The Real Difference Between Horsepower and Torque (Bit of a read, but worth it)

[elemein]

So I've searched this forum, and other forums for info on the topic, but besides one thread in this forum that gives an okay explanation of the two measurements (which I feel arent explained as thoroughly and precisely as they could be)...

I'm quite tired of people getting torque and horsepower wrong. I'm tired of hearing people say things like "horsepower is what accelerates you, torque is not", or things like "horsepower and torque are equally important", or an assortment of different things, here on this forum, and out there on other forums or even real life. I am very tired of it. I'm also tired of people explaining torque and horsepower wrong and I'm tired of the fact then when people google "what is the difference between torque and horsepower", all they get is vague sort-of-correct explanations.

Well I'm ending it.

First and foremost, I'm going to explain what torque is and what horsepower is, then I'm going to explain which is more important to you, then I'm going to answer some very good common questions regarding horsepower and torque. It's going to be a long write-up, but if you don't understand fully the subject of torque vs. horsepower, then you'll probably learn a thing or two.

So let's get started.

Right off the bat, I'm going to start all this off with a saying that we've probably all heard once or twice: "Horsepower is what sells cars, torque is what wins races". This could'nt be more true. Sure, it makes sense, but do we all understand what this saying is truly saying? Most likely not.

So let me explain. Firstly, horsepower doesnt exist. It does not. It is imaginary. It's a mathematical formulated number from RPM, torque and a constant. This may be hard to come to bear with at first, but let me explain.

Torque is a practical unit. Like lbs, or grams, or miles, or something that is absolutely measurable. Horsepower is a mathematical, theory unit, such as imaginary numbers. Horsepower CANNOT be imagined, visioned, used directly, or even be practical; torque CAN.

So let us imagine torque; I am going to use the torque measurement of foot-pounds, or ft-lbs. Come with me on this one. Lets say at 3000 RPM, your engine produces 100 ft-lbs of torque. This 100 ft-lbs of torque goes through your transmission (let us say you are in first gear, with a gear ratio of 3.5, which is very realistic), and is amplified by your first gear; 3.5. So your torque value out of the transmission is now 350 ft-lbs of torque. Now let's send this torque through your driveshaft or transaxle to the differential unit, and this differential ratio, or "final gear ratio", is 4, which is also very realistic. So let us amplify 350 ft-lbs * 4 = 1400 ft-lbs. Now, no transmission is perfect, and some power is lost through the gears, this amount is usually (but not always) 15-25%, so let us take 20% off our 1400 ft-lbs value. The result is 1120 ft-lbs. Now let's imagine your axle (we will be using a rear-wheel drive car for simplicity), imagine a metal, indestructable, weightless bar attached to your axle. This bar is one foot long (this is where FOOT-lbs start making sense). Now let's imagine 1120 lbs hanging off of this one-foot-long bar. The force of 1120 lbs pushing the one-foot bar down, is the force that is being put on your rear axle. We can imagine this, and if we were to try doing this in real life (putting 1120 lbs on the end of a 1ft bar at the end of an axle), the force that the engine puts out would be equal to the force of the real life demonstration of 1120 lbs on the bar (though 1120 lbs is sort of difficult to do in real life; but if we were to do this with a smaller unit, such as 10 ft-lbs, then we could do this easily in a real life demonstration). THIS is how torque works; torque is a twisting force-- the amount of how much leverage and force that you can put on to an axle or turnpiece. This is how we imagine torque. That is how torque works.

Let us try to imagine horsepower. Firstly, we cannot. Though this raises a common question: What if we imagine x number of horses pulling the car?: Well firstly, that's just nonsense. What breed of horse? How old is the horse? Is the horse well-fed? What is the mood of the horse? How long is the leash on the horse? Where is the horse pulling from? Is the horse wearing horseshoes--??-!- Just far too many variables to justify imagining horsepower practically. With torque, it is always absolute; a pound is ALWAYS a pound. A foot is ALWAYS a foot. Though this raises another question; What about translating horsepower into kW? Well firstly, kW is a measure of power; how much work can be done over a period of time. Amazingly enough, kW is an imaginary unit too. kW is made of up amps and volts, two practical units, in order to make one imaginary unit of energy consumption/production: kW. So you can go ahead and translate kW into HP and vice versa, but it simply doesnt change anything. This concludes that horsepower is imaginary, and impractical; while torque, is practical.

So how do we find horsepower? Simple. Horsepower = Torque * RPM / 5252. That is the formula for horsepower; two units and a constant. Let us find the horsepower of our last example engine. Horsepower = 100 (ft-lbs) * 3000 RPM / 5252. The result is 57.121 horsepower. Well would you look at that; we just found horsepower. We found an imaginary unit from two practical units. (By the way, RPM is a practical unit; it stand for revolutions per minute, this can also be measurable and is absolute.)

So where did horsepower come from? Thomas Savery, a miner in 1702 who wrote a book about mining. He was explaining how 1 HP is equal to the carrying power of one horse, and explaining how if an engine (steam) can produce the same amount of carrying power of one horse, then the engine has 1 HP. Makes sense, but let's be honest here. This measurement is from a miner over 300 years ago being applied to steam engines. That's a little far-fetched for us to be using it now. Though this raises another question of the "imagination of horsepower" though; it is said that one horsepower is 33,000 ft-lb/min, or 550 ft-lb/sec. Absolutely correct!-- BUT WAIT! Did I not just say horsepower is imaginary??? Well, I did indeed. Though look closer. I also said that HP is similar to kW in the measurement of power, which is how much work can be done over a period of time. Look at the measurement once more: 33,000 ft-lb(work)/min(time). Also, lets do a double take! 33,000 ft-lb(practical unit)/min(practical unit). So the imaginary number is STILL made up of two practical units, and it is still a measurement of power (how much work is done over a period of time).

Alright, alright, but if the measurement of horsepower is over 300 years old and applied to steam engines, which we hardly ever use today, why do we still use this measurement today?

Well the simple answer is:

Marketing.

Let's go back to our trusty old saying: "Horsepower sells cars, torque wins races". People have been trying to sell things since the beginning of time, and using imaginary numbers that most people dont fully understand, in order to trick people into buying something to believe is more powerful than it actually is, has been used also since the beginning of time (or beginning of numbers and measurement really...) Look at several car commericals; you will quickly realize that most car commercials throw around the word "horsepower", while most diesel super duty trucks throw around the word "torque". The two measurements are different, as we explained before.

So are the car manufacturers tricking us into believing that their car is more powerful than it actually is? Are they pulling numbers out of their rear-ends to make us feel happy about our purchase?

Not really. That would be scamming, fraudulent, and false advertising; which is bad. Horsepower is still mathematically measurable (HP = trq * RPM / 5252), and it is still a measure of power (work done over time), so it is still technically "valid".

Though this raises another question; why do we use horsepower at all then if torque is the practical unit and the marketing practices are still valid? Well, horsepower is generous. This sounds odd, and nonsensical, as how can a measurement of power be "generous"??

Well, let's use some examples.

200 HP = 350 ft-lbs * 3001 RPM / 5252

200 HP = 150 ft-lbs * 7002 RPM / 5252

Both engines "look" to be exactly the same power, but they both achieve different force (torque), at a different RPM. These engines look to be equivalent to say, a diesel truck, and a Wankel rotary engine. The diesel truck, would be the first example. Produces a LOT of force, but revs very low. The second example, the Wankel rotary, revs VERY high, but produces little force. So even though they both have the same power, they are both very different engines.

So, on the surface, both engines look equally matched; but that is very incorrect when put into application.(KEYWORD: APPLICATION. Remember, application is PRACTICAL, while just looking at the power of both engines is IMAGINARY) Let's say you're a young man looking to purchase his first car. You want something powerful, but fuel efficient as well, you also dont want to create too much noise. Which engine is best for you?

The first example.

But why? They both provide the same power and both do the same amount of work over a period of time. Well the answer is simple. Look at the RPMs. The first example is at 3001; a common RPM for highway cruising, is relatively fuel-efficient, and also relatively quiet. Then look at example two: 7002. Very noisy, VERY impractical (besides the race track or under very heavy loads, where else would you use 7002 RPM?!), and NOT fuel-efficient whatsoever!

Okay, so the first example has both fuel efficiency, practicality, and noise in check, but what about power? Again, the first example wins again! Just simply look at the equations; the first example has 350 ft-lbs whilst the second one has 150 ft-lbs; now I'm just a youngster, but 150 is lower than 350! The first example is more powerful as well!

Alright, so in most applications, the first car is the car to get, but does that mean that the second example engine is completely and utterly subordinate to the first example? No. In MOST applications, YES, it is, but-- in SOME applications; it is the second engine you want. (Also, before I forget, let me explain that horsepower is generous in the way that it is providing the same horsepower number as an engine with more force only because it can make more RPMs. Even though the second engine is actually WEAKER, it has the same HP rating. People like seeing high HP numbers, and will be more likely to buy a car with more horsepower, even though they don't fully understand what both torque and horsepower are. That is why it is generous.)

Well what are these "some applications"? Racing. In most forms of racing, it is high-revolutions that are very important, as well as force (remember, I said MOST forms of racing. If you're a drag racer with an old-muscle car and now thinking about getting a Wankel rotary-- dont. Your torque monster is what you want.) Though why are high-revs important?

For a number of reasons. Let's use ol' Newton's law to explain it: Things in motion tend to want to stay in motion, whilst things not in motion tend to want to stay that way.

Though what does that mean in application? Well simply, the higher you rev your car, the faster it will want to rev. Revving your car from 1000 RPM to 2000 RPM will take longer than revving it from 5000 RPM to 6000 RPM simply because the momentum and centrifugal force that the crankshaft has built up at 5000 RPM is greater than the force it has created at 1000 RPM. The higher you rev, the more the engine wants to rev. Which in turn makes for faster acceleration.

Let's use another example; nitro engines. Small RC nitro-cars use specialized engines called "nitro" engines with a "glowplug". These engines can rev up to sometimes 40,000 RPM! Though they produce very little force, they rev very quickly and provide much acceleration.

Though this raises yet another question; if RPM is all you want for acceleration, why arent all race cars using teeny-tiny nitro engines to get their cars flying around the track? Would'nt that be better than our gasoline engines that only go up to 6500 RPM (for a normal commuter car) usually?

Well no. RPM accounts for less than half of the story. The small little 40,000 RPM-revving nitro engines produce VERY little torque; in the same way that Wankel rotary engines can rev up to 10,000 RPM, but still produce little torque in comparison to the diesel engines that have a hard time revving past 5,000 RPM, but have enough torque to roll over an elephant. Let's use some more examples. I'm going to answer this question, along with another question at the same time: Why do race engines have high-horsepower, but low torque, and why do super-duty diesel trucks have low-horsepower, but high torque?

Well the simple answer is: Application.

A race car weighs usually under 1500 lbs; sometimes even under 1000 lbs, and they're purpose built with aerodynamics, light-weight materials, great weight-distribution, extreme traction, and a whole wack of other little things that make it so that not much force is needed to move the car. 100 ft-lbs of torque would do a LOT more work on moving this race car, than it would trying to move a heavy (usually over 4,000 lbs), unaerodynamic (diesel trucks usually arent very streamlined...), nose-heavy (the diesel engine in the front usually weighs a lot), traction-less (they don't have no traction, but in comparison to race cars which have super-specialized tires, and extreme aerodynamics, they have little traciton), super duty diesel truck, along with the 5000 lb boat it's hauling behind it.

So the race car needs RPM more than the diesel truck needs it so that it can accelerate faster and get up to the powerband faster. Most applications where a super-duty truck is needed; it rarely requires that the truck carry 10,000 lbs while going down a straight-away at 300 km/h; it just requires that is carries a helluva lot of mass.

So race cars need both FORCE and RPM in their application, while super duty trucks really only need FORCE (it requires more force than the race cars do).

Like we explained before, more revs = more acceleration; and we also know that hauling trucks rarely need to go over 300 km/h. Though if both RPM and FORCE contribute to acceleration, then why do trucks need more FORCE than RPM? Well the answer is again, simple.

Lets say you have a gearbox; the first gear has a ratio of 3:1, and the differential ratio is 3.5:1.

Let's say your engine makes 80 ft-lbs at 2000 RPM, and 150 ft-lbs at 6000 RPM (the numbers are quite realistic)

So let's see what we're making at 2000 RPM.

30 HP = 80 ft-lbs * 2000 RPM / 5252
tire rotation = 2000 / 3 (first gear ratio) = 666 / 3.5 (differential ratio) = 190. The tire is rotating 190 times per minute.

And now at 6000 RPM?

171 HP = 150 ft-lbs * 6000 RPM / 5252
tire rotation = 6000 / 3 = 2000 / 3.5 = 571. The tire is rotating 571 times per minute.

Notice, that NO MATTER the HP, and NO MATTER the ft-lbs of torque you are making; RPM is the ONLY contributor to speed. (Acceleration has many variables to it; mainly being RPM, FORCE, and WEIGHT, which we already have explained all and how they contribute to acceleration.)

So using the same gearbox (which is common, as most cars actually use very similar gear ratios and many cars use the EXACT SAME gearbox; such as the Getrag 282 gearbox which is used in the Chevrolet Cavalier, Pontiac Sunfire, Pontiac Fiero, Pontiac Grand Am, and a number of other cars; as an example of how many cars use the same gearbox, and similar gear ratios), 100 ft-lbs of torque and 5000 RPM will do a lot more for a light-weight race-car that needs to accelerate fast and go high-speeds, than it will do for a super heavy-weight hauling truck that needs to haul LOTS of weight at lower speeds and does not need to accelerate extremely quickly. On the other side of things, 500 ft-lbs of torque and 2000 RPM will do a lot more for the super heavy-weight hauling truck than it will do for the race car. As if the race-cars redline is 2000 RPM, then it will need to make very short gear-changes and have to have TONS of gears to reach a reasonable top speed (of over 300 km/h, which the super heavy-weight truck will never need).

Well there you have it. That is the difference of torque and horsepower, with many common questions answered. Though throughout this write-up, this makes horsepower seem very insignificant and useless; is it though? Not exactly.

In a perfect world, we would get dyno-charts for every single car we ever were thinking of buying to see where the engine makes x amount of torque at x RPM; and as a consumer, I would know which RPM range I would use most so I would know if the car I was to buy would be providing most of its power in the RPM range I would be using it most in, or if it would be providing it's power in an RPM range I would not be using.

Unfortunately, we often are NOT given dyno-charts for every single car we are thinking of buying; we are usually only given two numbers: HP and TORQUE and at which RPM. Well this helps a little, as a torque rating will always be at a lower RPM than at a horsepower rating, meaning that if I want more power at a lower RPM, then I should choose the car with more torque than horsepower and vice versa; but even then, without the dyno charts, the few numbers we're given are still quite vague and far-off from painting us an accurate picture of how the engine/car we are buying produces it's power.

In some cases, we are not even given the torque number, and in some cases, not even any RPM figures. At all. Sometimes all we get is a horsepower number; and while I'd usually not buy anything that gives me such little information; sometimes its all we get. So in cases like these when we have very little information; horsepower is a good enough indicator of how powerful an engine is when there is no other information availible.

Hell, even in our "perfect world" situation where we get a dyno sheet for every car we may buy; even then, dynometers are not 100% accurate. They do paint an extremely good picture of how a certain engine makes it's power, but it isnt exactly perfect; though it's the best we can do for now.

So that's my "rant" on the difference between horsepower and torque, as well as answering many common questions. If you have any questions feel free to ask. Hopefully this puts an end to many people misusing the terms horsepower and torque as they're not interchangeable at all, and they are a little tricky at first to understand. If you want to use this text for anything, feel free to do so, but please give me a notice of such acts as well as the credit for the text; it'd be much appreciated

So that's that!

- Sebastian Soeiro

[James45]

Thank you Sebastian for your detailed post on horsepower, torque, etc.

What about using pounds per square inch of thrust (to the final differential/axle measuerment) as opposed to the vague "horsepower" figure you described? Just a thought.

[elemein]

Originally Posted by James45

Thank you Sebastian for your detailed post on horsepower, torque, etc.

What about using pounds per square inch of thrust (to the final differential/axle measuerment) as opposed to the vague "horsepower" figure you described? Just a thought.

Thrust is a pushing force, so therefore it cannot be used in an automotive application (atleast not in this way). Torque is twisting, which can be used. Also, horsepower isnt a "vague" figure, it's a mathematical figure, and it's explained quite well Thanks for reading it though!