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Diesel Engine Operation FAQ
What is the "safe" RPM range for my engine?

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Q: Can I Leave the engine idling for long periods?

From the Cummins Webpage:

A. Do not idle the engine for excessively long periods. Long periods of idling (more than 10 minutes) can damage an engine because combustion chamber temperatures drop so low the fuel will not burn completely. This will cause carbon to clog the injector spray holes and piston rings, and can cause the valves to stick. If the engine coolant temperature becomes too low (60 degrees C [140 degrees F]), raw fuel will wash the lubricating oil off the cylinder walls and dilute the crankcase oil; therefore, all moving parts of the engine will not receive the correct amount of lubrication. (For more information, refer to Cummins Operation and Maintenance Manual section 1-5, Bulletin 3810205-12.)

From Josh Berman to the Cummins mail list:

The only time when extended idling becomes a problem is when it is cool-to-cold outside. At that point, the engine may not be able to generate enough heat to stay warm inside. When that happens, fuel can condense on the inner walls of the cylinders, washing away the oil film and causing accelerated wear. Carbon from the incompletely burned fuel can also clog injector tips and stick the piston rings.

If it was extremely hot, you had the A/C running full blast, and you were parked right up to a wall so there was no airflow through the radiator, you might have an overheating problem (a bad thermostat notwithstanding), but it's have to be pretty toasty outside for that to happen.
Joshua Berman           MidRange Service        Cummins Engine Company          Cummins Homepage:

Q: Can I lug my Cummins engine?

From: Josh Berman <>
Subject: Operating below torque peak RPM

The Cummins B-Series O&M (Operation & Maintenance) Manual says:

"Cummins engines are designed to operate successfully at FULL THROTTLE under transient conditions down to peak torque engine speed (edit by D Fritz: The torque peak occurs at 1600 RPM for the B series engine used in the Ram; the torque peak occurs at speeds as low as 1100 rpm for some larger engines).  This is consistent with recommended driving practices for good fuel economy. Excessive FULL THROTTLE operation below peak torque RPM will shorten engine life to overhaul, can cause serious engine damage, and is considered engine abuse."

So you're right, we don't recommend FULL THROTTLE operation below peak torque RPM.  However, just after the first caution, the manual says:

"Operation of the engine below peak torque RPM can occur during gear shifting due to the difference of ratios between transmission gears, but engine operation must not be sustained more than 1 minute at FULL THROTTLE below peak torque RPM."

So basically you've got 60 seconds of continuous FULL THROTTLE operation below torque peak RPM; if the engine can't handle the load and RPMs don't increase to above peak torque RPM, downshift to raise engine speed.

However, operation of the engine at less than FULL THROTTLE at speeds below torque peak RPM is  ABSOLUTELY OK.

I hope this helps clarify the issue,

-Josh Berman
Joshua Berman           MidRange Service        Cummins Engine Company          Cummins Homepage:

Some Rules Of Thumb:

Engine "lugging" is defined as operating under a load great enough that engine speed can not increase at wide open throttle (assuming that the governor is not cutting the throttle).

Generally, you can cruise around town in the Ram at 1100 rpm with no trouble, but you should avoid using much throttle at such low engine speeds.

To reduce engine vibration that loosens the 5th gear nut:
Avoid running below 1600 rpm in 5th gear. With an enhanced 12V or any 24V engine, keep the boost under 20 psi in 5th gear (OD) until the engine is turning above 1800 RPM.

Even with an unmodified engine, EGT can soar into the danger zone when the engine is run at full throttle for several minutes. Always keep an eye on the EGT when the engine has a heavy load and downshift when EGT reaches the top of the safe range (950 at turbo elbow, 1250 at manifold).

These are conservative guidelines that help maximize engine and transmission life.   Dave

Q: Can I just wind it up and run against the governor??

Subject:     Running on the governor
Date:         Wed, 9 Apr 1997 10:03:03 -0500
From:        Josh Berman <>
To:            cummins

> We found ourselves pulling against some strong winds this past weekend
> Buried in the carpet it'll do 65 in 3rd but it's right on the 2500 rpm governed limit
> and I'm not sure how good it would be to run it there mile after mile. Is running
> with the throttle buried is a problem?

Running @ full throttle for several hours should not be a problem. We routinely test the 6B at rated speed and power (ie: for a '96/'97 engine, the dyno pulls 215 HP out of the engine @ 2500 RPM) for several hundred hours at a stretch (shutting down only to change oil or measurement equipment) in our test cells, without problems.

Joshua Berman              MidRange Service                Cummins Engine Company Cummins     Homepage:

Q: What is the governed speed for various engines

Subject:   Re: Redline
Date:       Mon, 13 Jul 1998 11:38:43 -0500
From:      Josh Berman <>
To:          cummins

> I have read in several posts that the v8 navistar engine that Ford
> uses is a higher revving engine than the Cummins 5.9. What does the
> 5.9 redline at? Since the 7.3 navistar redlines around 3300, The 5.9
> should be much less per the posts.

The 12-valve B-Series engines in pre-'98.5 Dodge trucks have a governed speed of 2700 RPM (180 HP - rated is 2500 RPM) and 2800 RPM (215 HP - rated is 2600 RPM). Their maximum no load governed speed (aka: "high idle") is 2950-3050.

The ISBs in Dodge trucks have a governed speed of 3200 RPM (rated is 2700 RPM), and a maximum no load governed speed (aka: "high idle") of 3500 RPM.

The "governed speed" I spoke of is the highest speed that you can get any power at. Higher than the governed speeds, the power drops off _very_ quickly.

( thanks to Ken Scobel for some of the #'s I didn't have at hand)

        -Josh B.

Q: At What Speed does Engine Damage Take Place?

Subject:  "Don't go there" speeds
Date:       Tue, 21 Apr 1998 16:25:02 -0500
From:      Josh Berman <>

The ISB should survive a  _BRIEF_  excursion to 4000 RPM, and the 12-valve B to 3900 RPM. Do NOT do this on a regular basis - it is considered abuse. However, if you miss a shift, or some other mistake, your engine probably won't need major work if you don't go over those RPMs.

The valves begin to float at speeds a little higher than these, and that means the valves hit the top of the pistons, usually leading to expensive repairs...


Other interesting limits:

Q: Which axle ratio should I get, 3.54  4.10?

Subject:  3.54/4.10
Date:      Fri, 17 Apr 1998 19:31:26 -0700
From:    Dan Hepner <>
To:        cummins

Regarding whether it's "better" or not to run an engine at a higher or lower RPM, where better is defined as "engine not working as hard".  

Before we get wound up, here's a simplistic assertion: diesel engine wear is nearly totally a function of fuel consumption. But we need an accepted definition of "working hard".

The effort required to maintain speed is the rate of power required at any moment.  Presuming that the situation remains constant (flat ground, no traffic, etc) the rate of power required to maintain the speed will remain fixed also. Observe in particular that this isn't a measurement of torque. Torque can be more with fewer RPM, or less with more RPM, and achieve the same goal of maintaining speed.

Now back to the fuel consumption.  Sure, it's a generalization, and an indirect one at that. What really matters of course is the amount of time spent at high stress levels. Maximum stress levels are only achieved with the pedal down, and generally, the further down the pedal, the higher the stress on the engine, and the fewer the number of hours AND MILES the engine will operate.  Of course, if your engine will operate for 200k miles with the pedal always to the metal, you might not give a rat's if it is being "consumed" by the occasional stretch.  For this discussion however, that's irrelevant. We're trying to keep our engine from "working hard".

So from the above, accept that given two axle choices (3.54 vs 4.10) we're trying to minimize fuel consumption.

One of the problems we face when looking for hard references on this question is that most/all referenceable measurements are made at full throttle. That's not what we care about.  However, there is a clear trend: the best fuel consumption, as measured by (fuel volume / hp), is at or near torque peak. I've never seen any evidence that this does not hold at lower throttle settings.  So if we're trying to minimize fuel consumption to achieve a particular hp, we'd be best to get the rpm down to near torque peak.  Generally, this tends to indicate a higher axle ratio (e.g. 3.54 rather than 4.10).

Now, there's lots more to the story than that of course. Again, if your engine lasts twice as long as you own the rig, do you care that it doesn't last 3 times as long?

Maybe more importantly, when operating at higher RPM at light throttle, there's a lot more hp to be had by putting the pedal down than when operating at torque peak. The rig is simply more responsive, and many people particularly those who tow find this responsiveness to be an overwhelming part of their decision.

Dan Hepner



Last Update: September 3, 2001