a flat torque curve of 505 lb-ft from 1600 RPM up to (I can't recall exactly) something approaching 2700 RPM, so it really didn't have any torque rise as did the example engine shown on the torque curve I posted earlier. You were just riding the 505 lb-ft torque plateau down through the RPM band until the torque began to drop, and then downshifted to get back on it and ride it down again.

 

[AH64ID]

With all respect, that's not the way the 68RFE functions. You can pull a grade in 6th gear at full throttle with the torque converter locked up, and it will NOT downshift until RPM drops significantly. It will hold a gear and ride the torque curve down until a downshift is required - just like a fully loaded big rig pulling a grade (and, no, I don't have any large car chauffeur aspirations here. ) With 4. 10 gears, my truck will downshift from 6th to 5th under full throttle only after the speed has fallen to ~58 MPH (~1600 RPM).

Rusty

If it won't downshift out of 6th until 58 mph how do you maintain speed up a grade at 65mph when more power is required? With what you stated it wouldn't allow a downshift to 5th and would loose speed on any hill where the power to the ground in 6th was inadequate.

I haven't ever towed with a 68RFE, but what you have stated doesn't sound correct and sounds counter productive to being able to maintain speed on grades, and certainly wouldn't get the 68RFE the praise it gets when towing. But I can almost guarantee if you have cruise set at 65 (or just use your foot) and you hit a big grade and the throttle goes to 100% and speed drops below 65 mph the trans will down shift (even thou rpms are above 1600) and go to 5th to maintain 65 (or what your foot wants). If not that is a HUGE design flaw, you should not have to use the ± on the stalk, its an option for the driver not a manualmatic.

The 2001-2002 stock 24V HO is sort of a special case since the darned thing was programmed with a flat torque curve of 505 lb-ft from 1600 RPM up to (I can't recall exactly) something approaching 2700 RPM, so it really didn't have any torque rise as did the example engine shown on the torque curve I posted earlier. You were just riding the 505 lb-ft torque plateau down through the RPM band until the torque began to drop, and then downshifted to get back on it and ride it down again.

Yeah it's a nice flat torque curve.

The torque curves and all the reports I have seen show the 01-02 HO as having 505 from 1600-2700 as you stated, but the advertised hp is 245 @ 2700, which is only 475 ft/lbs. 505 ft/lbs at 2700 is 260 hp. So I am not sure which is off.

The 6. 7HO has the least flat torque curve of any ISB since 1998. 0, but has the most torque at any rpm so it makes up for it. I have a feeling the 2013 ISB will have a flat torque curve again.

The 04. 5-07 5. 9 was rated at 610 peak at 1600, with at least 555 from 1400 - 2900, and 590 at 2900 rpms, so is also very flat. It's rated at 440 ft/lbs at 1000 rpms, but good luck ever making that.

 

[RustyJC]

I haven't ever towed with a 68RFE, but what you have stated doesn't sound correct .....

I give up. I drive the bloody thing, and somehow I'm not smart enough to know how it behaves.....



Consider the typical towing mode for us - with the cruise control set at 65 MPH and letting the computers do their thing. On a Texas Hill Country-type grade that's slowly increasing up to 6% or so, the cruise will indeed allow the speed to drop before it downshifts. Understand, however, that the computers are sophisticated enough to detect the rate of decrease in engine RPM - if the vehicle speed is dropping rapidly on an extremely steep grade or continues to drop even after a downshift, then the transmission will indeed continue to downshift to put the engine speed as high as 3000 RPM. It's smart enough to go for peak BHP when it needs it, but it doesn't do it on every hill. Frankly, if you've ever felt a downshift in a 68RFE under full throttle in tow/haul towing 16K, you'd probably want to avoid them as much as possible. I really don't want the transmission slamming a downshift to 5th every time the speed degrades from the 65 MPH setpoint to 64. 8 MPH and upshifting to 6th at 65. 1 MPH, an operating characteristic generally regarded as "hunting".



Can I FORCE a kickdown? Yes, if I stomp the accelerator through the floor, but that's not the way I drive. I don't beat on my equipment. When I tow, if I need full throttle on a grade and don't have the cruise control engaged, I'll roll into the throttle gently. Under those conditions, I can get the accelerator pedal to the mat without inducing a downshift or unlocking the torque converter. If I need a downshift under heavy load, I often prefer to ease off the accelerator and toggle my own rather than have the transmission slam an automatic downshift at full throttle - it's much easier on the drivetrain that way.



Rusty

 

[Newt]

I use a Edge CTS which will tell you what gear the transmission is in and if the TC is locked. My experience towing ~15,000# in the mountains (we are currently in Colorado) with cruise control set is that the transmission will downshift from 6th to 4th when the speed drops to 5 MPH below what the cruise is set at. As Rusty says it is a violent shift. Due to that, when encountering an incline I know will require a downshift, I manually downshift to 5th. If I see the truck won't maintain speed in 5th, I will downshift to 4th. The reason being if the cruise is set, it sets full throttle to try to get back to the cruise speed setting. IMHO is you let the cruise dictate the downshifts, you will shorten transmission life and use more fuel.

Newt

 

[Jim W]

Have you changed to a cooler thermostat that stock? No I am reading my factory water gage and there might be some gage error in it from actual water temps.



Why so cool on the EGT's? Your leaving a lot of power unused.

Because I am deleted and this is where the engine will normally run in 6th gear pulling my 5er at 60 to 65 MPH on flat land. I try to keep the RPMS' in the 1800 to 2000 RPM range as the MBRP will start to drone quite load above 2000 RPM's My DW will complain about engine sound above the 2000 RPM limit.



Jim W

 

[AH64ID]

Can I FORCE a kickdown? Yes, if I stomp the accelerator through the floor, but that's not the way I drive. I don't beat on my equipment. When I tow, if I need full throttle on a grade and don't have the cruise control engaged, I'll roll into the throttle gently. Under those conditions, I can get the accelerator pedal to the mat without inducing a downshift or unlocking the torque converter. If I need a downshift under heavy load, I often prefer to ease off the accelerator and toggle my own rather than have the transmission slam an automatic downshift at full throttle - it's much easier on the drivetrain that way.

Rusty

This statement is a LOT different than this one. . and makes more sense. It's all in the wording, no need to get upset.

You can pull a grade in 6th gear at full throttle with the torque converter locked up, and it will NOT downshift until RPM drops significantly. It will hold a gear and ride the torque curve down until a downshift is required - just like a fully loaded big rig pulling a grade (and, no, I don't have any large car chauffeur aspirations here. ) With 4. 10 gears, my truck will downshift from 6th to 5th under full throttle only after the speed has fallen to ~58 MPH (~1600 RPM).

Rusty

It demon straits the beauty of an electronically controlled transmission. I am certainly not trying to start a 68RFE war, was just pointing out that the way you initially described it was not inline with how a transmission should operate.

No I am reading my factory water gage and there might be some gage error in it from actual water temps.

Keep in mind that the OEM thermostat on your 08 isn't fully open until 207°, and max allowed temp is 225°. 207°-215° on a grade is normal, and not excessive.