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Old 08-22-2019, 11:27 AM   #1
SummitPond
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Education please: Torque vs HP

Badbart in this post got me thinking (typically a dangerous thing): What's the difference (in application) between HP and torque. From my days in physics class, HP is the rate of doing work, whereas torque is a force applied some distance from a pivot point resulting in translation of an object around that pivot point.

Our friend Google doesn't really clear things up for me:
  • What is the difference between horsepower and torque? ... Horsepower is measured at higher RPMs and torque is the low-end “grunt” or low RPM pulling power of an engine. Diesel engines only make around 300-400HP in those big rigs, but they make anywhere from 1,500 - 2,000 ft-lbs.
  • What does horsepower and torque mean? ... Power is the rate of completing work (or applying torque) in a given amount of time. Mathematically, horsepower equals torque multiplied by rpm. H = T x rpm/5252, where H is horsepower, T is pound-feet, rpm is how fast the engine is spinning, and 5252 is a constant that makes the units jibe.
I can see where a Diesel engine can produce more torque as the "explosion" in the cylinder yields more force (and I guess a Diesel has a larger bore displacement). But to my simple mind a Diesel will allow you to produce the same HP as a gas engine just at lower RPM, which is why a gas engine must rev up to deliver more HP.

Other than having a screaming gas engine under the hood while trying to climb a steep slope (ignoring the other benefits of a Diesel) what am I missing?

If and when I upgrade our TV I would consider a Diesel. I'd like to be as educated as possible, knowing I won't be able to believe half (or more) of what comes out of the salesperson's mouth.

Thanks.
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Old 08-22-2019, 01:41 PM   #2
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Any of the big 3 Diesel trucks 2500 and 3500 and 4500 can pull a legal lode up any interstate grade in the USA at legal speed. And that is a conservative statement. Need I say more. Pay attention to the LEGAL load requirement as there are many factors that change the load / trailer towing configurations and weights. The Diesel also offers considerable downhill breaking.

Happy Towing
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Old 08-22-2019, 02:11 PM   #3
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https://en.wikipedia.org/wiki/Horsepower

https://en.wikipedia.org/wiki/Torque
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Old 08-22-2019, 02:25 PM   #4
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There are some good posts on YouTube explaining the differences.
One point is diesels have much higher compression which gives them a lot of force at low rpm's. A gas engine needs to gain a lot of rpm's in order to make the same kind of power. That's also why they burn through more fuel. I get nearly double the mpg from my diesel truck verses the previous gas truck.

Check out YouTube.
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Old 08-22-2019, 03:48 PM   #5
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Diesels are inherently higher in torque but also much more efficient than the gas counterparts. The air/fuel ratio advantages are explained here.


https://rentar.com/diesel-engines-fu...oline-engines/


The 2018 Freightliner I drive is so much more advanced from engines just a few years ago. It gets 8.5 to 10 mpg moving up to 80K pounds with 2:73 gears with the engine turning 1220 rpm at 65 mph. 15 or 20 years ago the norm was 5-6 mpg for semi trucks.
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Old 08-22-2019, 04:44 PM   #6
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Not scientific but I think of it this way;

If you want to win races, you want max horsepower.

If you want to drag a house up the road in comfort, you want max torque.

It is a wonderful experience towing your house up a hill at legal speed and the engine is puttering along at only a little above idle.
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Old 08-25-2019, 08:38 AM   #7
SummitPond
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Thank you all for the information. I guess it is the compression that helps tip the edge - more bang for the buck at lower RPM. As stated, my intent is to help educate myself for the time when our TV needs to be replaced. Maybe (ha ha) Ford and the others will have a reasonably priced, capable electric pick-up and I'll be back for more education.
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