Engine/Transmission (1998.5 - 2002) Cruising \ Towing RPM

[klaybus]

I was pulling this weekend (#6000) & at 70 MPH I was turning 2400 RPM. What is an acceptable cruising RPM for a 24 Valve CTD without harming the engine?



Thanks, Clay

 

[GLASMITHS]

towing rpm

The best is 2000-2200. Some one posted a while back that Cummins recommends 15% less your rated rpm. Isn't that 2600. That would be 2210. From my experience that is the best range; however never below 2000. If you need to run all day at 2800 it would be good for your transmission and engine, but hard on the ears.

 

[ACoyle]

2200 works for me. IMHO this is a number that you develope a feel for as you and the truck "Bond". I am only sorry that my number ends up to be about 82 MPH. Makes smokey mad.



AC

 

[RustyJC]

With our 4. 10 rear axle, towing our 5ver between 60-70 MPH gives an RPM range of 2000-2350 in 6th gear. Sustained 2400 RPM operation would only be about 72 or 73 MPH for us - I wouldn't be concerned to set the cruise control at this speed and let 'er rip! It's much harder on your ears than it is the engine!



Rusty

 

[klenger]

IMHO, that's the problem with towing with 3. 54 gears. 5th gear at 2500 RPM at 65 MPH (est) or 6th gear at 2000 RPM at 70 MPH (est). In any case, you won't hurt anything towing at 2500 RPM, and it's better than lugging the engine at 1800 - 2000.

 

[RKohn]

"lugging the engine at 1800 - 2000"

It'll comfortablly pull at a lot lower than that, but I'm sure you do't wanna cruise that slow.

 

[Boldt's Wagon]

I believe you get the best milage at 1800-2000 rpm.



I am also starting to form the opinion that the coolest exhaust gas temp for the same HP output while towing at constant rpm is at 2400 rpm. Below 2400 rpm the air flow drops with rpm. Above 2400 rpm the pressure drops and inefficiencies reduce the air per revolution.



Does anybody else have thoughts or observations on exhaust gas temp vs rpm?

 

[Gary - K7GLD]

I'm surprised your RPM is that high with 3. 54 differential gears - towing our similar 5th wheel, I see 2000 RPM at 70 mph with the samr rear gears and a 6-speed - is it possible the Torque converter slips that much at that speed/load?



Or am I missing something?

 

[klaybus]

RPM's

Gary,

I may have been a touch over 70 MPH but pretty darn close. Keep in mind this is in "drive" not O. D. while pulling. In O. D. empty I turn about 1875-1900 @ 70 MPH. Does that sound any better?



Clay

 

[GLASMITHS]

Best Mileage 1800-2000

Somewhere, someone has a chart on fuel consumptionconfused put out by Cummins and for the 24V the best rpm list was 2040. Anything below that is less, anything above is less. Now having said that, whether this was loaded or unloaded I know not. From experience, my best towing mpg was in 3rd direct at about 50 mph, about 2200 rpm going up the Natchez Trace Parkway. Also the egts' will be lower at high rpm. To an extent. 14. 6 mpg.



Somebody help me find that chart. One of the moderators posted it and I did not save the site #. It was fuel consumption for 24V.

 

[Boldt's Wagon]

The Excel spreadsheet on the link has fuel consumption data on one of the spreadsheet pages. The fuel consumption data was from a Cummins graph reproduced in one of the TDR issues:



http://www.nwbombers.com/board/ikonboard.cgi?s=3e1a18ec7b59ffff;act=ST;f=4;t=1049

 

[Gary - K7GLD]

"Keep in mind this is in "drive" not O. D. "



THAT explains it! We usually pull our 7000 lb 5er in OD in all but steepest grades...

 

[GLASMITHS]

Sorry Bolt, wrong one

And from the description anyway; I couldn't open it for some techie reason.

 

[Boldt's Wagon]

The link and download still works for me. You do need Excel on your computer.

 

[LSMITH]

Lugging is any speed that you add fuel and BOOST and/or RPM WILL NOT INCREASE. If boost and/or RPM will increase you are not lugging period. It can happen at 2800 just as it can happen at 1200. LOW RPM IS NOT LUGGING just because the engine is at low RPM. Lugging is so bad that Cummins recommends that you not lug for periods exceeding one minute. In other words if you are lugging you will know it, and you have plenty of time to figure out what you need to get out of the situation.



There is NOTHING wrong with pulling in OD over 1700 RPM, just keep the boost at or under . 01 X RPM with a stock transmission. 17 PSI at 1700 RPM, 22 PSI at 2200 RPM and so on.



It will be a cold day in hades before I pull for a long distance in Drive when the truck will pull the load fine in OD. The fuel economy and noise are just not worth it when it will not hurt a thing.

 

[GLASMITHS]

LSmith

I don't know where your information comes from, however, the response from Cummins that I read, stated that you should not run at full throttle, in a situation where you are not accelerating under the peak torque, which for the 24V is 1600 (i thinks). They did not say lugging. They said it constituted abuse. And that they would void a warranty if you had a failure relating to that.



Yeah, I know the noise is not pleasant for a Long day pull, but then I do not run high rpm for economy. The cost of the fuel is not even on the screen. It is the driveability and service life of the drivetrain. The GV and 3rd direct at 60 mph puts me at about 2250-2300. Just right. That gives me *7* forward speedsOo.



Edit: Who needs a 5 speed Allison?

 

[LSMITH]

Glasmiths, my definition is more leniant on the drivetrain than yours. Yours allows full throttle, mine simply says that if you add fuel and it will not increase RPM or boost you are lugging. Go read it again.



EDIT: someone that wants a transmission that is truely tuneable to what the customer wants. One that will downshift to the proper gear all by itself, and be in the right gear when you come out of a corner without stomping it to get it to finally downshift spiking torque through the drivetrain. It is also a transmission that knows when it is slipping and will tell the engine so it will back down a bit. Any and all shifts are programmable, making it work right. I would LOVE to have a 5 speed allison, but no one has made an adaptor to get it in my truck replacing the peak of 1985 technology that we are STUCK WITH!!!!!

 

[RKohn]

Assuming you mean a real Allison, not that thing that they put in the Duramax.



How about an infinitely variable drive, sorta like hydrastatic. Set your optimum rpm and let the drive train vary the speed? Subura had a cool variable drive in the late 60's kinda like an overgrown snowmobile clutch. Lots of concepts out there, maybe someday a few of'em will make it under the hood.

 

[pwr2tow]

How can lugging be concidered lugging??



Just a hypotysis here; If i have a stock truck that is lugging (not gaining speed) at 1900 rpm then I add a fueling module and it gains speed how is it lugging in stock form, is the engine being over powered more (fuel) with the module?



If lugging is just not gaining speed then all you have to do is add more fuel until you gain speed. There has to be a better explaination.



Ron

 

[Ray Torresdal]

LUGGING

I would also question LSMITH's definition of "lugging". I've sat on many a long hill, right foot to the floor, and 100 rpm below governed rpm. Not lugging in my book. Lugging would be at rpm below torque peak, and full throttle doesn't increase rpm. Full throttle and no acceleration at 12 or 1400 rpm on our diesels falls into my definition of lugging, 2000 rpm and up just means we have a load that matches or exceeds available power.



I'm not an engineer, just an old truck driver.



Ray



PS: Saw an Allison at my local transmission shop the other day. 40,000 miles, 4000 out of warrantee! That had to make him smile!