jeffteel

What is the magical RPM for the HO CTD...the RPM where I'll get the best fuel economy? I just looked at the torque/HP curve from Cummins, and it looks like peak torque is achieved from 1400-2800 RPM. To me, that would say that 1400 is the magical number because HP is lowest there. And I would think that fuel economy would be the worst at 2800 RPM. Does that sound right?

So...anything under 1400 is lugging it, and anything over 2800 is stupid, right?

Thanks!

DF5152

Theoretically the best fuel mileage would be gotten at the lowest RPM peak torque is developed at. BUt i dont think you can cruise there not without some massive gear changes

Shovelhead

Approx. 2,000 rpm is the Cummins "Happy Place"

jeffteel

Well...out of curiosity...why is 2000RPM the happy place, Shovelhead? Is this just from people's experiences here, or is there some reason?

And DF5152: Peak torque is developed at 1400RPM...can you not cruise there? I think that's about the RPM I'm at when I'm going 35-40 in 5th gear. It definitely doesn't feel like I'm lugging it unless I drop down below 35mph in 5th.

Shovelhead

All I know is that at 2000 rpm, approx 64 mph, (I'm running an auto & 4.10 gears) I get my best fuel milage, and the engine seems to be neither lugging nor straining.
If I need to increase speed, a simple press on the throttle is all that's needed.

The accepted rule is that under 1600 rpm is considered lugging the engine.
The definition of Lugging that I'm familiar with is when driving at a given rpm and load you press on the throttle and the rpm is slow to respond.

Mexstan

On mine there appears to be the contraversial 'sweet spot' (see previous topics on this) at around 2000-2200 rpm. However as I am normally in a hurry to go from A to B I often cruise at 2500 rpm for hours at a time. Yes, at that rpm there is drop in fuel consumption but it is still pretty good.

doug

There are some valid arguments in favor of a sweet spot, but this advantage is very small compared to the other very large factors that influence actual driving fuel economy.

Basically, you have the following:

1. The CTD engine does seem to have some preference for the lower rpms -- meaning that large fuel charges less often is preferable to smaller fuel charges more often. This is according to a local Cummins Intermountain rep I spoke to. That is to say, 100 HP output at 1400 rpm is probably more efficient than 100 HP output at 2200 rpm.

2. Required HP increases as the cube of vehicle speed. This is arguably the single most dominant factor -- keep the speed down!

3. the natrual, mechanical efficiency sweet spot of the CTD is masked by wastegated turbos and ECM controls. There may be a small (between zero and 5 percent perhaps) efficiency advantage, believed to be in the 1800 - 2000 rpm range but this has not been substantiated and remains in the emotional realm ;D I happen to be in that camp, and there are some valid arguments in favor of this position, but there is no proof of it.

the bottom line is that you'll get more improvement in efficiency by keeping both speed and rpms to a minimum, given the driving conditions, than you will by trying to find the sweet spot.

That said, "keeping rpms down" is far less important than keeping speed down. Indeed, those with 4.10 axles are claiming excelent efficiency while towing large loads, and this may be because this axle may optimize the engine's load over a wide variety of driving conditions for those who tow. I know that with my 02, instantaneous fuel economy tended to drop slowly as rpms increased above 2000, but dropped dramatically if rpms dropped below 1500 or so.

Commatoze

The "sweet spot" for me consists of a number of factors. Most important MPG, then noise comfort, and finally time to the destination; let's call it practically. I disagree with Doug. If I do 55, I'll get splendid fuel mileage. But it's not practical for long trips plus you're a hazzard on Interstates. For me 1700 RPM with a 3.55 rear puts me right in that zone. See I almost forgot speed, that's about 64 mph and I'll get close to 23 mpg on the flats doing it (unloaded). The older I get, the less I care about how fast I can go. I guess I'm turning into what I refer to as a "Hat". I can see over the steering wheel though. ;D

jeffteel

Thanks a lot for the info. This gives me something to think about. I guess I'm mostly worried about lugging it in city driving situations. Based on what I've heard, I probably need to use a lower gear once in awhile to keep above 1600RPM or so.

RamDan03

I've found I get the best mileage at about 2000-2100rpm. Any faster and the mileage seems to go down really quick and any lower you will have more turbo lag.

jeffteel

Me too. ???

bulabula

The under "1600 rpm is lugging rule" is news to me. In town, I'll torque around at 1600, or under easily. But under no load, the truck can easily accelerate from that rpm. Thats not lugging it. Our trucks can go through the gears with no throttle - that isn't lugging it.

Its detrimental when the engine is unable to accelerate from a given rpm (3200+ rpm not counting?).

Scotty

What I experienced as lugging is when the engine/truck is unable to sustain a speed and/or accelerate under load.
For example, pulling a trailer up a hill and the rpm's are dropping and so would boost and your egt would be climbing . Then, you will start to lose speed and if you press the go pedal and it does not repsond; then you are lugging and yes its detrimental to your engine as you are now in a situation where the engine cannot produce the torque needed. Yet the APPS is seeing it needs fuel. You can hear and feel when a diesel engine is lugging...you are pretty much choking it. This usually is most prevelant when towing. Less air and and more fuel...not good.

For me I found that the rpm range that suits my truck best...loaded or empty is 2000-2125. I have also noticed that when driving empty it 'seems' like this truck wants to go faster. ;D 8)

P Kennedy

I think Scotty may have said this lugging thing right, #1 it is a feel for the engine and what it is doing. If you are coasting through town loaded or empty and you are rolling easy you could be at 1000rpm but if you were to need to accelerate to make a light or on ramp you will find it is just not there to do it. Pulling on a hill is different again as you decelerate from weight and lose momentum the engine will find a place where it will maintain an RPM in gear where it will hold. The weight, grade and air temp will make a difference ie; some days it will settle at about 2400 in 4th gear on 6% with 20,000lbs eg. You may not accelerate but it will hold the load pulling, this is not lugging as you have no way to gear down without losing more speed than necessary but the engine is working within its torque/hp band. The lower the rpm the higher the EGT's however if you are just about to the top and the grade is flattening out I will allow the engine to come down to 1500-1600 with a lighter fuel pedal until it reaches breakover and starts to accelerate in that gear (approx.10-30 seconds). Another factor that comes into play like I said is temperature, if the water temp (directly affected by outside temp-ambient) rises and is maintained with the fan running intermittent or continously thats OK (200-210F) but if it starts to climb and is not cycling down from 210F+ gear down or back out of it. This will either cause you to shift down, slow down or both. Bombed trucks are more prone to this than stockers but as you decrease speed air rammed into the Radiator"s" and yes there are 3 of them will rely more on the fan to cool. Ideal "sweet spot" for pulling is different than for fuel milage again weight or load being the determining factor. An empty truck at 1800-2000 gets about the best fuel milage while travelling at a reasonable space, loaded trucks may do better at 2400RPM but theoretically heavy loads should be most efficient at the point where the torque curve crosses the HP curve ie; highest torque closest to high HP on graph. This will also change with engine ratings and years 12V vs 24V vs year/hp. Yes theoretically lowest torque would give you best fuel milage but as stated not practical, driving style is just as important as RPM. PK