Whats the difference between Torque and Horsepower

[retsmah]

...horsepower is a unit of measure of power.

Lets say you have two cars. Both are moving at 30mph. Both are putting out 300hp at this point in time. They are putting out different torques, and the engines are spinning at different speeds. Both will end up putting the same torque to the wheels (assuming they have the same size wheels, I guess a better way of putting it is that they both have the same force accelerating them)

Note that I am saying two cars moving at the same speed. If two cars are moving at the same speed, and both are putting the same power to the wheels, then they are definitley putting the same torque to the wheels. Power at the wheels is torque at the wheels times the rotational speed of the wheel, if the power and speed are the same then the torque absolutely has to be the same.


And yeah, I was ignoring frictional losses, two cars putting out the same power at the crank probably wont be putting out the same power at the wheels. I ignored it because it just makes the discussion of power and torque more complicated, and it's not really important in understanding them.

 

[retsmah]

Another way of looking at it, if a heavier car out accelerates a slower one, it has to be putting out more power than the slower one. Maybe their peak powers are the same, but if you look at how much power the heavier car is putting out during the entire run, it will be more. Significantly more if there is a 700lb weight difference between them.

 

[LeadSlead#2]

im sorry, but that is absolutly impossible. if it was, we wouldnt hear about hp & torque ratings...it would just be 1 rating --- I have no idea how you could possibly think that a car with 300hp and 200 lb-ft will be the same as a car with 300hp & 350lb-ft
2nd off speed has Absolutly Nothing to do with HP....gears & RPM's, yes -- but speed? not at all

Example car 1: 150 hp 150lb-ft 15.8 @ 87mph weight: 2800lbs
Example car 2: 160 hp 111lb-ft 15.9 @ 87mph weight: 2600lbs

why is car 1 just as fast? you tell me....these are real life cars & times driven by both

 

[retsmah]

Because car 1 is putting out more power than car 2 more of the time, regardless of what their maximum horsepower numbers are.

Thats why you cant take a peak power and peak torque number and tell me which car is going to be faster. Or at least you can't definitley say that, although pretty often just having peak torque and power numbers you can make a decent educated guess about which will be faster.



If I tell you how fast a car is going at a point in time, and how much power it is putting out at that point in time, you can tell me how much torque it is putting to the wheels at that point in time. You don't need to know anything about gearing and rpms.

If I give you how much torque it is putting out instead of power, then you need to know something about gearing and rpms. That's why peak power can be a kinda useful thing to know, it allows a really rough comparison of two cars without knowing much about them or having to do much analysis. Although the downside is that it's pretty innacurate.

 

[LeadSlead#2]

dude, you seriously NEED to look at some power-bands for cars...it is quite possible for 2 cars to have the same HP at a given RPM & for one to have more torque at the same RPM

 

[retsmah]

I know that. I'm saying at the wheels. Torque at the crank and at the wheels is not at all the same, because your gearbox multiplies torque.

 

[niky]

im sorry, but that is absolutly impossible. if it was, we wouldnt hear about hp & torque ratings...it would just be 1 rating --- I have no idea how you could possibly think that a car with 300hp and 200 lb-ft will be the same as a car with 300hp & 350lb-ft
2nd off speed has Absolutly Nothing to do with HP....gears & RPM's, yes -- but speed? not at all

Example car 1: 150 hp 150lb-ft 15.8 @ 87mph weight: 2800lbs
Example car 2: 160 hp 111lb-ft 15.9 @ 87mph weight: 2600lbs

why is car 1 just as fast? you tell me....these are real life cars & times driven by both

Hmmm... I think I know those cars. Is this about the old VTEC vs. SR rivalry?

But this is basically a power-curve related deficiency....



which is caused by low torque... yeah, that's right. 👍 ... after the 1/4th, though, car 2 will pull away.

 

[LeadSlead#2]

I don't know what an SR is... so ok... but no, car 1 will hit a higher speed (provided you have the computer reprogrammed to eliminate the speed governor) because, like I said before, torque will give you off the line and top speed...HP is all medium ranges (i.e., acceleration from roll) because torque will push your cars weight off the line and torque will push your car through the wind at high speeds

retsmah: your misunderstanding me... I'm telling you (not suggesting or guessing) that two cars can have the exact same hp at 6000rpm & one of them can have more torque at 6000rpm -- at the wheels

 

[niky]

I don't know what an SR is... so ok... but no, car 1 will hit a higher speed (provided you have the computer reprogrammed to eliminate the speed governor) because, like I said before, torque will give you off the line and top speed...HP is all medium ranges (i.e., acceleration from roll) because torque will push your cars weight off the line and torque will push your car through the wind at high speeds

retsmah: your misunderstanding me... I'm telling you (not suggesting or guessing) that two cars can have the exact same hp at 6000rpm & one of them can have more torque at 6000rpm -- at the wheels

The SR-VTEC rivalry is the first generation Nissan Sentra SE-R and the B16 Honda/Acura Integra GSR.

The SE-R has about 145 hp, 135 ft-lbs of torque, while the Integra has about 160-170 hp and 113 ft-lbs of torque. They're both pretty light, about the same weight. They're a more even match than the new SE-R and RSX, the SE-R is way too heavy, and the RSX is much more powerful.

In quarter-mile battles, the old SE-R would jump the Integra off the line, but the higher horsepower Honda would catch up either by the end of the 1/4th or just after. In a standing mile battle, the Integra would always win. Of course, the Integra is geared[b/] for its powerband, as is the SE-R.

 

[retsmah]

retsmah: your misunderstanding me... I'm telling you (not suggesting or guessing) that two cars can have the exact same hp at 6000rpm & one of them can have more torque at 6000rpm -- at the wheels

Sure, if they are moving at different speeds. (Or if you are talking about drivetrain losses)

I'm saying you've got two cars, at a point in time both are moving at exactly the same speed, and putting out exactly the same horsepower at the crank, at that point in time. I'm not talking about peak horsepower. They may be spinning at completely different engine speeds, but at this point in time they are both making the same power at the crank. In that case, they will both be putting the same torque to the wheels.

edit: to be entirely correct i should really be saying force at the tire, not torque at the wheels, unless the cars happen to have the same size tires... if they don't, then the tire becomes part of the overall gearing of the car. So these two cars could be putting different torques on their wheels, but the end result will be equal force at the tire, what i really should have been saying from the beginning... although I don't think that really changes the overal meaning and potential usefullness of the peak hp number.

 

[LeadSlead#2]

no,no,no,no If you honestly think that is true, then what is the point of buying a truck with a towing engine with say, 300hp and 550ft-lb of torque? why not buy one with 350hp & 350ft-lb of torque? torque and hp are different - yes hp may be some by-product of torque... I'm still not sure exactly how it's figured out, and it's really not important to me, but It's quite obvious that it is not the same...just look at what trucks can tow & what can't... do you honestly believe they have the same force at the wheels?

 

[retsmah]

I'm still not sure exactly how it's figured out, and it's really not important to me, but It's quite obvious that it is not the same

Clearly. I'm not claiming power and torque are the same thing, the point of that example is to show what the significance of power is. Either it's not a good example, or as I suspect power and energy concepts are too complicated to quickly explain.

The problem is that you keep ignoring all of the conditions I stated. I'm saying two cars, putting out equal power at that point in time, both moving at the same speed at that point in time, will put out equal force to the tires at that point in time*. If you say that's not true, you are either ignoring one of those conditions, or are wrong.

I'm not making any claims about power being more important than torque, or torque being more imporant that power, because neither is true, a torque curve and power curve are two different ways of looking at exactly the same thing, engine output.

*Again, I'm ignoring drivetrain losses, since they aren't really relevant to this discussion, as drivetrain losses are more a function of drivetrain design than engine output.

 

[Wolfe]

I think we need to look back to ExigeExcel's example, as it was probably the best one --

It's like an elephant will start pulling a heavy cart quicker than a few horses. But while the Elephant is still plodding along quite comfortably the horses will be galoping at speed.

A 150hp, 300lb.ft car will pull away from a 300hp, 150ft.lb car at the line, but the 300hp car will be much faster at higher speeds (and higher rpms). This is why diesel racecars are rare, because diesels are much torquier than they are powerful.

 

[LeadSlead#2]

Clearly. I'm not claiming power and torque are the same thing, the point of that example is to show what the significance of power is. Either it's not a good example, or as I suspect power and energy concepts are too complicated to quickly explain.

The problem is that you keep ignoring all of the conditions I stated. I'm saying two cars, putting out equal power at that point in time, both moving at the same speed at that point in time, will put out equal force to the tires at that point in time*. If you say that's not true, you are either ignoring one of those conditions, or are wrong.

I'm not making any claims about power being more important than torque, or torque being more imporant that power, because neither is true, a torque curve and power curve are two different ways of looking at exactly the same thing, engine output.

*Again, I'm ignoring drivetrain losses, since they aren't really relevant to this discussion, as drivetrain losses are more a function of drivetrain design than engine output.

If thats all your saying, your not saying much... why would anyone think to cars with the same power would have different power? if that is really what your saying this whole time... I would love to know what the significance is, & why you shared it with us in the first place... it really sounded like you were saying torque was irrelevant because hp is just torque multiplyed.... whcich couldnt be farther from the truth

 

[LeadSlead#2]

I think we need to look back to ExigeExcel's example, as it was probably the best one --



A 150hp, 300lb.ft car will pull away from a 300hp, 150ft.lb car at the line, but the 300hp car will be much faster at higher speeds (and higher rpms). This is why diesel racecars are rare, because diesels are much torquier than they are powerful.

I think we all realize with examples like this thats true...
what I'm saying is simply 2cars--- same(ish) power one weighs more, but has an equal portion(or more) torque (relative to weight) will be pretty much equal, except the one with more torque will probly be faster at high speeds, due to the extra torque

 

[Wolfe]

I think we all realize with examples like this thats true...
what I'm saying is simply 2cars--- same(ish) power one weighs more, but has an equal portion(or more) torque (relative to weight) will be pretty much equal, except the one with more torque will probly be faster at high speeds, due to the extra torque

Ah, I see. The one with a higher torque-to-weight ratio will indeed be faster at lower speeds (and rpms), but horsepower always wins at high speeds. Otherwise the new Z06 would have a much higher top speed than the new M5. And it doesn't. (~500hp Corvette = estimated 198mph / ~500hp M5 = estimated 202mph)

 

[LeadSlead#2]

Ah, I see. The one with a higher torque-to-weight ratio will indeed be faster at lower speeds (and rpms), but horsepower always wins at high speeds. Otherwise the new Z06 would have a much higher top speed than the new M5. And it doesn't. (~500hp Corvette = estimated 198mph / ~500hp M5 = estimated 202mph)

I got one word for you... Gearing

also, does either one have much more torque than the other?

 

[///M-Spec]

A 150hp, 300lb.ft car will pull away from a 300hp, 150ft.lb car at the line, but the 300hp car will be much faster at higher speeds (and higher rpms). This is why diesel racecars are rare, because diesels are much torquier than they are powerful.

You're using the units, but not stating at what RPM the engine is producing these units. I can easily draw a power/torque curve using your two sets of units and get whatever results I wanted just by changing the gearing.

Your 300 ft-lb car may be making this peak figure at 2,000 RPM, then for the rest of the engine range makes 175 ft-lb (until redline @4750 RPM). If that were the case, the 150 ft-lb car would only need to sustain this peak number to 7,000 RPM and would destroy the first car even out of the hole shot just using the appropriate gearing.

*EDITED numbers to workout

You've got to look at the area under the curve, and not focus on peak output. Engines only produce peak numbers for a brief period of time.

If thats all your saying, your not saying much... why would anyone think to cars with the same power would have different power?

He's saying more than you think he is.

Ah, I see. The one with a higher torque-to-weight ratio will indeed be faster at lower speeds (and rpms), but horsepower always wins at high speeds. Otherwise the new Z06 would have a much higher top speed than the new M5. And it doesn't. (~500hp Corvette = estimated 198mph / ~500hp M5 = estimated 202mph)

At high speeds, aerodynamic drag really the primary factor after horsepower. That's because as speeds increase, the amount of horsepower required to keep the car at speed goes up exponentially.

For example, at 150 mph it may require 160 hp just for an M5 to maintain that speed, leaving 'only' 320 hp to accelerate the car. At 190 mph, it may require 400 hp, leaving only a few to get us the rest of the way to 200 (when you factor in drivetrain losses). NOTE: These aren't real numbers. I guesstimated them up for illustration's sake.

The reason the C5 Z06 and E60 M5 have similar top speeds despite the Vette's huge power/weight ratio advantage would be aerodynamics. I would also imagine the Vette's mechanical rolling resistance from the huge tires is a factor as well.


M

 

[LeadSlead#2]

since it wont let me reply, i'll do it like this

1. we're all assuming same gearing for comparison's...(i hope) i am, anyway, because if you don't it makes room for unwanted side arguments that get people distracted from the real issues

2. he certainly didnt say anything else in his last post, so if you think you know what he's saying, and it's more than that, feel free to clarify

3. M5 7-Speed Corvette.. I personally won't be surprised if it goes over 198.. secondly, with different gears, would probly go faster

 

[retsmah]

I was attempting to show what power is and how... well, powerful, it can be. it often allows you to solve problems without getting into all the messy details... like I just told you these two completely different cars with different engines and drivetrains are puttnig out the same force at the wheels, just by using the power output and speed of the vehicle. If I told you the torque output instead of power output, you couldn't solve anything.

Unfortunatly power and energy are fairly difficult concepts, I'm clearly not going to be able to teach them here.

Really, the thread is posed wrong. If you are asking 'whats the difference between torque and horsepower', you are thinking about the whole subject wrong.

 

[niky]

At high speeds, aerodynamic drag really the primary factor after horsepower. That's because as speeds increase, the amount of horsepower required to keep the car at speed goes up exponentially.

For example, at 150 mph it may require 160 hp just for an M5 to maintain that speed, leaving 'only' 320 hp to accelerate the car. At 190 mph, it may require 400 hp, leaving only a few to get us the rest of the way to 200 (when you factor in drivetrain losses). NOTE: These aren't real numbers. I guesstimated them up for illustration's sake.

The reason the C5 Z06 and E60 M5 have similar top speeds despite the Vette's huge power/weight ratio advantage would be aerodynamics. I would also imagine the Vette's mechanical rolling resistance from the huge tires is a factor as well.


M

Nice post. 👍 But I would say 250-300hp to get to 150mph, about 400 to get to 170 or 180, then 500, give or take 190-200+, that last number being more heavily dependent on aerodynamics. 160hp usually only gives you about 130mph, where drag balances out that much engine power on most modern cars, no matter what the gearing.

It is an interesting conundrum, I'd have thought the Vette's long gearing would give it a higher V-max, but yes, those huge rear tires would cause a lot of drag.

 

[LeadSlead#2]

retsmah, the problem with telling me about two imaginary cars with different gearing & engines, travelling at the same speed, having the same power, is that your taking so much for granted, & not explaining half of the needed factors... so rather than make this an infinate discussion, I'll try this..

I understand that two vehicles with the same power in whatever gear necessary at a given speed will have the same force driving the wheels... except that is not true, because it Is possible for two vehicles to have the same HP at this speed, But for one to have more torque.... just because they have the same hp, (not peak) but at this speed (since you seem against RPM's) the same HP, one can have more torque, even thought they have the same HP, therefore, it would have more force at the wheels --

I'll look for some dynos.. at the wheel ratings, if necessary to show you this

 

[Wolfe]

I got one word for you... Gearing

also, does either one have much more torque than the other?

If gearing's the problem, how come the Vette's additional torque doesn't make up for it? It all comes down to aerodynamics. The 'Vette's Cd is .34, and the M5's is .32, which explains why the M5 is just a bit faster.

Corvette -- 475lb.ft
M5 -- 384lb.ft

You're using the units, but not stating at what RPM the engine is producing these units. I can easily draw a power/torque curve using your two sets of units and get whatever results I wanted just by changing the gearing.

Your 300 ft-lb car may be making this peak figure at 2,000 RPM, then for the rest of the engine range makes 175 ft-lb (until redline @4750 RPM). If that were the case, the 150 ft-lb car would only need to sustain this peak number to 7,000 RPM and would destroy the first car even out of the hole shot just using the appropriate gearing.

*EDITED numbers to workout

You've got to look at the area under the curve, and not focus on peak output. Engines only produce peak numbers for a brief period of time.

I was generalizing. Trying to simplify things, here.

At high speeds, aerodynamic drag really the primary factor after horsepower. That's because as speeds increase, the amount of horsepower required to keep the car at speed goes up exponentially.

For example, at 150 mph it may require 160 hp just for an M5 to maintain that speed, leaving 'only' 320 hp to accelerate the car. At 190 mph, it may require 400 hp, leaving only a few to get us the rest of the way to 200 (when you factor in drivetrain losses). NOTE: These aren't real numbers. I guesstimated them up for illustration's sake.

The reason the C5 Z06 and E60 M5 have similar top speeds despite the Vette's huge power/weight ratio advantage would be aerodynamics. I would also imagine the Vette's mechanical rolling resistance from the huge tires is a factor as well.

A. Power/weight has nothing to do with top speed. Just power. Case in point -- Lotus Elise.

B. I understand completely how aerodynamics/rolling resistance and top speed are related -- you've missed the point I was trying to make, which was that cars with more torque don't necessarily have higher top speeds than equivalent-HP cars with less torque. Again, this is based around generic peak-output numbers, and is in response to LeadSlead#2.

 

[retsmah]

Power = Force x Velocity. If they are putting out the same power, at the same velocity, then the force at the wheels has to be the same. I don't need to know gearing, i dont need to know torque, i dont need to know anything else. That's what's interesting about power. That's why I said its a roundabout way of measuring torque at the wheels.

I'm not 'against rpm', as ive stated several times if you really want to know something about an engine you need a torque or power curve. You know, torque plotted against rpm. I'm just trying to give people an idea of what power is, since most people don't actually understand what it means, at least in physics terms.


As another example, lets say you have two vehicles, one puts out a max of 18 ft lbs at the crank, the other puts out a max of 70 ft lbs. In addition, the one putting out 70 ft lbs weighs about 1/3 of what the one putting out 18 ft lbs. Based on those numbers, can you really tell me anything about which is going to be faster?

Probably not, unless of course you were to actually see the vehicles, as the one putting out 18 ft lbs is a Kawasaki Ninja 250R, while the one putting out 70ft lbs is a bicycle.

 

[LeadSlead#2]

what bycicle has 70 ft-lbs? I'd love to see the muscles for that

 

[retsmah]

what bycicle has 70 ft-lbs? I'd love to see the muscles for that

I can put that amount of torque out just fine. I weigh about 125lbs, I put all of my weight on a pedal that has a six inch arm on it:

125 lbf * .5 feet = 62.5 ft lbs.

You can imagine if you had a guy who isn't 5'6" you'd have a lot more than that. But the bike won't be putting out much in the way of torque at the wheels. Why?

Might have a little something to do with the fact that a human puts out less than .25 hp. Or, too look at it in a slightly different way, that 62.5 ft lbs is at 0 rpm, and it drops off pretty quickly, at like 300rpm or so I am probably no longer capable of putting out any torque.

But you wouldn't be able to tell any of that just looking at the peak torque number. If you had seen the peak power number, then you would be able to figure out why the bike can only get up to 25mph even with all that torque. With a ten speed transmission too!


Note: The physics of the torque at the crank on the bicycle is slighly more complicated than that because if I really want to crank on it I will be accelerating upwards, increasing the force on the pedal... so I can actually put out even more torque than that.

 

[Wolfe]

@retsmah: I think you've finally nailed what you were trying to get at. 👍

 

[retsmah]

...can't tell if you are making fun of me or not I personally think the bicycle vs. motorcycle example is good, I've used it before talking to people about torque and power

 

[Wolfe]

No really, I'm serious. If I was being sarcastic I'd use or

It's one of the greatest explanations of how power and torque work, and what they mean, that I've ever heard.

I was laughing because it was such a knock-out example.

 

[LeadSlead#2]

okay, I think I've got it... and if i'm right, we've been arguing different points that have nothing to do with each other...so hold on & try to get this, because sometimes my mind works in unbelievably odd ways, but still works

You are saying: a car with 300 HP at 8000rpm, although it has less torque per say, (lets say 120) has the same power as a car with 300 HP at 6000rpm, even though it has, say 200 ftlb of torque -- since both have the same amount of torque multiplyed by the RPM's=Horsepower..... correct?

I have been saying, a car with 300HP at 6000RPM will have more torque at 6000 RPM's than say, a car with 300HP at 8000RPM will have at 8000RPM's -- which is true, it will have to to have the HP at a lower RPM

I am also saying that a car with 300HP at 6000RPM and more torque will be faster... which if all other factors are identical, it will, because of a broader powerband...

and it seems that since we did'nt seem to have a clue what the other was trying to say, both arguments are quite stupid... due to how simple and obvious each is

so, hopefully this will end this argument, as hopefully I have phrased it well enough so that both arguments are understood, and its seen that we've been arguing about different things...
P.S. I must admit I feel kinda stupid for not realizing what you were saying, but hell my mind only turns on at 4a.m. soooo... oh yeah, I don't wanna be mean, but the bicycle point was horribly dumb... although it may be true, it would have made it much harder to understand if I was actually disagreeing with what you were saying... seriously, no offense have a beer if it's all figured out, if not give me my beer back!