Towing: Torque Vs. HP?

[sfpcservice]

I just found this dyno sheet on another site and was wondering: When pulling up a long steep hill is it best to keep the RPM where torque and HP intersect, at peak of Torque or peak of HP?

Obviously I wouldn't want to run my motor at 5800 rpm, but what is the most efficient for discussion sake?

[homer]

I'll take torque anytime before HP

[brian_bp]

When pulling up a long steep hill is it best to keep the RPM where torque and HP intersect...

There nothing special about the intersection point. Change either scale on the drawing and the intersection point changes, because the intersection is just a graphic coincidence, not a characteristic of the engine.

In fact, power = torque x speed, so you really only need one curve - the other one can be calculated from whichever one you have.

... at peak of Torque or peak of HP?

Those peaks are the speeds at which the maximum (of torque or horsepower) is available; there's no reason to operate the engine at that speed if you are not using the peak amount - unless you want it available by flooring the accelerator pedal and not having to change gears. It's an automatic and changes gears immediately and by itself under full power, so what does it matter if it would need to shift to allow the engine to deliver full power or torque?

Obviously I wouldn't want to run my motor at 5800 rpm, but what is the most efficient for discussion sake?

• The least frictional losses come from running at the lowest engine speed which produces enough power for what you are doing at the moment.
• The lowest pumping losses (and thus likely higher efficiency) comes from running at the lowest engine speed which produces enough power for what you are doing at the moment, because that means the highest load and most open throttle.
• A complicating factor is that any engine must be optimized for a specific range of speeds, since everything in design is a compromise. The most efficient speed will likely be somewhere around the peak torque, because that peak indicates that is where the engine is working particularly well. Which peak is that for the lumpy curve caused by VVT-i? My guess is the lower one, since the upper one results from lots of valve overlap.
• Finally, the transmission won't let you do exactly what you want, so it is not possible to have the throttle wide open and the engine speed low.

Aside from efficiency, there is a school of thought which says that a vehicle is best behaved with the engine speed moderately above the peak torque point, but slower than the peak power point - perhaps 3000 to 5000 rpm for these engines, but more like 3700 to 5000 for a consistent slope (due to the VVT-i effect). In that case, if an increase in slope or other load (soft surface, headwind, whatever...) slows the vehicle down, there is more torque available same throttle setting which somewhat compensates. The same thing happens in reverse (less load -> higher speed -> less torque -> not so much speed increase), so being in this range makes it easier to keep speed stable. The point of staying below the peak power point is that you can likely speed up without needing to shift. Modern features such as automatic transmissions, cruise control, electronic throttles, and variable valve timing make this less relevant that it was, and this speed range is too high for best efficiency unless the engine needs to be working hard. I only drive at over 3000 rpm if that's what it takes to climb the grade or accelerate at the desired rate, and it's pretty rare for me to see over 3700 other than for a few seconds of acceleration.

I think the end result is that the most efficient operation is to let the transmission shift itself, overriding it with one gear lower only as necessary to keep it from "hunting" back and forth between two gears, or as necessary to get enough power if flooring the pedal in the automatically chosen gear doesn't quite do it.