RPMs?

[AHarris]

What is the recommended operating range of the 6. 7? With the G56, I am at about 2450 running unloaded on the freeway at 80. Would frequent running at these higher RPMs affect the longevity of this engine?

 

[grasmo]

Technically yes. You must have 411's . Real world keep oil in it and run the hell out of it ... . I've got a 5. 9 also w/500k +++ on it & has had the hell runout of it by my workers and still runs as good as new. I think the 6. 7's will be just as good.

 

[RVTRKN]

The sweet spot for my 6. 7 C&C is around 1900 to 2100 RPM, at 2100 RPM I'm a little over 70 MPH and at 1900 its somewhere around 65MPH. When pulling heavy on grades, and I'm running at 2100, it just seems to never slow. But with that being said, I don't like to run that low of an RPM on a heavy pull up steep grades. I will keep the RPM's at around 2500 RPMs in 5th and if I'm slowed by another truck, I'll keep it at 2500 RPM in 4th if necessary. The higher RPM keeps the air to fuel mixture high, for a cooler exhaust for better EGT's. I have asked many techs about this and they all agree that you can run a Cummins at or below the governed engine speed all day long, but I run no higher than 2500 RPM. On my 325/600 CR and my 6. 7 305/600 CR, 3200RPM is the governed speed, and my preference is no higher than 2500. If I'm passing a slow truck I will not hesitate to take it to the governed speed, but only briefly and then shift back to 5th. 2500 RPM is about tops for me on a flat run and an empty truck, but there are times out in the desert on long stretches, were I'll go over that and hit the governed speed when passing, but then again it is very brief

 

[Coalsmoke]

Not to pull the thread off topic, but I was thinking last night, I've bumped the redline a few times here and there, playing around. Has anyone come across any data stating what the destructive resonance RPM levels are for this engine?

 

[WMonroe]

For the 5. 9 it was around 4200 rpms. It is probably similiar for the 6. 7.

Will

 

[HBarlow]

A while back I found the recommended operating band of the ISB6. 7 on the Cummins website and posted it. I can't find it right now and don't want to spend an hour looking but I think the recommended band was 1900-2400 rpm. IIRC, 1950 was the recommended rpm for most economical cruise.

The engines were designed and built to work at full throttle, full load, at governed rpm all day, every day, for 350,000 miles or more. If we leave them at factory power ratings and change the fluids and filter at reasonable intervals we can't hurt them in our Dodge applications.

 

[AHarris]

I've never had mine over 3300. When I did it was on a flat spot of open road where I wanted to see how fast this beast would go. I got to 102 (105 on the speedometer) indicated on the GPS and still had a little left but I didn't want to push my luck in case a cop came along plus I am usually pretty easy on this truck. I wasn't flooring it either. That's pretty damn fast for a 7000 pound beast!!

 

[RVTRKN]

For the 5. 9 it was around 4200 rpms. It is probably similiar for the 6. 7.

Will

Its 3200RPM

Not to pull the thread off topic, but I was thinking last night, I've bumped the redline a few times here and there, playing around. Has anyone come across any data stating what the destructive resonance RPM levels are for this engine?

Yes, while shifting, pulling heavy, I hit the red line all the time. to keep the power band right where I want it (HP + Torque). The trick is to shift just before and don't let the governor engage. Once your in the gear you want, pulling a grade heavy, you should settle back to your desired RPM. If you let the governor engage your engine is starved of fuel and you will lose the momentum of the forces at work.

 

[WMonroe]

Its 3200RPM



I read somewhere in the past that while the redline is 3200 rpm, destructive harmonic vibration occurs at 4200rpm.



Will

 

[RVTRKN]

Its 3200RPM



I read somewhere in the past that while the redline is 3200 rpm, destructive harmonic vibration occurs at 4200rpm.



Will

The governor will limit the fueling at 3200RPM unless it been modified. Even the Drivers Handbook states that you need to be careful about that with a manual, especially with a manual trans. The engine will scramble, at what the RPM is, I don't know, but its the valves springs that are the first to go.

 

[Coalsmoke]

Mine will go past the redline like its not even there. The owners manual doesn't say that I could find what the magic RPMs are for breaking things. It would be nice to know to program it in to the comp.

Thanks in advance if anyone finds some hard factual data on this.

 

[RVTRKN]

Mine will go past the redline like its not even there. The owners manual doesn't say that I could find what the magic RPMs are for breaking things. It would be nice to know to program it in to the comp.



Thanks in advance if anyone finds some hard factual data on this.

Is this when you are down shifting or engine braking going over the 3200 RPM mark? If yours goes over the red line when excellerating then you might want to take it to a dealer and have that checked. If your is stock, then there is a problem.

 

[Coalsmoke]

500HP / not stock.

 

[RVTRKN]

500HP / not stock.

Gee, did something fall off as well?

 

[Coalsmoke]

Let's just say that the EGR, Nox, and DPF systems are heavier than I would have thought.

 

[AHarris]

Speaking of RPMs, I've been told (either read on this site or heard it somewhere else--don't remember) that when pulling a grade under load (such as towing a trailer) the RPMs should be up over 1800 or so to ensure you don't get the EGTs too high and melt something. So when I'm pulling a steep hill around 50 with the trailer I drop down into 5th to keep the revs up and the EGTs down.



Best towing speed with this truck seems to be around 65 where the RPMs are up high enough to avoid high EGTs on hills (doesn't even slow down if cruise is set) but still within safe limits for towing a camper (with new tires and regularly serviced wheel bearings and trailer brakes).

 

[RVTRKN]

AHarris, you probably heard that from me, I have posted many times about that subject. Its because the more RPM, the more air will be mixed with the fuel at combustion time, and that will help keep the EGT's safe. I don't have a EGT gage, but my trucks are stock. My diesel tech says in stock form, there is no need, as long as you don't lug the engine. There are several write ups about that, and the theory of diesel burning properties under compression in many different mags as well as the TDR mag. I just don't remember which mag or which article. A manual trans is easier to do this, being we shift when we want and what RPM we want to run at.