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Old 08-11-2013, 12:42 PM   #1
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Torque, Horsepower, diesel, 3.8, 3.6, etc.

I see folks repeat some fairly common misconceptions about torque vs. horsepower on this forum, particularly when discussing the merits of a diesel engine vs. gas or the 3.6 vs. other gas engines (3.8, 4.0 in particular). I found this web site, which I think explains things fairly well. The web site is not a masterpiece, but the description seems reasonably accurate and easy to understand:

Debunking Horsepower and Torque Myths

I think the main takeaway is that, given arbitrary control over gearing, an engine capable of producing more horsepower over a given RPM interval will always out-accelerate a comparable engine that may produce higher torque numbers but lower HP numbers at the same vehicle speeds. One way to conceptualize this is to realize that the torque output at the rear wheels can be manipulated by gearing.

That said, it is really only when you have your foot mashed completely to the floor that this has any bearing.

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Old 08-11-2013, 12:57 PM   #2
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Nice find, Thanks!

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Old 08-11-2013, 02:42 PM   #3
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I think it really helps explain some of the driving characteristics of different engines. E.g., perceived lower effort of diesel engines when hauling a load, improved first-gear performance and reduced shifting at sustained speeds when doing a regear for larger tires, etc.
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Old 08-11-2013, 08:33 PM   #4
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That is an interesting article. I'm not sure I agree with this one part:


Myth 4: An engine that has more torque than horsepower is better for towing

Again, depending on the overall gear ratio, horsepower is the most important measure. Because of the math, an engine that is tuned to have peak horsepower below 5252 RPM will ALWAYS have more torque than horsepower, and engines with peak horsepower above 5252 RPM will ALWAYS have more horsepower than torque. However, other factors when towing, such as fuel economy, probably tip the scales toward the lower RPM (e.g., diesel) powered vehicle.

Here is my logic. I owned a 2012 Ram 2500 Crew Cab 4x4 with a 5.7 Hemi that made 399 HP and had 3.73 gears. I'm not sure what the torque rating was on that engine, but it is less than the other truck I am comparing it to. I currently own another 2012 Ram 2500 Crew Cab 4x4 with a 6.7 Cummins that makes 800 foot pounds of torque and has 3.54 gears. I'm not sure what the HP rating is, but it is but than 399 HP.

So if HP is more important than torque, the Hemi truck should tow better according to this article. Except in real life the diesel truck with less HP, but more torque tows much much better.

Someone care to explain this to me.
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Old 08-11-2013, 09:26 PM   #5
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Originally Posted by MOPWR2U View Post
That is an interesting article. I'm not sure I agree with this one part:


Myth 4: An engine that has more torque than horsepower is better for towing

Again, depending on the overall gear ratio, horsepower is the most important measure. Because of the math, an engine that is tuned to have peak horsepower below 5252 RPM will ALWAYS have more torque than horsepower, and engines with peak horsepower above 5252 RPM will ALWAYS have more horsepower than torque. However, other factors when towing, such as fuel economy, probably tip the scales toward the lower RPM (e.g., diesel) powered vehicle.

Here is my logic. I owned a 2012 Ram 2500 Crew Cab 4x4 with a 5.7 Hemi that made 399 HP and had 3.73 gears. I'm not sure what the torque rating was on that engine, but it is less than the other truck I am comparing it to. I currently own another 2012 Ram 2500 Crew Cab 4x4 with a 6.7 Cummins that makes 800 foot pounds of torque and has 3.54 gears. I'm not sure what the HP rating is, but it is but than 399 HP.

So if HP is more important than torque, the Hemi truck should tow better according to this article. Except in real life the diesel truck with less HP, but more torque tows much much better.

Someone care to explain this to me.
First question - by "tows better" do you mean full power acceleration, or just perceived effort? If you mean perceived effort, the diesel produces more torque - or, phrased differently, produces more power at lower RPMs. Think of a "torquey" engine as one that makes POWER at lower RPM. So, you do not need to rev the diesel as high to get enough power to maintain speed or accelerate the load. Not to mention issues with heat, fuel economy, engine durability, etc.

If you mean full power acceleration of two vehicles with an equivalent load, it is a mathematical certainty that the faster-accelerating vehicle produces more average power for the duration of the test. That said, diesel engines often have "flatter" power curves, so a 260 HP diesel engine may very well produce more AVERAGE power during a given acceleration test than a 280 HP gas engine.

(note in particular a diesel engine will do particularly well in first gear relative to a gas engine. this is because the gas engine doesn't produce much POWER at low RPM).

Are you familiar with integrals? If so, work is the integral of power over time. You can envision a first power function of the gas engine with a peak at 280 HP and relatively steep downward slopes on the side of the peak, and a second power function of the diesel engine with a peak at 260 HP with relatively flat downward slopes on the side of the peak. If the difference is sufficient, the diesel can out-accelerate the gas engine even though the gas engine has a higher power rating. It can be difficult to really compare two engines with only the peak HP values.
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Old 08-11-2013, 10:17 PM   #6
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You ask if I am describing "perceived" ease of towing, vs "measured" acceleration. Actually what I am describing is the speed at which the engine can climb a particular grade. I live about 20 miles from a long 10% grade in I-15 called Black Ridge. I've climbed it countless times towing various loads. For any given load the diesel truck with a lower horsepower rating can maintain the 75 mph speed limit longer than the gas truck. The diesel truck can climb the grade at a faster speed than the gas truck.

It sounds like you're saying that just because the gas engine truck has a higher horsepower rating, doesn't mean it has more area under the curve when power is measured throughout the RPM range. And because of more area under the curve, the diesel engine truck pulls better in real world situations.
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Old 08-11-2013, 10:28 PM   #7
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Originally Posted by MOPWR2U View Post
You ask if I am describing "perceived" ease of towing, vs "measured" acceleration. Actually what I am describing is the speed at which the engine can climb a particular grade. I live about 20 miles from a long 10% grade in I-15 called Black Ridge. I've climbed it countless times towing various loads. For any given load the diesel truck with a lower horsepower rating can maintain the 75 mph speed limit longer than the gas truck. The diesel truck can climb the grade at a faster speed than the gas truck.

It sounds like you're saying that just because the gas engine truck has a higher horsepower rating, doesn't mean it has more area under the curve when power is measured throughout the RPM range. And because of more area under the curve, the diesel engine truck pulls better in real world situations.
Foot to the floor?

Yes, that is exactly what I am saying. In fact, if you just know how much torque an engine was making over a given time, you cannot say how much work the engine has performed during that time (e.g., accelerating a vehicle is work). However, if instead of the torque you know the power, you can determine how much work was performed. And, if you double the power, you double the work - regardless of how much torque is produced at the engine.

A simple example - take an engine that produces 100 ft lbs of torque at 1000 RPM at 10 mph (so the RPM doesn't change). Now double the engine speed and reduce the torque by half - 50 ft lbs of torque at 2000 RPM at 10 mph. Both engines will accelerate a load at the same rate. In fact, they will both produce the same amount of torque at the axle - in order for the second engine to spin twice as fast, it needs a mechanical advantage factor of 2 somewhere in the drivetrain.

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