Engine/Transmission (1998.5 - 2002) Sustainable RPM's

[JPLB22]

I spent some time searching forums on this subject. Came across a thread saying that an e-mail from Cummins said you could sustain 2850 for "all day and all night". I placed two phone calls to Cummins (1-800-diesels) for a little more info. Both people I talked to said definately do not hold over 2500 for long periods. As in it is possible to float the valves. They said 2500 ok for "all night and all day". I'm just wondering what most people will stop at as there personal RPM limit for long periods. After engine break in of course.

 

[Hohn]

Since the other ISB 5. 9s are rated governed at 2600 RPM, that's what I would consider my max SUSTAINED rpm.



Peaks over that are fine. Rumor has it that things things don't come apart until 4300 rpm (vp44 limit?).



HOHN

 

[Diesel Freak]

2200-2300 max for log periods

 

[SlyBones]

Whats a long period? OK its not "all day and all night", but what is a long period Hours? Obviously its not just running thru the gears.



Seems interesting in that many use these engines to haul big loads over passes where they are in a low gear, with the R's rev'd up and going for it for what could be a 1/2 hour at a time. Would Cummins consider this a long period of time?

 

[Boondocker]

sustained RPMs

I think there are a number of practical and preferential considerations here, of which, length of time probably doesn't make the top 5.



All day and/or all night is hardly a reasonable time frame unless you're talking about a marine application where it might be possible to travel for many hours with operational and environmental factors staying within a nominal range. Even when I'm driving a long trip, I can barely use cruise control because conditions like the road and traffic won't stay the same long enough. So from that standpoint, I can't sustain an RPM long enough for it to be an issue. However, during these brief moments, I've pondered the issue.



What really makes me think about it is when I'm pulling my little travel trailer and I can't go fast enough to warrent 6th gear (speed limits, traffic conditions). I believe lugging the engine is as detrimental as over-revving. So I don't like to be under load (carrying weight up a grade) with the RPMs in the teens. With RPMs in the low 2000's, engine response is better, boost is higher and EGTs are lower.



You can debate all day about "engine sweet spot", I've heard some deny such thing, as for me, I believe. Emperically, I think you could make a case for the RPM at max torque as being the sweet spot, that is when the engine is most efficient in terms of energy output (power) for energy input (fuel).



I don't like cruising at 2500 RPM. It's loud and just doesn't feel right. If I'm pulling hard, it's ok, if I'm just maintaining speed, it's uncomfortable. I have cruised for many hours around 2100 RPM, which equates to about 75 mph in 6th gear with 3:54 in the diffs. (at the speed limit, not towing) and felt quite comfortable. But with the torque monster under the hood, I don't want to say how many times I've tried shifting into 7th



Neil

 

[Snow man]

The redline doesnt start on the tach until 3250. I was told by the local Cummins shop you couldnt hurt the ISB,it was governed well below the true redine and could be run at 3250 indefinetely. I ran my truck to Fla lst fall,with 4. 10 gears I cruised at 2300-2550 the entire time 17 hrs,both ways ,if anything the truck ran better after the trip,and my oil sample was pulled from that oil,came back perfect,all wear metals,and contaminants well within normal for 7500 miles on the oil.

 

[Tom W]

Just a question.



Why would time have any effect? Would 10 minutes at a time for 10 times be less harmful than one time for 100 minutes? These engines don't get tired and obviously don't get rested when slowed down. Seems to me like other factors such as lugging and loading are primary concern.



Tom

 

[JPLB22]

The only reason that I brought up time is, sometimes under hard exceleration I hit 2800 and 2900 RPMs easily. This feels comfortable to me. But I would not feel so comfortable towing a 8000# trailer up Parley's Canyon at 2850. Parly's Canyon is quite steep and in the hot months you see many vehicles on the side with their hoods up (ie: gassers, cars,eighteen wheelers). At 2500 in third I do 65 mph. and it will take me about 30 - 35 min. to reach the summit. Do anyone think I am over doing it at 2500.

 

[RustyJC]

No. 2350 RPM is 70 MPH in our truck (4. 10's). We pull a 36' 5th wheel cruising at 70 MPH all day during the summer in the Texas Hill Country at 21,180 lbs GCW (21,500 lbs GCWR). 2500 RPM for 30 minutes isn't even worth worrying about.



Rusty

 

[DocStressor]

Just think about marine applications. Marine diesels are governed at 2400 to 2600 rpm and typically run all day at or close to full throttle. Pushing a semi-displacement hull through the water is like towing a 5ver up hill. The typical time before overhaul is 10,000 hr for the Cummins in marine use.



These engines are designed to work under load. They have huge bearing surfaces, high oil flow capacity, and heavy duty cooling systems. Short trips and frequent startups are much harder on this type of engine than running under load near the governed rpm limit.

 

[Hohn]

I never ran higher rpm much until I tried it once for a whole tank. i would cruise at 2100-2600 instead of upshifting. Engine seemed to smooth out and is now much happier having done it.



Engine also smoothed out even more with my switch to Redline oil. I wring its neck now, and I am starting to believe what they are saying about not being able to hurt the ISB. Heck, the ISBe doesn't even reach rated power until 3200 rpm (or was it 2900?) It's higher than our ETH/ETC motors @ 2700 rated power.



When I really wind it up (3k) it feels more like the VP44 is unhappy than anything else. Sounds like a good reason to use a lubricity additive.



Hohn

 

[JJW_ND]

BP.....Before power

Before I found some extra power I pulled several very tall, very heavy loads in 4th gear (would not hold 5th) for 700 miles straight through stoping once for fuel running 2700 rpms the entire way. I got between 4. 5 mpg - 5. 5 mpg overal each trip! That was 150K miles ago.



This was under a good load. Engine pulled up to its defuel rpm on flat road. EGT ran 1200 the entire time. Running empty at the time I could pull 3,000 rpms.



Running these engines under a load and empty are like apples to oranges.



I would not worry about running WOT day and night (if you can aford the fuel) under a good load. Running WOT empty is not possible as even with our open country you would have trouble keeping it on the road at 110 day and night.



jjw

ND