Shift points for the highest torque?

Just read several interesting definitions for horsepower and torque in one of the post below.

I have always wondered how to find out where the highest torque level is for a tractor or truck. At what RPM is the engine putting out the most pulling power?

Also how about when shifting gears on a loaded truck. What is the best rpm to shift at to get the most power. For example: your pulling onto an interstate entrance ramp pulling a trailer with tractor on it. You want to get up to speed as fast as possible. I am guessing most the responses will be you go by feel of your truck or tractor. Anyone have any other ideas of finding the best shift points on the tach. Race car drivers would probably call this the highest power curve on a graft.

I have kind of thought on a tractor if your tach shows your engine is running at a speed that gets the pto up to 540, then you are probably hitting the highest power curve at the optimal rpm. Not sure just a guess.

Looking forward to hearing the wisdom of this board. We got some sharp guys on it.

Thanks
Cliff

The exact shift points would depend on the torque and power curves for a particular engine. As mentioned below, maximum pulling power is at the engine's maximum power. The torque curve also has considerable effect on acceleration and lugging ability. Another consideration is torque rise, which is the percentage increase in torque from torque at rated speed to torque at maximum. This draws the torque curve. As an example, my Ford 7710 has a Ford 268 turbo engine. The engine is rated at 103 hp gross, at 2100 rpm. That equals 257 lb-ft of torque at rated speed. The maximum torque is about 300 lb-ft at about 1400 rpm. (Just a rough guess, as I don't have the exact specs.) This leaves a torque rise of 17 percent, and a relatively flat torque curve, with a steep power curve, and maximum power at 2100 rpm. There is roughly 80 gross hp at maximum torque. This engine is rather balanced for lugging ability and acceleration. Some engines have much higher torque rise, with steeper torque curves, and flatter power curves. An example would be the Cummins 350 Formula in my truck. Max power is 350 at 1800, while max torque is around 1300 lb-ft at say 1400 rpm. This engine has much higher torque rise, so it lugs better, but it can't accelerate nearly as well. I find the cummins has a much better chance of picking up the gear when the engine is kept as high as possible (out at 22, and in at 17). The big advantage of the flatter torque curve is that the old Ford can pick up anywhere above 1200 rpm, which is below the maximum torque, but still very close. It would take a miracle for the cummins to do that. One of the big advantages of the electronic controls and wastegate turbos on new engines is that they can compensate for a lot of the problems I mentioned in both of the above examples. The Ford can be made to lug better, while still accelerating, and the cummins can accelerate better while still maintaining it's ability to lug. Torque and power curves are now less dependant on cam profile, to some extent. Well, since I now confused some and bored the rest to death, I'll sign off.

Rod

We have a new 2003 Dodge 4x4 gas motor with a standard transmission. It has plenty of power unloaded, but when pulling a trailer, I find my self kind of sorry for not spending a very little more and getting some more horses under the hood.

It is kind of slow getting up to highway speeds with a 7000 pound load. Just glad I only do it every once in a while. Overall I think it is going to be a great truck.

Thanks again for your post.

Have a good week.

Cliff

This forum is one the best on the internet. We have some very positive folks who are always willing to share their thoughts in a very positive and non-demeaning method. That’s what makes this one of the best boards! IMHP :>)

Thanks

truck and tractor engines have "fairly" flat torque curves. as opposed to a highly tuned and modified 2 cycle engine like a motorcycle. the curve goes up sharply, peaks and goes down sharply all within a few hundred rpms.theory would be to accelerate to just a few rpms past the peak, then shift hoping not to fall back down the curve diring the shift to loose torque. in my old scca days most transmissions/flywheel/clutch/gear combinations would loose about 400 rmp on a shift. so you would run the engine rpm past the peak of the curve by about 300 rpm. shift and if all went well you were just about 100 rpm on the "left" or start of the peak. on most engines (not all) the red line has little to do with peak torque. it depends on what rpm the peak is tuned. that has to with the whole engine dynamics (carb bore and distance to the inlet valves, exhaust bore and distance to outlet nozzle, valve size/location/lift/and duration. red line has to do with the weakest sections of the engines starting to vibrate at critical frequences. you can tune a peak curve with what you've got. to raise a red line of an engine,,,well you've got to redesign components of the engine..big money. and finding the peak? you are right seat of your pants,,kinda. the engine should be dymonitored to find peaks on graph paper, but then it boils down to spending some "quality time" with the engine/drivetrain/atmospheric conditions/track conditions. been out of the game for some time. there are probably computer systems that allow you to monitor and make decisions on all the stuff we use to do by scientific trial and error. these days i'm not looking for max accleration, i'm looking for max fuel ecomony. thats on another curve by the way.

Since power is a function of torque times rpm, an engine that produces maximum torque at a low engine rpm will have a relatively flat power band. That is, the horsepower will be almost constant across a wide range on engine rpms. As the rpms increase, the torque decreases so the horsepower remains almost constant. This describes most truck engines. Such engines are relatively insensitive to shift points, since the horsepower doesn't drop significantly when you upshift. Still, you'll probably get best acceleration when you wind the engine up to the redline before shifting.

On the other hand, engines that produce maximum torque at a high rpm have a very narrow power band. They produce enormous power, but only when they're really wound up. This describes most high performance automotive engines. They must be kept at high rpms, because they don't produce much power at low speed. Hence you need to redline them before shifting.

In the case of your tractor, you're correct that it probably produces maximum horsepower when the PTO rpm is around rated speed (540 or 1000 rpm). It's designed this way. Note that maximum torque will be produced at a much lower rpm.

In an aside, it's interesting to note that piston steam engines produce maximum torque at zero rpm. That's why they don't need transmissions.

One small clarification. Torque is not work and is more precisely indicated in terms of lb-ft and not ft-lb. While it is common practice to refer to torque as ft-lb, it is not technically correct. This is to distinguish it from the work term of ft-lb, which is of course, a force acting through a distance; something moving, as you correctly indicate. Torque is nothing more than a force multiplied by a moment arm. It is a force term, not a work term.

Rod