What is a better measurement of power? HP or Torque (answer)

My dad and I were driving a Ford 7.3l powerstroke pulling a trailer. When we were going up a hill in sixth gear at 1600 RPM (which is maximum torque for that engine) we could not maintain speed. But if we downshifted to fifth, we could accelerate.

My dad asked me how we could accelerate when we were making less torque. I knew it had to do with RPM but could not get over the fact that we were making less torque. We were making more HP. And I had to admit the engine was making more "power".

I started doing research and found a lot of interesting stuff.

-Torque is a measurement of force and does not necessarily move anything.
(you can put 500 ft lbs of torque on a bolt and it might not ever move)

-HP is a measurement of power
(1 HP is the power needed to move 33,000 ft lbs one foot in one minute)

-The key to HP is that it takes into account time

-200 ft lbs of torque at 200 rpm is a lot less power than 200 ft lbs of torque at 400 rpm

So I found that torque is used to calculate HP based on RPM but is not a measurement of power.
(1 HP = Torque x RPM / 5252)

So an engines peak torque is basically useless in measuring an engines power.

Now I know what you all are thinking, why does a diesel car with less HP win a race against a gas car with far more HP?

A diesel creates its torque very early in its rpm range so that its average HP is higher over its RPM range than most gas engines.

A gas engine that creates 1000 HP at 7000 RPM may have a very low average HP.

But a diesel that only creates 750 HP may create enough HP at low RPM to make its average HP higher than the gas engine.

High school physics ... which a lot of people have forgotten ...

Jesus Is Lord said:

My dad and I were driving a Ford 7.3l powerstroke pulling a trailer. When we were going up a hill in sixth gear at 1600 RPM (which is maximum torque for that engine) we could not maintain speed. But if we downshifted to fifth, we could accelerate.
My dad asked me how we could accelerate when we were making less torque. I knew it had to do with RPM but could not get over the fact that we were making less torque.

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I think the lower gear ratio in 5th is what made the difference. The whole torque vs. HP thing is a bit complicated and I'm no engineer but my Golf TDI has more torque than my V8 Touareg and weighs half as much but my Touareg will beat it in a race every time because it has almost 400 HP.
.

As has often been said: "People buy horsepower, but they drive torque".

If you run out of torque then you need HP and a lower gear to do the job.The same truck with a 6.8L V-10 should out pull a healthy stock 7.3 even with less torque.Horespower wins the race towing or unloaded.

Depends if your engine is working correctly.The 7.3's can have less power with age.Injectors working at 100% can make a world of difference to those engines when at the stock power level or modded.

Also depends on auto or manual trans and axle gearing/tire size.

Vince Waldon said:

As has often been said: "People buy horsepower, but they drive torque".

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Except Honda Civics,

correct me if im wrong but if you are going up hill now a fraction of your weight is now going to be a force acting the opposite direction of the way your truck is moving. when factoring in air resistance and everything when you are are peak torque and still decelerating, you have a larger force pushing you back. example: i live at the top of one of the steepest hills in pittsburgh. my old jetta 2.5 had about 100ftlb less than my tdi and if i shift into 2nd i more or less couldnt accelerate going up the hill. this is because i ran out of torque, where as with my tdi in 2nd i'll still be pulling. both have 150hp. but if i down shifted to 1st in my jetta i could pull until redline. since the gearing is now reduced the engine might be making less at the crank but because its being reduced, there is more torque at the wheel. HP is unaffected by gearing. think of it this way you take usain bolt vs magnus samuelsson in a 100m, bolt is going to win. now you take both of them but tell them to carry a 300lb rock, samuelsson is going to do it in about the same time unlike bolt who will be doing it much slower if he can even do it

Well, guess what? HP and torque are interrelated. Just different ways to measure the same thing. Diesels are great because they have more torque for any given HP than a gas engine.

andy2 said:

If you run out of torque then you need HP and a lower gear to do the job.The same truck with a 6.8L V-10 should out pull a healthy stock 7.3 even with less torque.Horespower wins the race towing or unloaded.

Depends if your engine is working correctly.The 7.3's can have less power with age.Injectors working at 100% can make a world of difference to those engines when at the stock power level or modded.

Also depends on auto or manual trans and axle gearing/tire size.

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I disagree about the 6.8L. I've driven them loaded before, they don't have nearly the hill climbing power the 7.3L does. Not even close. And I have actually moved a bunch of RVs with someone (dealer moved, and I got recruited by a friend that works there to help move a whole bunch of them). A Duramax crew cab/long bed 4WD 3500 dually towing the biggest damn 5th wheel camper trailer I have ever seen passed my sorry ass on a hill with ease, and I was in a 6.8L E450 Class C motorhome that was a smaller one.... I had it floored, it had already chosen a lower gear, and it was not happy. Even the 3.0L V6 diesel Sprinter cab-chassis based Class C was better at climbing the same hill.

Funny, that Silverado moved about 30 5th wheel camper trailers that day and it never missed a beat. Never even needed to stop for fuel.

Now, an empty F250 6.8L? Yeah, those haul ass. And they'll do pretty good even with a moderate load (although they drink gas like crazy). But really loaded down on hills? Nah. Just a 5.4L with a couple extra cylinders.

You never want to be at peak torque for extended periods, this is your lowest power threshold for pulling (flat land).

I know for my big Cummins it's specific to never apply full throttle below peak torque, and full throttle above that is unlimited...


HP at a given RPM is always what matters... This accounts for torque.

Most diesels produce more available HP from idle to 4000 rpm's than most gassers, that's why they are faster in that range, and feel quicker because they are. But once the gasser gets into the 4000+ range you'll be seeing tail lights.

HP at a given RPM is always what matters...

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Bingo....

Horsepower is calculated.

Torque is measured by a load cell on a dynamometer and the computer takes into consideration the amount of input from the load cell and over RPM to display the estimated horsepower.


As for the towing part, mechanical advantage from gearing is why its possible to accelerate after downshifting.

From High School Physysics book: Torque is work or energy (F*d) not Force (F).

F=m*a (mass x acceleration) and is measured in Newtons

so T=m*a*d to give you Nm (or ft-lbs depending on unit choice)

Gearing of the Truck will affect when the sweet spot occurs. Let's say the gear spins 10 times and then the wheels turn once. All that work and energy is being supplied into one rotation of the wheel. Now in a higher gear, the gear let's say spins twice to turn the wheel once, less energy used but not as much energy being transferred to the wheel. In a basic example, try pulling something heavy up a hill, do you take long, slow strides or short baby steps?

Several factors at work here. you have rotational forces (gearing, axles, etc.) plus linear force (truck moving on a hill), working against the negative acceleration of gravity. That's how it makes sense to me in the mathematical world at least.

Jesus Is Lord said:

....we were going up a hill in sixth gear at 1600 RPM (which is maximum torque for that engine) we could not maintain speed. But if we downshifted to fifth, we could accelerate.
My dad asked me how we could accelerate when we were making less torque.

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You were making less torque, but you were making it more often (quicker rpm). Force times Rate is Power.
The increased rate of production from shifting down a gear more than offset the reduction in quantity.
Maybe you were making 10% less torque that far above the rpm at which peak torque is made, but if you were spinning 20% faster you are still making 10% more power.

This is a bit of a simplification, but horsepower is what makes you go fast, and torque is what breaks drivetrain components. Not that an abundance of torque is a bad thing.

OP you answered your own question. Horsepower is a unit of power and can be converted to watts. Torque is... well a torque measurement and is measured in units of ft-lbs or N-m. You already know the relationship between the two HP = Torque * RPM / 5252.

horsepower is what makes you go fast, and torque is what breaks drivetrain components.

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LOL, That's great!

Lug_Nut said:

You were making less torque, but you were making it more often (quicker rpm). Force times Rate is Power.
The increased rate of production from shifting down a gear more than offset the reduction in quantity.
Maybe you were making 10% less torque that far above the rpm at which peak torque is made, but if you were spinning 20% faster you are still making 10% more power.

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Thank you
You explained that perfectly.
I definitely understand this a lot better now.

fifth gear should be one to one ratio and sixth gear is a overdrive, lugging a diesel for extended periods of time is bad and costly

Key thing here are the gear ratios that multiply power from the engine to the wheels. With same amount of engine power you are having more power To the wheels with smaller gears.

There's only one measurement that matters - power.

Go take your torque wrench, set it to 100 ft-lbf and put it in a vice and pull on it. There, you have 100 ft-lbf and ZERO power. No work is done.

Maintain this 100 ft-lbf while turning the wrench and now you have power and work is being done.

There's only one measurement that matters - power.

Go take your torque wrench, set it to 100 ft-lbf and put it in a vice and pull on it. There, you have 100 ft-lbf and ZERO power. No work is done.

Maintain this 100 ft-lbf while turning the wrench and now you have power and work is being done.

Torque multiplication. 6th gear is .72:1, while 5th is 1:1. Going from 6th to 5th gave him roughly a 39% increase in torque multiplication.
From engine to tire (if 4x4, transfer case in 1:1, rear end 3.73 for example), effective gear ratio:
6th gear: 2.69
5th: 3.73

I know it's been answered, but horsepower is the energy an engine makes and torque is the ability to use that energy to do work. This is why people obsess over the crossing points on motorcycle power charts, and where power bands come from. You need both. This is further complicated by gearing: lower gears need less energy per spin but have a revolution limit based on the engine. In sixth, the engine was struggling to produce enough energy or horsepower to spin the flywheel because of the load. Downshifting reduced the energy requirement (by 39%) and likely increased the energy output of the engine by increasing RPM. This then allowed more work to be done because the truck accelerated, increasing the torque. Engines don't make torque, it's a calculated reflection of the engine-gearbox-train-wheel-tire system and its ability to generate force.

So for an engine, horsepower. For a car, torque.

Efchou said:

From High School Physysics book: Torque is work or energy (F*d) not Force (F).
F=m*a (mass x acceleration) and is measured in Newtons
so T=m*a*d to give you Nm (or ft-lbs depending on unit choice)
Gearing of the Truck will affect when the sweet spot occurs. Let's say the gear spins 10 times and then the wheels turn once. All that work and energy is being supplied into one rotation of the wheel. Now in a higher gear, the gear let's say spins twice to turn the wheel once, less energy used but not as much energy being transferred to the wheel. In a basic example, try pulling something heavy up a hill, do you take long, slow strides or short baby steps?
Several factors at work here. you have rotational forces (gearing, axles, etc.) plus linear force (truck moving on a hill), working against the negative acceleration of gravity. That's how it makes sense to me in the mathematical world at least.

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Is this quote literally from a high school Physics book? Would like to know which one.
Your two paragraphs of prose are quite correct but equating torque to work is not correct. The units are the same and this can make things really confusing. However, to have work you have to have torque through an Angle.
Torque is a unit of force acting about an axis of rotation. Torque is NOT work nor is it energy.

Here are some linear ~ rotational analogies

Potential to do work
Torque ~ Force


Work
Torque * Angle ~ Force * Distance


Power
Work / time
or
(Torque * Angle )/ time ~ (Force * Distance) / time
What I noticed about my TDI is that I can drive around the city quite comfortably, accelerate from a stop, out of a roundabout, etc. The strong torque of the Diesel allows for this. However, on the highway or freeway passing another vehicle can be a problem because the car literally runs out of power. At highway and freeway speeds, the 90HP limit is a real thing when I ask the car to accelerate.

https://en.wikipedia.org/wiki/Torque#Relationship_between_torque.2C_power.2C_and_energy