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Old 10-17-2012, 07:34 PM   #15
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Peak torque means peak power to pull a load and staying at that RPM is important when going up a grade. That is why the rear end gearing is so important. GM rates the towing capacity of its trucks with the gas engine and 4.10 rear end at 25% higher than with the 3.73 rear end.

Tractor diesels are most fuel efficient in the 58-64 MPH range and when they can use the momentum gained going downhill to help them up the next hill on the road. Very few drivers run at these speeds anymore so fuel costs are clearly less of a concern then getting to their destination.
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Old 10-18-2012, 08:39 AM   #16
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Quote:
Originally Posted by elkhornsun View Post
Peak torque means peak power to pull a load and staying at that RPM is important when going up a grade.
That's over-simplyfied. Torque by itself won't give you "power". You have to have enough horses hooked up to pull the load as well as plenty of torque to maintain the speed.

On my 7.3L PowerStroke, the peak torque was at 1,600 RPM. But at 1,600 RPM, there weren't enough horses to pull the load without constant downshifting for every little bump in the road. With a stock tune, I had to cruise at 1,900 RPM to get high enough on the HP curve to tow my 8,000 pound 5er without constant downshifting. With a 60-tow tune, I still had to be at about 1,800 RPM to tow the load without constant downshifting. My torque curve was flat as a pancake from 1,600 to 2,600 RPM, so anywhere in that range produced about the same torque. But the HP curve showed there simply weren't enough horses available at the torque peak to tow my load.

So you need both torque and HP. Study of the HP/torque curves can help you arrive at the best RPM for towing a heavy load. Towing through the Hill Country can confirm it. If your rig downshifts for less than a 4% or 5% grade, you need more RPM to get up higher on the HP curve.
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Old 10-18-2012, 09:08 AM   #17
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Originally Posted by SmokeyWren

That's over-simplyfied. Torque by itself won't give you "power". You have to have enough horses hooked up to pull the load as well as plenty of torque to maintain the speed.

On my 7.3L PowerStroke, the peak torque was at 1,600 RPM. But at 1,600 RPM, there weren't enough horses to pull the load without constant downshifting for every little bump in the road. With a stock tune, I had to cruise at 1,900 RPM to get high enough on the HP curve to tow my 8,000 pound 5er without constant downshifting. With a 60-tow tune, I still had to be at about 1,800 RPM to tow the load without constant downshifting. My torque curve was flat as a pancake from 1,600 to 2,600 RPM, so anywhere in that range produced about the same torque. But the HP curve showed there simply weren't enough horses available at the torque peak to tow my load.

So you need both torque and HP. Study of the HP/torque curves can help you arrive at the best RPM for towing a heavy load. Towing through the Hill Country can confirm it. If your rig downshifts for less than a 4% or 5% grade, you need more RPM to get up higher on the HP curve.
From what I've read (and correct me if I'm wrong) torque is what you need to get the truck and trailer moving from a stop and HP is used when you are cruising along the hi-way and have to pass another vehicle or climb a hill. I'm still having a heck of a time finding the curves for my truck online, I'm not sure if ford had any done up for this truck.
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Old 10-18-2012, 09:25 AM   #18
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It's generally beneficial to operate between the torque and horsepower peaks of the curves when in hilly terrain. This allows one to take advantage of a phenomenon called TORQUE RISE which is briefly explained below. Although the following explanation deals primarily with industrial applications, the principle of utilizing Torque Rise to your benefit holds true for automotive applications as well:

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Torque and Torque Rise
Horsepower is a rate of doing work. Torque is "the rotary force in a mechanism" according to a dictionary definition. The two are related (Torque lb-ft = H.P. X 5252 / RPM) but torque is often misunderstood. Since there is a fixed relationship between HP (or kWm) and torque, two engines having the same horsepower at the same RPM will have the same torque. However, at work, the two engines may perform very differently. The reason for this is they may have very different torque rises. Therefore they respond differently to the demands of the load.

The length of piston stroke, number of cylinders, rotating mass and other factors, affects torque rise. Newer electronically controlled engines are able to produce torque characteristics that could not be achieved with mechanical fuel controls.

Engine horsepower curves often show the torque curve or "pull down torque" as well. This curve illustrates the amount of torque available from the engine as a load is applied that exceeds the engine's rated torque at the operating RPM. The difference in the torque at the rated RPM and the maximum or peak torque is the "torque rise". It is usually expressed as a percentage. (Peak Torque - Rated Torque / Rated Torque = Torque Rise X 100)


In this case the rated torque is 477 lb.-ft. and the peak torque is 657 lb.-ft. @ 1200 RPM. The torque rise is: 657 - 477 divided by 477 = 38%.

Note that although there are two horsepower curves, continuous and intermittent, only the intermittent torque curve is shown. The assumption is that if the engine is "pulled down" (reduced in RPM by the load), then the engine will be performing on its intermittent rating curve. Torque curves are usually available for any published horsepower rating.

What all this means in actual usage is that, in many applications, the engine with the greater torque rise will do its work more quickly. It will seem more powerful and responsive. This difference will be very evident in applications where the engine is routinely pulled down from its rated speed by the load. Examples of this are a drilling rig lifting the rod string, a grinder processing a stump or a loader digging into a rocky bank. Even applications not generally thought of as sensitive to torque rise such as generator sets and marine engines can, under some conditions, benefit from good torque rise characteristics. (Pulling a heavy trawl net, bucking a current or starting a motor load, for example.)

Torque and torque rise are very important considerations in many applications particularly those where the engine is routinely pulled down from its rated speed owing to the effects of the load. Greater torque rise allows the engine to run at a higher r.p.m. under load and thereby accomplish its work more quickly. In extreme conditions, inadequate torque rise will prevent the engine from accepting the load and it will stall.

There is a lot to add on the importance of torque. "Droop" and "isochronous" governing, electronic or mechanical controls and other factors enter into the aspects to consider. Again, if the intended usage is clear, the best option will usually be apparent.
For those who are interested, the entire article may be found HERE.

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Old 10-18-2012, 10:15 AM   #19
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OK...If I am understanding the comments on this string let me see if I have the flick....

Let's use this chart for my points

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Some theories for ya'll to shoot at. LOL

1. This engine would normally achieve best MPG at about 1200 RPM. This would be appropriate for nearly flat land, no wind, no need to accelerate quickly and not being at maximum weights. In general, when cruising along without any challenges. The low HP might cause the tranny to shift more often in rolling hills and longer minor uphill situations.

2. This engine would have the most pulling power at about 2000-2100 RPM. You would want to be there for the BIG climbs especially at high weights. This is where proper gear selections really pays off to get and stay there.

3. I would think that 1300 RPM would probably be the "sweet spot" for most applications with minor challenges such as rolling hills and/or lower total weight and would minimize tranny shifts when compared to 1200 because of the peak torque and bump in HP.

4. When approaching a big climb 1800 RPM would be the lowest RPM to be at. If RPM drops below 1800 then downshift to get in the 2000-2100 range.

These are very general thoughts. Any big holes in my thinking?
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Old 10-18-2012, 10:28 AM   #20
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4. When approaching a big climb 1800 RPM would be the lowest RPM to be at. If RPM drops below 1800 then downshift to get in the 2000-2100 range.

These are very general thoughts. Any big holes in my thinking?
Your torque peak is way down at 1300 RPM. As the engine speed decreases from 1800 RPM, available torque increases, so you're wasting available torque rise by downshifting that early (1800 RPM). There's every chance that the engine might find a point of equilibrium down around 1500 RPM or so and be perfectly happy as you're climbing that hill.

Having said that, there are peripheral considerations such as water pump speed, engine cooling fan speed, transmission temperatures, etc. that might necessitate a downshift at higher RPM, but if you take a lesson from the big rigs when they climb a hill fully loaded, you can listen to them ride the torque curve down the RPM range until they absolutely have to downshift. You might try it both ways (operating at high RPM versus riding the torque curve down) and see which method your engine prefers.

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Old 10-18-2012, 06:41 PM   #21
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simple test for best RPM

There's a simple test to find the best gear (RPM) for any situation. Note how far you have to push the throttle to maintain your speed (up a hill for instance). Shift up or down to change the RPM and note whether you need more or less throttle position for the same speed. The "happy place" for your engine/trans/rear axle/load is the gear (RPM) that requires the least push on the throttle to maintain the desired speed.

this lesson was learned towing boats over the rockies with an 85 F150 with the 5.8.
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