Mpg. Question when towing

[RVTRKN]

I'm referring to operating the engine above 2700 RPM were the torque falls off on the other end. If you look at the charts that were provided by Don/TX, it seems to me that the RPM I like to run at is just before the peak HP and well before the torque falls off. That is probably why my performance seems better to me at the RPM range I run at. It seems to me that running at 2200-2300 RPM pulling heavy over grades (21K+) is the best performance range and the CTD and is strong enough,(structural) and if there is more wear on the CTD than were you run yours, it will be only minimal at the worst. I guess only time will tell me wether I'm right or wrong but if I get 300K miles at which I expect to get and sell them or retire them, then it doesn't matter.



I just ran from So. Cal. to Las Vegas with my 04. 5, with a light trailer at about #3000 and set the cruse at 57 MPH and never even had to down shift, for it never lugged going over the Baker pass. It is when I'm towing heavy I run them at the range I mentioned.

 

[AClayton]

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!

.

I have only placed about 15K on my odometer with the CTD and seem to learn something everyday on these threads. Originally,I thought that this was a dumb question and feared some sarcasm but I just said 'the heck with it'and asked anyway. I'm glad I did now!!!From now on ... . 'the only dumb question is the unasked question' ... is my motto! Thanks again.



Alan

 

[DonTX]

In days of old, and it may have NOTHING to do with our Cummins, the rule of thumb was that the RPM where the torque curve and the HP curve CROSSED was the "sweet spot" to run at for best performance and economy. You may not want to try that, though it is just another old wives tale. That would be 2500 rpm on my ol 12v, and it don't seem to be a sweet spot at all for it. For you 24v guys that would be 3200 rpm!

 

[HBarlow]

In days of old, and it may have NOTHING to do with our Cummins, the rule of thumb was that the RPM where the torque curve and the HP curve CROSSED was the "sweet spot" to run at for best performance and economy. You may not want to try that, though it is just another old wives tale. That would be 2500 rpm on my ol 12v, and it don't seem to be a sweet spot at all for it. For you 24v guys that would be 3200 rpm!

Don,



I think computer controlled engines probably changed that. I never owned one before the '01 24 valve and only drove a diesel-powered light truck a few times before that so have no experience with the old mechanical injection engines that you speak of.



RVTRKN,



Our engines don't produce more torque at higher engine speeds but do produce higher horsepower at higher rpms. I think that is what you are feeling and is the reason you like the higher rpm. If I'm pulling heavy and look up and see a long steep grade several miles ahead and put my foot in it so that I hit the bottom of the grade at 2200 rpm it will hold the speed better than hitting the bottom of the grade at 1600 - 1800 rpm. I know I burn more fuel doing that but it is a quicker way to get over the hill.

 

[RVTRKN]

HBarlow are we talking about the same thing. I don't run at 2200-2300 unless I'm on the grade, but usualy I'm running at 70 MPH and 2050 RPM and as the engine RPM drops to 1800 I'll shift to 5th and run the RPM at 2200-2300 climbing the grade.

 

[BDW]

From my experience I would have to concur with Mr. Barlow. At 70 MPH my RPM is 1750. Pulling my 5ver, it loves o/d and hills. I'm not sure on my weight but hope to find out in a few weeks. My last trip I average a little over 12 MPG.



I have the 6. 7 with 6 speed auto and 3. 73 gears.

 

[LRutigliano]

Just got back from Cannon Beach, OR.



I got stuck in traffic from I-205 South to I-5 to 217 to HWY 26. Took me 1 hour to go 30 miles. I knew my fuel mileage would suffer.



Got home, filled up the truck. Got 10. 8mpg with no running around w/o the trailer. All tow miles.



Had the Smarty on LEVEL1, with stock TM and stock RP. Saw my EGTs drop by approx 100-150F (1250-1300F before and less than 1150F with Smarty) under full throttle, full load, up a hill in OD. Engine also seemed quieter or less straining up the hills with Smarty.



Going to Kahneeta for the 4th. Will check mileage again.



Louis

 

[RHestand]

Since I have installed my new SBC clutch and solid one pc. flywheel I'm very aware of RPM below 1600.

The stock dual-mass setup very much masked any slight lugging conditions at 13-1400 RPMs.

With the SBC it fills the cab with harmonic sounds, especially in 5th gear.

I called Peter at SBC and he said," it's normal, and would get quieter over time. "

At least the clutch does'nt slip any more, but I do nothing under 1600 RPM, except accelerate from a stop.

Ray

 

[AH64ID]

Older diesels had tq curves like pikes peak, that is not the case any more. The 325 5. 9 make 440 at 1000, peak is 610 at 1600 at at least 555 from 1400-2900... thats FLAT. Yes Tq is what gets you moving, but hp keeps you moving. Which is why I can't pull a hill in 6th at 1600 but can pull it with ease in 5th at 2150, there is hp to accompany that tq... also there is more boost and more fuel available. Also with the enigine working at a lower load your economy will not suffer as it has an easier time making the power.

I have a SGII in my truck and there isn't much if any difference in 1600 WOT and 2150 65% throttle in fuel burn. The engine is now not working as hard, and running cooler. 70% load is considered the line between normal and severe use, so 1600 WOT is considered severe use and 2150 at 65% is not.

My truck pulls very well when towing from about 1400 up, thou I don't load it very high below 1600, and never WOT. Now around town 1400-1500 is my shift point.

Each gen engine has a different sweet spot. The HPCR 325 is around 2000-2200. I can't speak for the 24V or 12V...

Here is the graph on the 6. 7

#ad

1998 - 2000 ISB engine specs

I wish they would update that site with HPCR info.

 

[AClayton]

Since I have installed my new SBC clutch and solid one pc. flywheel I'm very aware of RPM below 1600.

The stock dual-mass setup very much masked any slight lugging conditions at 13-1400 RPMs.

With the SBC it fills the cab with harmonic sounds, especially in 5th gear.

I called Peter at SBC and he said," it's normal, and would get quieter over time. "

At least the clutch does'nt slip any more, but I do nothing under 1600 RPM, except accelerate from a stop.

Ray

I believe this to be my issue as well... . only difference is yours is manual and mine is auto. I never noticed the lugging before because the factory convertor must have slipped a little,whereas,my ATS 5-Star does not at all. When this thing locks it feels like a race car with a shift kit in the trans. The feedback generated from this thread is basically telling that any mileage lost from spinning at 2000rpm loaded will be marginal in comparison to the egt build-up from lugging at 1300 or 1400rpm. I also read a thread in the 24V forum,where one member was complaining about engine driveabilty concerns... . and a bunch of his symptoms match my truck..... the member feedback suggested a new VP:{:{ I may have other problems to contend with also