Lets Talk RPMs

[JWalker]

Can anyone tell me the RPM range were I can get the most power (torque). I have a 2007 Ram DRW with the 6.7, 6 speed auto and 3.73 gear ratio. I found that in certain driving conditions, I can manually go down a gear and the rpms would climb to around 1500-1600, the power and responsivness was alot better. Is it hard on the transmission to drop a gear to get into a better rpm range?

Thanks
Jamie

 

[GOTTOYS]

I have the same drivetrain. I let the computer decide what gear works the best. I tried running in 5th instead of 6th and my mileage went down. It's not hard on the transmission if you drop a gear. If it needs more power it will downshift automatically. The slower the engine is turning the less fuel it will use.

 

[branson4020]

Jamie,

The 6.7 Cummins torque curve is flat from 1500-2800 rpms at 650 lb-ft. Max horspower is at 3013 rpm. Its not harder on the transmisssion to run at a little higher RPM, just the opposite actually.

 

[Moose]

A picture is worth a bunch of words. LOL

This engine is rated with 350 horsepower at just 3013 rpm. The 6.7L Turbo Diesel achieves a peak torque of 650 lb-ft at 1500 rpm for the automatic transmission configuration and maintains that peak torque all the way to 2800 rpm (a "best in class" capability for a heavy-duty diesel pickup) and a full 610 lb-ft peak torque for the manual transmission.

 

[Duramax1]

My experience with the Duramax and Allison transmission is somewhat different.

I have found that by using the tow haul mode that the transmission does not go into 6th gear as you have to be going above 60 mph for the transmission to upshift from 5th to 6th. I usually travel at about 58 MPH as this is the sweet spot for managing my torque, horsepower and mileage.

If I turn off the tow haul mode the transmission will shift into 6th gear and the rpm will decrease slightly.

But............ while the RPMs go down slightly, the accelerator will increase to provide the engine with more fuel to compensate for the decrease in RPM's. So "the slower the engine is turning the less fuel it will use" does not work for me. I have verified this result by looking at my ScanGuage which provides me with an instantaneous fuel consumption readout.

So, I have always left the tow haul mode on and let the duramax/allison make its own decision.

 

[lwmcguir]

We use the tow/haul mode anytime we have 10K or more behind us. Always with the camper in tow. We usually settle in at 65 mph with 1500 rpm with the F350 6.7. At 1500 rpm the transmission is locked in the second overdrive. Only downshifts on large hills or high winds. Average is about 12 mpg without significant wind.

 

[jayc]

With my old truck, which was a '02 GMC, Duramax and Allison, I used tow-haul when towing and like Duramax1, the tranny would not shift into overdrive until about 64 mph. If I was traveling in a 55mph speed zone, I would speed up enough to make the transmission shift into overdrive and then slow down to the speed limit. My normal speed for towing is 62-63 mph and I consistently got 11-12 miles per gallon.

I put 220,000 miles on that truck with no issues with the transmission. I did keep it serviced at the required intervals and expect likewise service from my new truck.

 

[dieselengineer]

Jamie
The best economy RPM for any engine is the point on the RPM curve that the BSFC (Brake Specific Fuel Consumption) is the lowest point.

 

[caissiel]

I have a 6 speed standard with the 6.0L engine. The high torque on this engine is at 2000 RPM and you can feel it when it hits that RPM.
With the scangaugeII I did lots of test while driving and for me 2000 RPM is the fuel most efficient spot but it is where I cannot keep my engine at, because under load its a fuel hog and I would never be able to afford the fuel while towing. So I keep it at around 1750/1800 most of the time.

I was getting bad torque points on 6th (Overdrive) speed and loosing speed in the hills so I installed an E-Con by Hypertech at any setting to have the torque required to keep the speed in the hills.
So now I stay in 6th speed from 55 MPH to 70MPH and have no effect on fuel mileage. I have observed that when I need to shift from 6th to 5th (High) the fuel rate almost doubles going uphills and not very efficient on mileage above 55 MPH.

By luck and reason for driving with low turbo pressure I observed the best mileage on a long trip driving at 60MPH and no more then 10PSI turbo pressure. As soon as the pressure increased behound that the fuel flow realy climbed so with the programer I am able to keep the speed in the hills while holding back on turbo pressure.

Like the advertise says "The programer mows the hills and gives you plenty of torque not to use your lower gear that saves on fuel".

The best fuel savings I have seen is to drive steady speed and let the truck slow down a bit while going uphill, thus keeping the turbo lower. And if it has to shift down you will definitly pay in fuel mileage.

I found that at 55MPH and under I just leave it on high gear and sometimes I get more then 18MPG at that speed on 5th.

My signiture says the rest.

 

[Willym]

[QUOTE=dieselengineer;158609]Jamie
The best economy RPM for any engine is the point on the RPM curve that the BSFC (Brake Specific Fuel Consumption) is the lowest point.[/QUOTE]

This is the right technical answer, but, unfortunately most people won't understand it. Manufacturers don't always publish BSFC info either.

Using BSFC alone will not give tell you the best RPM for fuel economy. For the truck trailer combo, there will be a highway speed where mpg is optimum depending on engine speed and hence BSFC, and the horsepower required to overcome the aerodynamic drag and drive train friction. I've calculated this for diesel powered ships in the past, but I've never seen it done for land vehicles.

Generally though, keeping the road speed down will get you the best economy. I got good economy on our Alaska trip, but it suffered as soon as we headed home and hit interstate highways again.

 

[JWalker]

Unfortunatly, I,ve not towed the North Trail yet. I was driving around empty in a section of Pa were there was alot of elevetion change. It seemed as I entered a grade (maybe 5-7%), there wasn't enough load on the old Dodge to have it downshift from 6th to 5th gear. When I manually down shifted, the rpms hit around 1700 and it took off like a rocket. Let me get something straight, even in 6th gear, it had plenty of power. But in 5th, WOW. I turned around and did the run up the mountain again, this time I left it in 6th and went around 67-70mph and the rpms were a little higher and it cruised along like I was on flat ground. I will try the run the next time with the tow/haul mode on. Both runs were on a 3+ mile run with around 800-900 ft elevation change. I think it is posted around 7% grade.

Thanks for all the info.
Jamie

 

[caissiel]

Empty is never a problem with power to go up mountains but but a trailer in the back with 14K or more and wind in the front it will surely take the fuel and the engine will work harder. 7% hills are no problem I do them in overdrive most of the time, its the 11% hills we have on some of our roads that realy hurt.

I use my scangaugeII to show me the fuel flow, torque, Coolant and Oil temperature. While towing I keep them in the very comfortable range with my truck. And will never tow without it, specialy with the weight I am towing. I have no intention getting an other truck neither.
So far never saw any alarming indication that slowed me down while towing.

 

[Ricoh]

I have an 05 chev 2500 Duramax with a 5 speed Allison. Last year I was pulling a 3055 bighorn. I would set my tranny in tow mode, get the rig up to 100 KMPHR ( about 61 mph) and set the cruise. My truck would stay at 1850 RPM all day in 5th gear. The only time it would downshift is when I would pull out to pass or it would take a pretty good grade and a stiff headwind to make her downshift. Fuel mileage on a calm day or with a tailwind was good. Against a stiff headwind it was brutal.

Rick

 

[dieselengineer]

The biggest issue with the Dodge six speed standard NV5600 while towing is maintaining a low tranny oil temperature. While running in 5 gear (direct drive, 1:1) the temp runs about 100* over outdoor temp (100* day = 200* oil temp). When towing in overdrive (6th) the temp keeps on climbing. So I normal tow in direct drive, which puts the engine RPM in the middle of the torque curve (about 2200 rpm) Runs like a race car, very responsive actual fun to drive. Fuel mileage is about the same, and a lot less strain on the drivetrain. Those low RPM power spikes can be killers, like a jackhammer to the drivetrain.