JKE

I just read the "uh oh..." tranny thread. someone stated that 3.73 rear and 265 tires is running 73 mph at 2000 rpm.

Not my truck. I might be a RCH over 65 at 2000 rpm. Is this just a difference in auto to 6 speed or did they get the invoice wrong? ???

Thanks for any input,

John

doug

about a 6% difference between autos and manuals. At 73 mph, the auto is running at 2050, give or take tac calibration accuracy, and tire wear. The manuals run 2168, give or take.


you should be at about 68 mph pulling 2000 rpm

JKE

Thank you for the wisdom Doug.

After I posted and was on the way home, I ran at 2000 rpm in 6th...she was hitting 67 mph.
I am glad the paper work is correct.
Also, I NEVER run my truck over 2000 rpm on the slab...I feel like I am abusing it. I know they can take it, but I can't bring myself to do it (gasser syndrome )...and yes, I get passed by everyone :.

Thanks again.

John

Haulin_in_Dixie

Hey take it out on the freeway and run it a couple of hours in direct at highway speeds, it'll probably cure you. You will be doing the engine a favor to run it up now and then and don't be afraid to downshift and run it up to 3000 to pass someone.

Copenhagenjunkie

I agree with Haulin In Dixie. My truck is completely stock so 3000+ to pass. I have always run er hard so don't ask me about fuel mileage ;D. My old man drives around lugging his truck and I tell him he better run er hard at some point.

JKE

The truck is very strong, no doubt. This is part of the reason I do not run hard. You don't have/need to.

13,900 miles so, I bet it is not broken in yet from what you guys say on here. I will try to FORCE myself to run it up a bit. It's just that extended periods at high RPM are making my mind say "don't do it" "don't do it". "it will smoke, rattle and knock like an old chevy v-8 in 2 years if you do"

I may do 5th for a short period so I am not speeding too much and slowly get myself out of this mind set. Tickets are a bad thing....and then I have to explain to the wife :-[

Later

John

Haulin_in_Dixie

JKE, when I had the five speed, I was always worried about the pulling hard in OD, so ran a lot of miles in direct at 2700 to 3000, shifts were always at 3000 plus with the five speed and heavy weight. With the truck stock there were areas like on I81 where it would stay in direct at near or more than 3000 for many long periods of time at wot. Cummins said that it was fine, it could run at wot at the govenor max all day long. The same engine in a mid sized truck may be only geared for 65 mph and the drivers will keep the pedal on the floor for hours on end on the freeway. These commercial engines are made to run high and the govenor protects them. I once put a thread on here about rebuilding a Detroit engine. The run in sequence was ten minutes at a little above idle, ten minutes at a mid rpm and then 30 minutes at wot with the truck sitting there. That means place a brick on the pedal and go have coffee! After that sequence, go load the truck and run it. Did this many times. Look at it this way, many smaller engines in cars are running close to 3000 just going down the freeway on the way to work. A 69 Chev C65 that I had for a few years with a truck 427 was governed at 4200 and used every bit of it. I recalibrated the govenor to 3500 to keep it together longer and save fuel, the drivers were killing me with gas costs on a wrecker. 3000 sounds high on the Cummins but is really not all that wound up. You will not break the thing. If pulling heavy it is easier on the engine would up than lugging at 2000.

Boy am I going to get some comments on this one

JKE

Haulin',

I spun it up running through the gears yesterday afternoon. Pretty quick for a big truck...and did I mention that my exhaust is loud! It sure sounds good at 3/4 throttle and 2800. Music to my ears. That might be enough to get me to run it a little harder.

John

bulabula

2000+ rpm abuse? I certainly wouldn't worry about running it over 2000 rpm, 3200 maybe.

Heck, in my truck if I did that (under 2000 rpm) on the slab around here, I'd have to worry about someone hitching themselves up to my ball mount - about 62 mph....

Unloaded, I get about 16.5 (Overhead readout) on cruise control at 2400 rpm (~75).

doug

a few thoughts:

keep in mind that the governed speed (3200) is well under the true red line for the engine, which is well above 4,000 if memory serves. the red line on your tac is probably chosen to correspond with the governed speed, not engine red line. you can be sure that DC and cummins have chosen the governed speed very carefully and that 3200 is no where close to engine breakdown speeds.

don't forget about warm-up. run her easy and don't pull anything while she's cold. this might be incovinient and even impossible at times, but it would be better to run around for 5 min empty before hitching up. The warm up period should be characterized as moderate to easy driving while empty. If you have to hitch up, stay in low gears and baby her until she warms, and don't ever apply a full load until she reaches 190 degrees.

yea, the CTD likes to work hard, and you can run all her day at 3000 without putting long term reliability at stake. Its also true that the governed speed is 3200 but I wouldn't say she "likes" to run in this region. Actually she doesn't run very efficiently above 2900 and makes only 233 HP at governeed speed (3200). peak HP is 305 and occurs at 2900. You're better off to run at 2900 if you want her to give you maximum HP. In fact, there is zero benefit from going over 2900 in a towing situation, for example, unless you are compelled for some reason to maintain a certain speed. For example, you're running a 4.10 automatic up a very steep grade at WOT, and you HAVE to go over 45 mph if possible, AND the best you can do (given your load and the grade) is 3rd gear with TC unlocked (you automatic guys, is that realistic?). Assuming thats a realistic situation (I don't know, I've never driven an automatic), my calculations show that this should yield around 48 mph at the governed engine rpm (3200). you won't hurt anything, but you'll consume fuel at the rate of about 3.55 mpg.

I like the comments about ring seating and that you should be unafraid to use the higher rpms to make sure that ring seating occurs at the longest possible piston excursions. piston acceleration goes up pretty fast with rpm (if memory serves, the square of rpm), and peak acceleration of the piston occurs when piston speed is instantaneously zero (TDC, BDC). therefore, the forces acting on the piston at TDC and BDC are very large, and will translate to miniscule stretching and bending of several components (rod, wrist pin, for example), and some miniscule give in the crank and rod bearings themselves (the oil film gets thinner). I'm not sure that the ring itself would travel more on the piston with increasing rpm (seems like its peak tolerance would be acheived due to friction with the cylinder walls, and not rpm related), but any engine experts out there can please correct me.

I'm not an engine designer so I really don't know how to quantify all these factors in a real engine design -- I'm only saying that from a materials engineering standpoint, things stretch and squish under large forces, so I claim it is a falicy to assume that there is no change in ring peak-to-peak travel over the extreems of rpm.

breakin depends on miles and load. It takes up to 50,000 miles to break in a CTD if it is lightly loaded or running empty. A freind of mine has a 99, and at 65,000 miles his fuel economy still improved. He runs empty most of the time, or pulls a lightweight utility trailer or a 5,000 lb boat/trailer. Thats hardly enough to phase the CTD.

As for breakin loads, Cummins told me that the first thing they do to a completed engine is put it on the engine dyno at full load. So while I agree with the 500 mile conservative driving suggestions (before full loads) on a new truck, I think this has as much to do with the rest of the drive train rather than the CTD itself. especially for 6-speeds, the clutch needs to wear in properly when new, or you'll affect its behavior for the rest of its life.

finally, while we spend all this time fine tuning our breakin and running techniques for maximum engine life, note that many people don't care about any of this detail and their engines STILL outlive them :-).

JKE

I am no load all the time with rare exceptions. I guess I am the 50,000+ break-in engine considering how easy I am on it. I am very careful to warm it up and take it easy on a cold motor.

so, WHY did I buy it?? Because I WANTED A CUMMINS. Does not matter if it is needed or not. Now I have no limitations.

Thank you for the input.

John

doug

hee hee hee. sure you have limitations! for example, you can't pull most 5th wheels that have bedroom slides. the hitch weight puts you above GVWR.

so there -- now do you feel better?

JKE

Yeah, but I can pull uncle cooter's mobil home across the farm and relocate it for him.

Boy, the horses would get a kick out of that.

as a matter of fact...I FEEL GREAT about this truck...and the wife likes it...

John