Cummins smoking at low rpm's under load.

[cerberusiam]

Bearing problems on a Cummins? Even engines that are close to the million mile mark don't have issues.

Any engine that gets even remotely close to a million miles has been driven correctly, or, it doesn't even get close to a million miles.



As for the bearings, one can wipe out the thrust bearings and the rods\mains will still look pristine. Doesn't matter though, once the thrust bearings are gone and end play is over spec its done. Typical spot check on a 12V is the crank end play, especially if it used to really work hard.



The torsional harmonics have been much less of a problem since the advent of the 24V, chiefly because the engine just doesn not want to pull in the harmonics zone. The CR's help that a lot with multiple events also.



The gear ratios of the 12V naturally put those engines into the harmonics zones of 1500-1700 rpms a lot of the time and usually with realtively heavy loads behind. That just hammered the thrust bearings and drive trains to pieces on them.



The harmonics have been reduced but not eliminated on the newer trucks, its impossible to completely get rid of. The better drive trian isolation from passenger cabin has also made it a lot less noticeable unless one is looking for it. However, all it takes is a bad u-joint, a load, and running the truck at 1700 rpms to see the harmonics are there and the potential for damage.



My 12V under a heavy load a 1700 rpms SOUNDS like it is eating the transmission, at 2000 rpms it is nice and quiet. :-laf

 

[AH64ID]

How does low rpm equate to thrust bearing stress? I know the angle cut front gear train keeps some pressure on the thrust bearings as does holding the clutch in, but just low rpm? Inquiring minds want to know



Nick

I believe it has to do with the length of time the power-stroke occurs, and the pressure it builds.



At 1400 rpm's each stroke is twice as long (time wise) as at 2800 and that is more stress on the crank.

 

[cerberusiam]

I believe it has to do with the length of time the power-stroke occurs, and the pressure it builds.

That would only be true if force applied to the piston was constant at different rpms. In a lug condition the force is more than normal but not anywhere near the same generated at 2800 rpms.



That would still only contrubute to rotational forces not longitudal where the torsional harmonics cause the problems. Thrust bearings are longitudal not rotational.



Yes, excessive lug conditions can wipe out rod \main bearings, but, 1700 rpms is not neccessarily a lug condition. It is however dead in the middle of the worst torisional harmoncs and the thrust bearings are no wher enear as durable as rods\mains.

 

[AH64ID]

It's the duration thou. Torque is torque right? So is 600 ft/lbs for a . 2 seconds better or worse than 600 ft/lbs for . 1 second?

I realize that's still vertical, but I thought that it also effected the trust bearings at those kind of loads.

 

[cerberusiam]

It's the duration thou. Torque is torque right? So is 600 ft/lbs for a . 2 seconds better or worse than 600 ft/lbs for . 1 second?



I realize that's still vertical, but I thought that it also effected the trust bearings at those kind of loads.

Tq is TQ at full load, you won't see full load TQ at those rpms because the turbo is not spooled and without air its just gonna get hot, not make all the power it would at higher rpms.



A 20% increase in rpm and the point is moot while load is higher. A 20% decrease in load time is not going to make that much difference when the load is actually higher at reduced load time.



There is always some longitudal load and it will increase with engine load, the thrust bearings handle that fine. What they don't handle is the harmonics which rapidly slams the crank back and forth on the thrust bearings. It is not unusual to find the thrust bearings not only worn but broken in those conditions.



The results are exactly like cavitation in a gear pump. It will live a long time at 150 psi with enough flow so there never is any cavitation. At 20 psi with inadequate flow it hammers itself to death in a short time.



Bottom line is the less time time spent under 1800 rpms under a load results in a dramatic increase in engine life.

 

[AH64ID]

If you look at dyno's you can see full torque can come in at low rpms, they are rated at 1600 rpms on our 05's. It only takes 182 hp to make full torque at 1600 rpms, where at 2800 rpms it takes 320 hp.

 

[Prairie Dog]

Keep it over 1800? I guess my engine is doomed then. It has spent most of its life towing at 1600~1700 RPM's on the highway (where it runs quiet and smooth BTW). Towing any faster would violate local laws in many states and exceed the speed ratings of ST tires .
I wonder why Dodge and Cummins would engineer a power train to be damaged so easily under normal conditions.

 

[cerberusiam]

If you look at dyno's you can see full torque can come in at low rpms, they are rated at 1600 rpms on our 05's. It only takes 182 hp to make full torque at 1600 rpms, where at 2800 rpms it takes 320 hp.

Ratings from an engine dyno are not indicative as installed in the chassis. The process of developing engine dyno graphs is totally opposite of how the engine is actually used.



What is left out of engine graphs are all the safety features that limit TQ and HP at low rpms in the truck. All the safeties are off, the engine is at full load and pulled down to kill point in those graphs.



Yes the engine will develop 600 ft\lbs of TQ at 1600 rpms under FULL LOAD. Now try that as instaled in the chassis and you will see the difference.



All the transmissions used in these trucks are rated at 550 ft\lbs or less of TQ for acceptable operational life. Is really logical that limit is exceed on a regular basis?

Keep it over 1800? I guess my engine is doomed then. It has spent most of its life towing at 1600~1700 RPM's on the highway (where it runs quiet and smooth BTW). Towing any faster would violate local laws in many states and exceed the speed ratings of ST tires .

I wonder why Dodge and Cummins would engineer a power train to be damaged so easily under normal conditions.

Huh???



ST tires are rated to 65 mph and thats not out of the OE designed rpm band, especially with a manual.



Where in the US is a towing speed limit at less than 55 mph? The only place I know it is 55 mph is California and you get run off the road trying to drvie that slow, towing or not. :-laf



The factory configuration of these trucks is for average speeds of 65-75 mph to keep them at or slightly above peak TQ and BSFC. Thats well above 1800 rpms even with the automatics and well within towing regs for just about anything out there.



I never said towing at 1600-1700 rpms are damaging, I said (interpretation can be subjective) that heavy engine loads at 1600-1700 are the culprit. Its not the same thing.



There is a huge differnece between cruising along at 1700 rpms on flat roads and accelrating the same load to road speed and not exceeding 1700 rpms.



The load differences are not even comparable. However, purposely gearing the vehicle to put the 5. 9 at those rpms under heavy load for extended amount of times is NOT as they were designed. For a very good reason, they don't LIKE it.

 

[Prairie Dog]

In many places you are limited to 60 because your over 10000 lbs are to be considered a "truck". That equates to 1600 to 1750 RPMs with 3. 73 gearing. No matter what the speed posting, towing at 60 is not unusual.
Then there's the driving through town scenario. I'm generally cruising along very comfortably at 1200-1400 RPMs under a very light throttle. I've done this for many hours on end and there's certainly no eng damage.
Maybe I misunderstood, what I gathered from your posts is that we should never operate below 1800 RPM - in reality I rarely ever have cause to tow above that RPM.

 

[AH64ID]

That's becuase of your tires, and why I went back to stock size in the summer for towing. 60 is 1800 with stock tires and 2000 is 67, not too bad.

If my rpms are below 1800 and the road is anything but a downhill I will downshift, it's much easier on the motor, trans, etc and fuel economy is better for me.

With 285/75's you are only at 2275 rpms to do 60, about the same as a stock truck doing 75. That's where I would run, not 1600-1700 with a trailer.

I don't recall what the truck laws are in Washington, but in Idaho it takes 5 axles to be a truck regardless of GCW.

EDIT: Just looked it up, anything over 10K is a "truck", what a joke.

 

[Prairie Dog]

I was speaking of a stock tire configuration - I know my gearing is slightly higher.
Running on the flats at that high of RPM in 5th would be a complete waste. The truck cruises effortlessly (even with stock power) in 6th on the highway and with a very light throttle.

 

[cerberusiam]

Yeah, light throttle to maintain speed is way different than heavy engine load conditions. Even 1200-1400 is not bad at light throttle.



What I am talking about is short shifting all the time under heavy throttle and purposely gearing to keep rpms in the 1500-1700 rpms range even pulling grades. That works on an ISL or Big Cam, not on a 5. 9.

 

[AH64ID]

It does appear to cruise effortlessly, but at 60 I burn less fuel in 5th than 6th.

The nice thing about a stout clutch it is will tell you when your rpms are too low

 

[cerberusiam]

It does appear to cruise effortlessly, but at 60 I burn less fuel in 5th than 6th.



The nice thing about a stout clutch it is will tell you when your rpms are too low

Now THAT is some solid proof that low rpms do not always equate to efficiency. What is the rpm differnece and range?

 

[AH64ID]

60 at 1800 or 60 at 2450.

Usually I will run 67-70 or back down to 55. I also get about (± . 5mpg) the same mileage at 68-70 in 6th at 55-60 in 5/6 (trying to hold 6). Lately I have been towing at 55 and 68 as my 2 speeds depending on terrain/limit and get the best economy that way.

My theory is that at lower rpms the motor is more efficient in a very light load condition, but when it needs to make power to move 18K lbs it does it on less fuel with some rpms.

 

[Prairie Dog]

I definitely burn less fuel when running in 6th and towing - by about . 7 or better mpg. Probably depends greatly on the load though.

 

[AH64ID]

I was going to add that my upper rpm mileage really benefited from the low back pressure produced by the Garrett, so on a stock charger that's probably true.