Upper RPM limits on HPCR ??

[JPLB22]

When I asked this question when I owned my '02, I got the same answere from Cummins and TDR, 2600 rpms is the highest I would want to sustain for long a period. I also understood that it was the VP44 that was part of the problem with sustaining high rpms.



So what is the upper limit of sustainable rpms with HPCR ???



In my uneducated mind I don't think the CP3 would care very much what the rpms are. And I would think that electronic injectors should be able to fire faster than the VP44 fired the injectors.



As long as my temps are within limits, would it cause any problems to hold 2800-2900 rpms in 4th gear ( NV4500 ) for say 10 min. ???



TIA



Jeff

 

[FDavid]

i drove my truck at 3600 rpms for over 5 minutes and it was begging for more.

Dave

 

[SRadke]

Run it like you stole it!

 

[mberry]

Jeff,



You can run your engine at 3200 all day long as long as EGT and coolant temps are within limits. It won't hurt a thing.



Could have done the same thing in your '02 also. The limit on the '02 would have been fuel delivery, not necessarily the VP-44 itself.

 

[JCleary]

Cummins engines are designed to run at redline indefinitely, as long as your temps are in check.



Similar engines are used for 10,000+ hrs. in construction applications, running at WOT almost the entire time.

 

[MMeier]

Rev it till it revs no more !!! As long as temps are in check.

 

[Vaughn MacKenzie]

Cummins engines are designed to run at redline indefinitely, as long as your temps are in check.



Similar engines are used for 10,000+ hrs. in construction applications, running at WOT almost the entire time.

Yeah but those engines are governed to 2600 RPM. If you look at other applications of Cummins B-series engines you'll see they're almost always limited to 2600.



I'm sure our CTDs can run to redline for hundreds of hours, but the laws of physics don't go on vacation and likely it will take a toll on the rod bearings after awhile.



Vaughn

 

[JPLB22]

I was real pleased with the responses until Vaughn rained on my parade .



It seems to make sense that if I run 2900 rpms everywhere I go, it will put a little more wear and tear on the old Cummins. As compared to 2000 rpms. But I'm thinkin' that 10 - 15 min. at 2900 rpms once every 3 or 4 weeks when we go camping won't make much of a dent in the long life of the Cummins .



Jeff

 

[JHardwick]

Yeah but those engines are governed to 2600 RPM. If you look at other applications of Cummins B-series engines you'll see they're almost always limited to 2600.





Vaughn

How much of this is a powerband issue?

 

[EBottema]

Running the engine at higher RPM's puts more load on the bearings. If you run a cummins at 1000rpm for its life it will definatly last longer than one ran at 3000rpm for life. Rule to stand by is, occasionally run it at 3000 only when needed but for daily use, keep it at 2500 max...

 

[JHardwick]

If you run a cummins at 1000rpm for its life it will definatly last longer than one ran at 3000rpm for life. Rule to stand by is, occasionally run it at 3000 only when needed but for daily use, keep it at 2500 max...

I'd say surface speed (SFM) of the bearings would make this statement true on any rotary mechical device. Not loading.



Also, there is a savings of total revs to perform the same work.....

1 hour @ 3000RPM = 180,000 revs

1 hour @ 2500RPM = 150,000 revs



From 3000RPM to 2500RPM you've accomplished the same work, with less total revs and at a slower bearing speed.



I'd being willing to bet that the 2600RPM limiter has at least something to do with the power curve. I don't have any dyno charts in front of me, but I'm guessing 2600 is in the meat of the torque curve.



This is only my best guess

 

[EBottema]

If the 2600rpm is the top of the power curve, then there would be no reason to run at 3000 rpm since all you are doing is putting more pressure and working the engine harder. For that extra work you are really getting nothing out of it.

 

[JHardwick]

I'm not really sure what your saying there Erik.



Here is my take after looking at a dyno chart ... ... ... ...



Power and torque start to fall off drastically @ 3000RPM



So, in a constant RPM situation (generator, marine, dozer, etc) you can accomplish the same work at 2600RPM that you can at 3000RPM, therefore there is no need to turn the extra revs.



In a variable RPM situation (Ram P/U), that extra 400RPM will get you a few MPH more before the next shift, which will get you a few more RPM at the next gear, decreasing bearing load on the bottom of the next gear.



That's my final guess, I'm going back to bed :-laf



Oh, I should have said that the torque is basically the same at 2600RPM as it is at 3000RPM.

 

[Vaughn MacKenzie]

I was real pleased with the responses until Vaughn rained on my parade



Jeff

Sorry Jeff



When talking about RPM and load on bearings, I believe the pressure put on them when the pistons change direction goes up exponentially with RPM rise, and bearing pounding is more of a concern than bearing speed.



I know at 3000 RPM that big heavy piston and connecting rod make 50 complete round trips up & down the cylinder PER SECOND



JHardwick I think in other applications the power curve is set up with more of it at lower RPM. If you look at Cummins' website and read through all the applications and engines listed, many have higher torque but less HP, but governed RPM is 2500-2800.



Buses: http://www.everytime.cummins.com/every/applications/isb_shuttle_ratings.jsp

Motor Homes:

http://www.everytime.cummins.com/every/applications/isb_motor_ratings.jsp

Emergency

http://www.everytime.cummins.com/every/applications/isb_fire_ratings.jsp

Medium Duty Trucks

http://www.everytime.cummins.com/every/applications/isb_ratings.jsp

Agriculture

http://www.everytime.cummins.com/every/applications/b_series_ag_ratings.jsp





Vaughn

 

[Turbo_Bob]

So what is the upper limit of sustainable rpms with HPCR ???

As long as my temps are within limits, would it cause any problems to hold 2800-2900 rpms in 4th gear ( NV4500 ) for say 10 min. ???

2900 RPM's is PEAK torque & power, yes it would be fine if temps are under control.

 

[Big_H316]

Myself and two other guys are in the same hot-shot business. One has an auto( ) and my boss has the six speed, same as me. only difference between us is, the other two clowns will run their motors at 2500-3000 for the duration of the trip and cry about poor fuel economy..... DUHHH!

I run mine in 6th gear no matter how heavy the load as long as I can keep the rpm's at or above 2000. With the engine @ 2000 or above it will pull just about anything you need it to. Above that rpm,(2400 +) in my opinion, you are wasting fuel and the life of the motor. The guy with the auto refuses to use overdrive and my boss refuses to use 6th gear because both of them say it will destroy the transmission... . not so in my opinion, as i have pulled over 25k lbs of load for 194k miles... all using 6th gear. :-laf

Finally changed the transmission gear oil the other day and other than smelling a bit foul there was nothing wrong with it.

 

[RKerner]

Sorry Jeff



When talking about RPM and load on bearings, I believe the pressure put on them when the pistons change direction goes up exponentially with RPM rise, and bearing pounding is more of a concern than bearing speed.



Vaughn

Bearing forces are a function of the rotating imbalance to offset part of the reciprocating imbalance (piston and part of the rod. ) These forces are mostly part of the shaking forces made by the piston side pushing on the cylinder wall during the power stroke and the crank webs trying to balance this motion/force and the moments created by the distance the rotating imbalance is from the engine center of gravity. The bearing forces are mostly non-linear functions of sine and cosine and increase as rpm goes up, but not exponentially. Another adder to bearing forces is the pressure curve due to combustion. Up to peak torque, it increases bearing forces and afterwards, it decreases inline with BMEP, I think. One good thing is that inline sixes are inherently balanced and the shaking forces, and the resultant bearing forces are kept in check in a rather orderly fashion due to symmetry. Couple that with a properly designed mounting system and RPM is not a limiting factor in bearing life. At high rpm, piston mass is a big deal and acceleration rates of the piston just before and just after TDC and BDC are of concern, but that's another discussion.

 

[LDobie]

My question is, WHY is there a need to spin one of these at 2900 rpm?? I can see if you were humping a real big load up over a steep mountain pass in 4th gear at like 70-75mph?? Doesn't she have enough muscle to hump that load at 22-2300 rpm in the next higher gear??

 

[JPLB22]

RKerner ... that was a pretty interesting paragraph, although I had to read it 4 times, it was interesting thanks !



LDobie ... The reason I wondered about pushing it to 2900 for a little while, is because I've heard from so many people to NOT tow uphill in 5th gear overdrive ( NV4500 ). I read a tip about watching the load in 5th, someone said do not maintain anything over 20 psi boost while towing in 5th. It seemed like a pretty good tip, but I can hit 20 psi empty if I push it up some steep canyons. But I certainly am NOT going to downshift while going uphill empty. But I figured that with a load it might be a good idea to downshift while going uphill, but I also didn't want to be limited to 60 mph maybe 65. I would like to be able to do 70 mph ( I'm only talken about towing maybe 4-5000 lbs here, so I feel 70 is safe ).



I pushed it to 2850 rpms in fourth this last weekend ( empty ) which put me at 70 mph, and it felt real good and smooth. Seemed like it wouldn't harm it to do that for 10 min. or so.



Jeff

 

[Tim]

I have held mine to the floor with the wastegate open for several minutes, slowed down, and dropped the hammer again without issues. I can't belive you guys hold your trucks back to 70 MPH. I can't believe how fast my truck accelerates from 70-85.