Running the bfg 315s 6 speed I usually run 1500 rpm @55 mph on our local hwy alternates hilly and flat pull the hills fine empty and with 3000# camper still can acclerate but sometimes worry that Im lugging ? Think I should try to run little faster ? also drops to 13-1400 quite a bit due to slowing for curves I want max. miles out of this rig planning on keeping at least 15 yrs.
THANX

 

I'm interested in opinions on this subject as well. I'm not a speed demon on the highway, plus most highways near me you'd be lucky to do 55-60. If the torque peak is at 1600rpms, how would running at that rpm be lugging the engine?????????

 

Well I also wonder why every one thinks the sweet spot for rpms is 1800 when torque is flat from 1600-2400 THANX

 

I very rarely exeed 1600-1700, only when I get on the interstate. My last tank of fuel I got 23 mpg and none of it was on the highway(interstate). It was all country back road 35-50 miles per hour. I believe the as long as you push the pedal down and you can excelerate that the engine is not lugging. I am very light footed and I climb hills regularly between 1000 and 1300 rpms. I know I can go faster but I'm usually at or above the speed limit. Just my opinion

 

I'm in the same boat. I very rarely clear 2000RPM, and that's down in 5th while pulling a good hill. I usually let it lug down to 13-1400 and then grab 5th and try to stay around 1800 until I can hit 6th again. I know that these things are supposed to be able to handle 3kRPM all day every day, but I just don't like it... I too plan on keeping this thing well into the double digits in years, and I figure as long it's not bogging down (i.e. I can accelerate), I'm doing OK.

 

If the engine can accelerate freely without flooring the truck, it's not lugging. What you want to watch out for is running up a steep hill with a heavy load. If the truck starts to slow down even when floored, you need to drop a gear.

 

Look at what the minimum rpm is on the autos - from a quick look at a friend's truck the auto seemed to keep the revs above something 1400 rpm.

 

Then run 55 at 2000 and take the load off the bearings and get better fuel mileage. Your engine will love you for it. I would not think of running my engine below 1500 or 1600. You just put a bigger load on the entire drivetrain.

 

Hum, What is it about lugging that is so hard on it? You say bearings, but i guess in my mind, The engine has to make this much power to go this fast ect. Hows it put more strain on it when the engine moves slower?

 

When you are running it full power and bring it down below1300-1400, it would be considered lugging. When you are at full power below about 1100, you will start hammering out the rod bearings because the oil film on the bearings will break down and damage the babbit on the bearing shell. Different people have different opinions about where lugging starts and when bearing damage starts. The oils we use today are really good and protect the eng from wear, so the bearing damage may occur at different rpm's with different oils. These engines should be able to run 200 rpm below peak torque safely without any premature wear. If you are worried about it, then put a higher limit on your lowest rpm, wont hurt a thing.

 

Ok how about this, at 2000 and direct gearing instead of overdrive, each piston stroke has to put out 25% less power to move the truck. Overdrive costs you fuel mileage for the energy of turning two sets of gears, instead of not having to turn any. And if you try it, you will find that the mileage will be better barely cracking the throttle in direct instead of heavier throttle in overdrive. Higher rpm is easier on the engine bearings because there is less pressure on them.

Another example is fifth gear on the nv4500. You can run the thing with a load all day long so long as you keep the rpms at 2000 or better, but start loading it down at lower rpm's and the nut backs off. Too much torque and less horsepower at lower rpm's.

That is what is mentioned so often on the board, the "sweet spot" Why do you thing it goes up a hill easier in a lower gear?

 

Consider this--
At lower RPMs there is lower oil pressure. I would assume that having lower oil pressure at the range where torque is the highest is not a good thing for the bearings. I'm sure that there is a point though where too high RPMs would cause stresses that overcome the lubrication provided. (Another topic.)

My driving habits are what I would assume are average regarding RPMs/speed/load, etc. I try to get the highest MPG while commuting and I giverhell when I'm pulling heavy. I try to keep the RPMs up alittle while pulling for the HP and the oil pressure. I know of professional haulers getting multiples of 100k miles driving like I do with loads varying up to max rating.

I'm in agreement that if you can not accelerate easily you need a lower gear.

$.02

 

Some wear tests that I've run across concluded that the most wear occurs with high loads and low engine speed, which is lugging. It's especially bad to lug a diesel since the compresion and consequently the torque is so high, as not only does the engine take a beating the rest of the powertrain does too. We've all heard about 'million mile' Dodges right ? The vehicles run up lots of and lots of highway miles, sometimes towing heavy stuff, so it seems apparent that you don't need to worry about 'too many rpms while running in the sweet spot.

 

I let my 48RE pick the rpm (no choice really), and I see it drop to 1300-1400 on occasion. I thought EGT's were an indication of lugging? I don't see high EGT's at 1400 rpm. Towing or grades are a whole different story of course. There it seems to stay between 1800-2000. If EGTs climb and I downshift I'm usually moving to 2200 or so (and EGTs will drop 150 degrees almost immediately).

I am curious about the mechanics behind the other engine wear quoted for "lugging". Compared to a gasser the CTD initially shocked me with how low it rev'd all the time. I'm not sure I know what lugging sounded like on a diesel... and with the auto, can't really figure it out. Does it sound/feel like a gasser or is that different too?

 

The separation of bearing and journal is not a function of engine pump developed oil pressure.
It is a function of the relative sliding surface speed and the ability to build up the "oil wedge". The faster the relative motion, the thicker wedge and separtion of sliding surfaces.

A further factor is that of viscosity.

below from an old text

Hydrodynamic principles, which are active as the shaft rotates, create an oil wedge that supports the shaft and relocates it within the bearing clearances. In a horizontally split bearing the oil wedge will lift and support the shaft, relocating the centerline slightly up and to one side into a normal attitude position in a lower quadrant of the bearing. The normal attitude angle will depend upon the shaft rotation direction with a clockwise rotation having an attitude angle in the lower left quadrant. External influences, such as hydraulic volute pressures in pumps or generator electrical load can produce additional relocating forces on the shaft attitude angle and centerline position.