The Definitive Torque vs. Horsepower post

[holstein13]

Year after year, I keep seeing the same old debates about torque vs. horsepower. Everyone seems to have a general idea that we are referring to power, but most don't really understand the relationship of torque to horsepower. Therefore, I've decided to try and explain it as best as I understand it. (caveat, I am not an engineer so feel free to clarify and correct any mistakes I make below.)

First of all, a bit of math. (Torque x Revolutions Per Minute / 5252 = Horsepower). Even if you aren't good with math, the important thing to note in this equation is that Torque times RPM divided by a constant equals horsepower. If we ignore the constant, we see that torque times RPM determines horsepower. In other words, horsepower is dependent on RPM and the faster your motor turns at a constant torque, the more horsepower you will get. (In practice, torque falls off, the faster the engine spins)

OK, so what is torque anyways? Let's look at a simple example of a bicycle. The motor on a bicycle is typically your legs and assuming you don't have clips on your pedals, the maximum amount of power you can apply to a pedal is determined by the weight of your body. In other words, if you stand on one pedal and apply all your weight to it, you will apply the maximum amount of torque to the pedal and gears. But here's the funny part. If your bike is in top gear and you start from a stand still, you could apply your entire body weight as torque to the pedals and still not move. Assuming the bike is held upright, you could conceivably stand right up on one pedal all day long generate maximum torque and generate zero horsepower. Why, because your RPMs are zero and as we have already seen, torque times zero RPMs equals zero horsepower.

So how about if we started off in 1st gear on our bike. As most of you know, 1st gear is the easy gear and lets you move uphill or get started with very little effort. The downside is that your bike won't move forward very fast in 1st gear, but you can apply your torque in such a way as to get the RPMs going and hence, generate horsepower that will move you.

Thinking about our bike again, try to imagine that you are comfortably cruising along in 4th gear at 10 miles per hour and you change gears to 5th gear; what happens? If you maintain the 10 miles per hour, you will find that your pedals will turn slower, but you will need more power in each stroke to maintain the same speed. To get the maximum speed out of your bike, you need to apply your maximum torque to the pedals and then turn the pedals as fast as you can.

Well it's exactly the same thing with internal combustion engines. The torque from your engine comes from the combustion of fuel in your cylinders. In general, the bigger the cylinder and the more efficient the combustion chamber and the more energy your fuel source has, determines your maximum torque. But like we saw in our bike example, lots of torque times low RPMs doesn't generate much power. I like to think of torque as the amount of power generated by a single revolution of the engine. (That is not technically true, but that's the way I like to think of it)

In the great gas vs. diesel debate, it often comes down to this. Diesel engines typically produce more torque and run at lower RPMs. Two comparable gas and diesel engines could produce the same amount of horsepower, but typically, the diesel will run slower and would theoretically be more efficient because it is wasting less power in the form of heat.

But there is one other very important thing that usually no one considers when looking at horsepower. The horsepower rating that you always see quoted by everyone is the horsepower at the maximum torque and RPM. Meaning, you only get that maximum horsepower in top gear at top speed and depending on how your vehicle is geared, you may never be able to achieve that top gear at maximum RPM because your vehicle is too heavy or has too much wind resistance or has some other friction. This is why gearing is so important.

So what are the key takeaways from all this?

1) Horsepower is a function of RPM times torque. The faster your engine turns, the more horsepower you will have.
2) You will probably never see the maximum horsepower your engine produces because you will rarely, if ever, go to your maximum RPMs in top gear.
3) In general, the larger the cylinder, the more torque that cylinder will produce.
4) Diesel fuel has more energy in a given quantity than does gasoline and by default will produce more torque, all else being equal.
5) Gearing is important.

People often say that it's horsepower that moves us up mountains and that is 100% true. The difference is that a high torque engine will produce that same horsepower at a lower RPM which often results in quieter operation and more fuel economy.

[AH64ID]

Look at turbine engines, they still use the same equation for hp and torque. Aircraft are rated in shp, at least helicopters.

One example is a twin engine helicopter, at 100% torque (yes called torque) it's only getting 355 ft/lbs from each engine.. or 710 ft/lbs.. that is over 100 ft/lbs less than my truck makes. I could come nowhere near to spinning the rotor enough for flight because I am over 1100hp shy of what is required.

So what's the other side? 1426 horsepower.. and that's how it flies... and a LOT of gearing reduction.

I'm a diesel guy, I like torque but it's not the only important number. Torque gets you moving, horsepower keeps you moving.. horsepower is a function of rpm, and the rate at which the torque can do it's job.

Diesel makes more hp at lower rpms, that's why it has torque and that's why they drive better, lower rpm hp/torque.

Here is another example. Let's assume the engine has a 100% flat torque curve and makes 500 ft/lbs, and is operating at WOT. So now lets only change the gearing (gearing is very important, I think we all agree). So the constants are torque, tire size (265/70R17), transmission ratio (1:1), and speed (60mph).

4.30:1 rear end: 2829 rpms 269hp
4.10: 1 rear end: 2698 rpms 256hp
3.73: 1 rear end: 2454 rpm 233hp
3.55: rear end: 2336 rpms 222hp.

Okay, so same constants.. but this time the torque required changes to 1000 ft/lbs at the wheels. Lets see what that does to hp/tq at the engine.

4.30:1 rear end: 2829 rpms 125hp/232tq
4.10: 1 rear end: 2698 rpms 131hp/243tq
3.73: 1 rear end: 2454 rpm 144hp/268tq
3.55: rear end: 2336 rpms 151hp281tq

So the same work is being done by the 4 setups, but as rpms drop the hp required increases. That's a function of gearing, but the same thing would happen with 4.30's if you slowed down and still required 1000ft/lbs at the wheels. What requirement is increasing? Hp or Tq? Or Both?

They are all putting out the same torque, but the hp changes quite a bit. Why do you think that 4.30s would be rated to tow more? The torque is the same, so all you torque is the only thing that matters people, why??


Bottom line, you cannot argue one over the other. They need each other to be effective. Yes a 300hp 10,000 rpm Honda engine can put the same power to the ground as a 300hp 2,000 rpm diesel.. but getting to 10,000 rpms is painfully slow, lots of gearing (heat) and other issues. Which one will tow better?

To everyone who is comparing personally owned gas and diesel trucks the comparison is very hard to make fair unless both rigs where rated at the same hp, and you operated both at rated rpms. If you weren't, then it's not a fair comparison. The diesel still winds because it makes more per per rpm than a gas engine, thus more power available at average rpms.

I'm a diesel guy, so I think torque is king.. if it's made at the proper rpms. Which means more hp at lower rpms, which means more torque.. Going in circles, but that's how it works. The highest torque in a useable rpms get the work done the quickest, that's explained as horsepower....

[RCMAN46]

With most dynos used today this statement is not correct!

"The HP curve is derived from the torque curve"

The inertia dyno system provides a fixed inertial mass flywheel and computes the power required to accelerate the flywheel (the load) from the starting to the ending RPM.

This is the type most used on most small trucks and cars.

Torque is not measured but calculated.

What is measured is the time required to rotate a mass to a set rpm.

This gives the power or HP required to do that.

A computer then samples various points comparing the rpm of the flywheel to how long it took to change rpm from point to point.

That is a known HP required to do that then torque can be calculated.

The rpm and time are directly measured during the test.

[lbrjet]

You can put RPM's on the left side of the equation as well. Why would you want to? Because you can I guess. HP is calculated using RPM and torque. Without torque there is no HP. They are directly related. The HP curve is derived from the torque curve. Sure you can use the formula above to calculate torque from HP once you have the HP already calculated, but you need the torque to calculate the HP in the first place so what is the point?

[pnichols]

Hmmmm ... you must have read the link wrong or aren't aware of how equations can get at the truth if you look at them from various angles. :h

I thought the equation was this:

Torque (ft-lbs) = horsepower x 5252 / speed(RPM)

By the way, here quoted are the last three sentences from the link:

"In summary, there are a lot of misconceptions about horsepower and torque. Remember that it depends on the gearing, horsepower and flatness of the horsepower curve - but very little on the torque number by itself. If two engines have the same horsepower, with the proper gearing they will perform very similarly, but the same general statement can’t be said for two engines of the torque."

[rhagfo]

pnichols wrote:
Well ... I hate to throw water on any diesel-lovers parade, but a quick read of this just might help separate fact from fiction regarding why the equation for an engine's torque depends directly upon it's horsepower:

http://lainefamily.com/hp.htm

pnichols wrote:
You might want to rearrange that equation to put torque on the left all by itself. You'll then see how torque is directly proportional to horsepower. :C

This equation is what is in the document you provided the link to.

"Horsepower = speed (RPM) x torque (ft-lbs) / 5252"

Sorry just quoting your document. :B

[bobx2]

Yep, I've read all that before. You can show me all the equations and theories you want, but real world experience trumps a bunch of words and numbers on a piece of paper. No way, no how, will anyone ever make me believe a gasser will keep up with a diesel towing a load here. I've driven both, pulled with both, and see it every time I head up in the hills.
Who's to say that article isn't a myth, like they claim everything else is?

[pnichols]

Have you read this link yet:

http://lainefamily.com/hp.htm

[bobx2]

Just not buying into the whole, gas can pull as well as diesel. Traded in my 3/4 ton Suburban with a fuel injected 454 for an '03 Silverado Duramax. Pulled the same trailer up the same mountain roads with both. Suburban was down to about 20 mph and turning about 4,600 rpm on a hill that the Duramax pulled at 45 mph in 4th gear, maybe 1,500 rpm. I for one will never pull with a gas engine again. They just don't compare.

[pnichols]

You might want to rearrange that equation to put torque on the left all by itself. You'll then see how torque is directly proportional to horsepower. :C

[rhagfo]

pnichols wrote:
Well ... I hate to throw water on any diesel-lovers parade, but a quick read of this just might help separate fact from fiction regarding why the equation for an engine's torque depends directly upon it's horsepower:

http://lainefamily.com/hp.htm

You may want to restate that statement!

Horsepower = speed (RPM) x torque (ft-lbs) / 5252

Seems to me the Equation states that HP is a product of speed and Torque. :B

There is a point that to get the RPM necessary for low torque engines to make the necessary HP, it will exceed the engines red line.

So you run taller than stock tires to improve fuel mileage, maybe final of 4.30 from the 4.56 of the differential, but you are only moving 11,500#. I pulled a two horse GN trailer (about 5,000#) about 250 miles home, and needed to constantly check the mirrors to be it was still there, Total GCVW about 12,500 to 13,000#.
This example is just that same rear gears, gearing needed to get necessary torque limited speed to 25 mph.

JIMNLIN wrote:


My '03 truck= 305 hp Cummins.
Buddys '04 truck= 305 HP Nissan

My '03 truck = 555 ft lbs Cummins.
Buddys '04 truck = 379 ft lbs Nissan.

Both trucks have 3.73 axles pulling the same 4800 lb 21' bass boat/200 Merc over the same roads to the same lakes for the same bass tournaments.

The Nissan drags down to 25 mph in low gear and won't accelerate on one long steep highway in the hills.
The Cummins keeps the 50 mph highway speed limit at 1900 RPM an can still accelerate in the same gear on the same hill.

It is true that you can't have one without the other. Given the choice (Oh yes I was given the choice :)), I would rather have higher torque than HP as as long as the difference is reasonable it is a much more relaxing driving experience.

[pnichols]

Well ... I hate to throw water on any diesel-lovers parade, but a quick read of this just might help separate fact from fiction regarding why the equation for an engine's torque depends directly upon it's horsepower:

http://lainefamily.com/hp.htm

[hone_eagle]

Sport45 wrote:
I figured that. Just josh'n ya!

WOW you been around since 2001 !
very cool
-ol timer-
🙂

[rhagfo]

JIMNLIN, LOL :), comparing your Cummins to a Nissan!!

#1 At 4,800# you are not even starting to work the Cummins, heck I pull 11,000# with the same engine with about 270hp, and less torque with 3.55 gears and still don't get pulled down on hills.

How can you have the audacity to state that torque will win the race to the top of the hill!!
You need to listen to the gas owners on the forum, it is all about HP/REVs!!! Doesn't hurt to have 4.30 or lower gears. Ford and GM could get their 6.0 and 6.2 gas engines to pull 18,000# or better with a set of say 5.10's and 4 mpg! :B

Sorry, Sorry, it's Monday morning the Devil made me write that!! :S

[JIMNLIN]

My '03 truck= 305 hp Cummins.
Buddys '04 truck= 305 HP Nissan

My '03 truck = 555 ft lbs Cummins.
Buddys '04 truck = 379 ft lbs Nissan.

Both trucks have 3.73 axles pulling the same 4800 lb 21' bass boat/200 Merc over the same roads to the same lakes for the same bass tournaments.

The Nissan drags down to 25 mph in low gear and won't accelerate on one long steep highway in the hills.
The Cummins keeps the 50 mph highway speed limit at 1900 RPM an can still accelerate in the same gear on the same hill.

I've pulled for a living in the carburetor era with small blocks and big block gassers in mostly one ton DRW trucks pulling in the 25k-28k GCW range. In many cases the small blocks had the same HP as the big blocks in the same brand truck with the same driveline/etc. Diesels back then were all anemic in the hp/torque numbers and weren't a serious player yet in the LDT size trucks.
One thing I learned when actually working the different trucks/power plants for hundreds of thousands of miles is when pulling the same size load and having equal hp is he who has the most torque gets to the top of the 17 mile long grade first and is waiting on the truck with less torque and is struggling to "catch up".