Economical Towing - HP intersects Torque?

[FredMaryLou]

Hi,



I read somewhere that the most economical RPMs for towing is where the Horsepower Curve and the Torque Curve intersect. We are preparing for a 7,000 mile tow. So, now is a good time to ask a couple questions.



If that is true (or not), at what RPM do they intersect on my 2008 6. 7 Cummins 6-Speed automatic?



Does that mean downshifting to 5th or 4th to tow at that RPM intersection point is recommended?



FYI . . . I usually tow our 14,000 pound fifth wheel at 1,500 to 1,700 PRMs in 6th gear with a 4. 10 axle.





Fred

 

[Gary - K7GLD]

Greatest engine efficiency point in RPM is not necessarily also it's most economical point as well...

 

[Boondocker]

If you were towing on a dyno, we could give you a good answer. Since conditions on the road constantly change, there isn't one answer.
I like towing because it's a constant balancing act that keeps me fully engaged in the driving experience. For me it's all about engine load - trying to keep EGT, coolant temp, boost, RPM, and speed (pretty much in that order) all in their acceptable if not ideal ranges.

The sweet spot on my truck is around 1900 rpm. Lower rpm tends to push EGT, higher rpm tends to push coolant temp. I'll usually err on the side of higher rpm as it's easier on the drive train.

Watch the gauges, tune in to your rig, you'll get familiar with its comfort zone.

 

[DonTX]

In the old old days that was bible. It appears to me that now with the more modern engines, that is worthless information. Still hard for some of us old timers to get used to the curves on these newer engines. I think the only semi valid guidance would be that you need to be in your max torque area for best results. My opinion only.

 

[HBarlow]

On a Cummins engine the torque "curve" is flat from 1500 - 1600 rpm to about 2700 - 2900 rpm so the hp curve only intersects the torque curve at peak hp which occurs at 2900 rpm. I don't think towing at 2900 rpm would be very efficient.

 

[DonTX]

But don't forget, that is for the 24 valve only. My 12 valve has a considerably different curve as I recently learned. The 24 valve torque curve is a mighty hansome one, I am still envious of it.

On a Cummins engine the torque "curve" is flat from 1500 - 1600 rpm to about 2700 - 2900 rpm so the hp curve only intersects the torque curve at peak hp which occurs at 2900 rpm. I don't think towing at 2900 rpm would be very efficient.

 

[HBarlow]

The original poster's signature indicates he has an '08.

 

[Hoefler]

I find if you let the truck do its thing, you will find that it will do what is best for its self. Set the cruise at a comfortable speed, use Tow/Haul mode and let it do its thing. Pay attention to what is going on, if it is hunting gears, slow down or down shift ,if not both. Find a gear that it is happy in and a speed you are comfortable with, you will find that everything will fall in place. Just remember, speed and constantly shifting in and out of an ideal gear will kill your efficiency and mileage. I find that between 1900 and 2200 yields the best performance for me. I traveled 3400 miles like this and it rarely shifted out of sixth and pulled like a mule.

 

[klenger]

I fail to see how anyone could make any kind of "rule" for this as the units of measure for torque and HP are different. You can make the curves cross most anywhere you want to by changing the scales.

 

[CUMMINZ]

Forget where you think the power is. Don't lug your engine. Learn to use the shift buttons. After running a while you will learn just by looking up the hill whether you downshifted on a like hill the last time. Try not to make shifts on the hill under hard load. Just at the bottom of the hill go ahead and downshift to 5th. while you are just about coasting. Once in 5th. lay into it again. This causes less strain on converter lockup. Just at top of grade shift to 6th. Drive by the feel on your bottom and your ears. :-laf

 

[BDW]

I find if you let the truck do its thing, you will find that it will do what is best for its self. Set the cruise at a comfortable speed, use Tow/Haul mode and let it do its thing. Pay attention to what is going on, if it is hunting gears, slow down or down shift ,if not both. Find a gear that it is happy in and a speed you are comfortable with, you will find that everything will fall in place. Just remember, speed and constantly shifting in and out of an ideal gear will kill your efficiency and mileage. I find that between 1900 and 2200 yields the best performance for me. I traveled 3400 miles like this and it rarely shifted out of sixth and pulled like a mule.

Pulling my 5 ver I find this to be correct except my truck seems to get the best mpg @ about 1500 rpms. This equates to about 60 mph in O/D with the cruise set.

 

[SBeretz]

I agree with the others, stay in the 1800-2100RPM range and you will gain the most economy for miles pulled. IF it is dead flat, you can pull with less RPMS and not lug the engine. On majore roads it works for me to stay at 1900 which is about 62-64mph and the truck will pull every hill it comes to and turn out the best mileage. Scotty

 

[Pavemen]

and this is why I love my 5spd with gear splitter. I can find a sweet spot no matter what the conditions are at the time.

 

[Loren]

Fred,



I would like to reiterate for emphasis what klenger said (more succinctly) in an earlier post.



THE HORSEPOWER AND TORQUE CURVES DON'T CROSS!



Horsepower is measured in foot-pounds per second (or some other measure of power). Torque is measured in foot-pounds (or some other length times force units).



They don't plot on the same graph. You see them on the same graph in sales literature because it saves printing a second graph. The scale for power is along one vertical axis and the scale for torque is along the opposite vertical axis. It appears to be one graph, but it's not. You can make those two lines look much different by choosing different units for your scales.



Loren

 

[RHestand]

I have read several times over the years that internal combustion engines HP & torq. at curves intersect between 52-5300 RPM.

If this is true, most of us Cummins guys will not be dealing with this calculation, especially for economical reasons.

Ray

 

[Bob Cochran]

Are we really talking about graphs crossing? Or in reality looking to find out at what point you get maximum torque developed and what that simultaneous horsepower. I suppose for towing we are concerned at what RPM develops the maximum torque or in the 2nd Gen the range of RPM and secondarily where is the maximum horsepower in this same range. Yes the graphs are merely pretty pictures to attempt to visualize for us what we think is right but actually wrong. In the case of the 2nd gen when towing for fuel economy the maximum torque at the lowest RPM thus better MPG. If that RPM results in a speed lower than you want to drive you speed up, increase RPM and lower MPG. This makes sense or am confusing everyone and ME!!!