Horsepower versus torque

Frank

Certified not a majority
Veteran
While cars are advertized with having a certain amount of horsepower, that is losing significance all the time. Wouldn't it be better to state the amount of torque instead? Because horsepower is only interesting for the top speed, and has no real relevance without taking the weight and air friction of the car into acccount. And just about all of them will go much faster than the speed you can (normally) ever drive it in any case.

Torque is about fast acceleration and towing. And turbo charged, diesel and electric engines have so much more torque than normally aspirated gasoline engines for the same amount of maximum horsepower, that it is close to ridiculous. Twice the amount, in general. And at low rpm (or any, for electric), so that skews the picture even futher.

A car with twice the amount of torque and the same weigth will accelerate roughly twice as fast. And that's nowadays just about the only thing you notice or care about, power wise.
 
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DiGuru said:
... And turbo charged, diesel and electric engines have so much more torque than normally aspirated gasoline engines for the same amount of maximum horsepower, that it is close to ridiculous. Twice the amount, in general. ...
Turbo-charge the gasoline engine - and then you can at least compare like with like, except of course the gasoline engine will have even more power....

... And at low rpm (or any, for electric), so that skews the picture even futher. ...
Not so. Electric motors develop maximum torque at zero rpm, at higher speeds the torque is correspondingly reduced.
 
Any true car nut looks at the torque curve before buying already, right? And I thought most diesels had gobs of low-end torque but peetered out thereafter - great for getting a large mass moving but not so great for getting a car up to 120 km/hr quickly - for that a flat torque curve is better, no?
 
nthd said:
Turbo-charge the gasoline engine - and then you can at least compare like with like, except of course the gasoline engine will have even more power....
That's what I meant. But turbo-charged diesel engines do even better. And you can build those for a wide variance of specs, for the same amount of cubic inches. Because they work differently. They give power proportional to the amount of fuel injected (within margins).

Not so. Electric motors develop maximum torque at zero rpm, at higher speeds the torque is correspondingly reduced.
Well, electric motors can want an almost unlimited amount of power supplied to them when at zero rpm, until something melts down, because they're mostly short-circuited when not moving. But at any higher rpm the good ones (AC and brushless) are very lineair in their efficiency. The amount of power is not directly related to the rpm and amount of fuel injected, unlike with (gasoline) combustion engines.
 
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Mize said:
Any true car nut looks at the torque curve before buying already, right? And I thought most diesels had gobs of low-end torque but peetered out thereafter - great for getting a large mass moving but not so great for getting a car up to 120 km/hr quickly - for that a flat torque curve is better, no?
Inertia. So having the largest amount of torque at the lowest rpm is best for accelerating from standing still.

But yes, the amount of torque at the current rpm is what defines the acceleration. So a flat curve is better if you don't shift.
 
To explain a bit better what I mean: here in Europe the horsepower figure has become almost meaningless. Like, if you buy a new BMW or Mercedes, you might see a 3 liter V6 gasoline engine with 250 hp, next to a 2.5 liter turbo-charged diesel engine with only 180 hp. While the gasoline engine only has 230 Nm of torque at 6000 rpm and the diesel engine has 400 Nm at 2300 rpm.

Which one would the consumer buy, looking at the spec sheet? Which might only state the amount of horsepower. Or they might not know the relevance of torque, as most people don't.
 
Mize said:
Who cares about 0-20 anyway :)
Not many people, or they would all drive a 4wd car :)

But that turbo-charged diesel engine will drag you (or launch you, depending ;) ) to 60 mph much faster than that big V6 engine.
 
DiGuru said:
Not many people, or they would all drive a 4wd car :)

But that turbo-charged diesel engine will drag you (or launch you, depending ;) ) to 60 mph much faster than that big V6 engine.

That's why I have a big V8 ;)
 
Mize said:
That's why I have a big V8 ;)
:)

Then again, in racing, you're only allowed a turbo-charged engine that has at most half the amount of cubic inches as the naturally aspirated one, and it has to suck the air through a smaller diameter hole.
 
nthd said:
Turbo-charge the gasoline engine - and then you can at least compare like with like, except of course the gasoline engine will have even more power....
Turbocharged gasoline engines typically do not compare to turbocharged diesels, not even close in fact. Particulary not at low RPMs, as many gas engines hit max torque above 4000RPM. Most people rarely revv their engines that high, ever.

Unfortunately, diesel fumes are full of that microscopic particle crap that clogs up our lungs, so they're not ideal from every aspect either... :p

Not so. Electric motors develop maximum torque at zero rpm, at higher speeds the torque is correspondingly reduced.
Well, even so, electrical motors seem to have much flatter torque profiles than piston engines.
 
From the Tesla car (assuming equal power, which isn't specified, but looks on target):

motor_torque_curve.gif
 
I dunno, when it comes to cars reading the stats on a sheet just never was enough for me....I gotta drive it to see if I like it or not.

I think the HP numbers are just the equivalent of an e-penis thing after a while.
 
As Mize said, the reduction of the gearbox is the main factor in comparing acceleration for gasoline/diesel and turbo engines. And if you want the longest (flattest) power range, turbo-charged gasoline engines win hands down from any kind of diesel engines. Because they like high rpm. And shifts work by lowering the reduction. So, if you want to drive fast with an engine that works best at low rpm, you need much larger reductions, which require the engine to supply much more power at each rotation.

And when you have to shift once more accelerating to 60 mph, you'll probably lose. But on the other hand, if you keep the rpm low, you use less fuel and that diesel engine supplies more power.

So, for all-out power, the electric engine wins hands-down. And the turbo-charged gasoline engine comes second, followed by the turbo-charged diesel engine.

But for efficiency, the electric engine stil wins, but the (turbo-charged) diesel engine comes second, followed by the normally aspirated gasoline engine. And the turbo-charged gasoline engine comes last.
 
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DiGuru said:
:)

Then again, in racing, you're only allowed a turbo-charged engine that has at most half the amount of cubic inches as the naturally aspirated one, and it has to suck the air through a smaller diameter hole.

In racing there are all sorts of goofy rules that restrict all sorts of things. I can assure you the airflow into the naturally aspirated engine is also restricted.

And just for a basis of comparison here.

In 1971 Oldsmobile was producing naturally aspirated (Carburator) gasoline engines that produced 365HP and 500 ft./lbs of torque. HP vs. Torque is a matter of tuning, not a matter of fuel or induction type.
 
DiGuru said:
While cars are advertized with having a certain amount of horsepower, that is losing significance all the time.
It isn't.
Why are you claiming that it is losing significance?
Wouldn't it be better to state the amount of torque instead? Because horsepower is only interesting for the top speed, and has no real relevance without taking the weight and air friction of the car into acccount. And just about all of them will go much faster than the speed you can (normally) ever drive it in any case.

Torque is about fast acceleration and towing.
Torque is about towing. But Horsepower is about acceleration.
Just compare a lot of car specs and you will see that a car with more horsepower but less torque will accelerate faster than a car with more torque but less horsepower. (0-100 km/h)
And turbo charged, diesel and electric engines have so much more torque than normally aspirated gasoline engines for the same amount of maximum horsepower, that it is close to ridiculous. Twice the amount, in general. And at low rpm (or any, for electric), so that skews the picture even futher.

A car with twice the amount of torque and the same weigth will accelerate roughly twice as fast. And that's nowadays just about the only thing you notice or care about, power wise.

So you are comparing a 3 liter turbo charged diesel engine to a 2 liter gasoline engine, (otherwise you won't get the same amount of horsepower) and then you are suprised that the diesel wins?
You are comparing apples to vigs. (not oranges because your comparison is too far off for that...)

Compare a modern 2 liter diesel to a 2 liter gasoline engine and you will see that the diesel will have more torque, but the gasoline will have more horsepower.
Also the diesel will be more expensive, and will have an economical lifespan of 250.000 km while the gasoline engine will last 500.000 km.

Of course if you compare it with a diesel engine that costs twice as much, the gasoline will loose.


Then back to your original statement that torque tells you more about performance:
It is simply wrong!
Torque tells you ABSOLUTELY NOTHING about the performance of your car.

Torque only becomes a meaningful value when you know at what rpm it is produced, and what the maximum rpm of a car is. The maximum power of the engine is then determined by multiplying Torque with rpm.

Let me give an example:
An engine with a Torque of "4" with a rpm of 2000 will have a maximum "power" of 8000
An engine with a Torque of "2" with a rpm of 5000 will have a maximum power of 10.000

As most people will know, gasoline engines have lower torque, but use higher rpm. Because of this, the engine can produce just as much power as a diesel engine.

So this "power" value is a far better value to compare engines with.
And not suprisingly this "power" value is the amount of HORSEPOWER of an engine.
And that is why HORSEPOWER is usually used to compare engines.

There is an exception to all this though: Towing.
If you need to move a heavy load, then you start from 0 rpm. You cannot compensate for lower torque by using higher rpm, because you don't move fast yet.
So to get a large load moving, a high torque value is important.

(Although you can compensate for that with different gearbox ratio,s but lets not make this too complicated)

There is a different torque value that would actually be interesting: The torque on the wheel axis.
The rpm value doesn't play a role here anymore, because the rpm for accelerating from 0 - 100 km/h is the same between cars. (as long as the wheels have the same size)
You then also take the gearbox and other factors into account.

Unfortunately those numbers are almost never included in the specs of a car.

I have found that many people do not truly understand what torque and HP means. So I advice everybody here to read the following page:
http://www.squadra-tuning.com/English/EN_what_is_torque_and_performance.htm

It explains clearly what it means, and what the graphs you see in the specs of cars tell you.
 
Because horsepower is only interesting for the top speed, and has no real relevance without taking the weight and air friction of the car into acccount.
Well, I wouldn't say that is completely true (or rather, that it's true, but not the whole truth). The thing is that top speed is also a question of being able to make power at high rpm as well. The drivetrain and everything needs to stand up to it, of course, but then torque is also kind of implied in that.

What's important isn't that one peak horsepower figure, but the entire power curve over the rpm range. If you can make higher power at lower rpm, that's the thing that translates into high torque. That's basically what diesels do -- they can't go way up in the rpm range as easily as a gasoline engine can, but the main thing is that a diesel of half the peak power of an NA gas engine can make way more power at say, 2000 rpm than the gas engine could, and thus be far more efficient in normal driving conditions.

It isn't.
Why are you claiming that it is losing significance?
I think it's bad wording on his part. A single horsepower figure loses its meaning when compared to other drivetrains which could theoretically have lower peak power figures but actually drive as though it's many times more powerful. A 50 KW electric motor would run circles around the 120-hp 4-cylinder I have because the torque will be that much greater. Sure, the 4-cylinder would beat it on a track or dragstrip, but I don't take a racetrack to work.

A more fair comparison would be to look at the entire powercurve, but that will never show up on a spec sheet, nor do I expect the average car buyer (especially, especially, especially, especially not in America) to be able to comprehend the difference.

That's what I meant. But turbo-charged diesel engines do even better. And you can build those for a wide variance of specs, for the same amount of cubic inches. Because they work differently. They give power proportional to the amount of fuel injected (within margins).
That's one of the many things that makes diesels so much more efficient -- the power you make is largely controlled by the amount of fuel injected. Direct-Injection gas engines also achieve this to a point, but there's still a lower limit because it's usually still spark-ignited rather than instant photo-ignition like a diesel.

Diesel is pretty near dead here in the US (in the mass market, that is -- obviously industrial and shipping is a different story) because of the EPAs immeasurable stupidity. The emissions standards out here are based on fractional rates per unit of emissions, whereas in EU, they're based rates per unit mileage. That makes the emissions of diesels look inflatedly bad. Well, it's true that diesels do have NOx problems, but those aren't insurmountable. The problem is that it involves replacing more than one fluid regularly when filling up fuel -- something that the EPA believes (and I agree on this) that Americans are too absent-minded to be able to handle.

Not many people, or they would all drive a 4wd car
4WD is too expensive. Most everything is FWD because it's cheaper to produce. RWD is only starting to make a comeback as more and more people are demanding more peak horsepower (even though what they really are effectively asking for is torque). Of course more peak power can feel like more torque because everything including torque may be higher.
 
mjtdevries said:
It isn't.
Why are you claiming that it is losing significance?

Torque is about towing. But Horsepower is about acceleration.
No, torque is about accelerating, horsepower is about maximum power.

Torque is the amount of power delivered for each stroke, horsepower is the maximum amount of power delivered in total, at a specific amount of rpm.

Just compare a lot of car specs and you will see that a car with more horsepower but less torque will accelerate faster than a car with more torque but less horsepower. (0-100 km/h)
Yes, that's possible. It depends on the maximum torque, the weight of the car and the ratio of the gearbox used.

So you are comparing a 3 liter turbo charged diesel engine to a 2 liter gasoline engine, (otherwise you won't get the same amount of horsepower) and then you are suprised that the diesel wins?
Nah. For starters, that diesel engine would have about three times the amount of torque of that gasoline engine. At least. It would just have that maximum amount at less than half the rpm of the gasoline engine.

But that gasoline engine can simply burn more fuel at higher rpm. Not more efficient, just more. Which translates into more power at those higher rpm.

Both engines lose efficiency fast when rotating and burning fuel faster. But one of them doesn't need to do that to produce maximum power.
You are comparing apples to vigs. (not oranges because your comparison is too far off for that...)

Compare a modern 2 liter diesel to a 2 liter gasoline engine and you will see that the diesel will have more torque, but the gasoline will have more horsepower.
Also the diesel will be more expensive, and will have an economical lifespan of 250.000 km while the gasoline engine will last 500.000 km.
It does? I have always been under the impression that diesel engines last more than twice as long as gasoline engines. Although you might have to replace the turbos at some time, if they have those.

Of course if you compare it with a diesel engine that costs twice as much, the gasoline will loose.
I'm only looking at the performance characteristics. I don't think the sales price of the engine has much to do with that.

Then back to your original statement that torque tells you more about performance:
It is simply wrong!
Torque tells you ABSOLUTELY NOTHING about the performance of your car.

Torque only becomes a meaningful value when you know at what rpm it is produced, and what the maximum rpm of a car is. The maximum power of the engine is then determined by multiplying Torque with rpm.

Let me give an example:
An engine with a Torque of "4" with a rpm of 2000 will have a maximum "power" of 8000
An engine with a Torque of "2" with a rpm of 5000 will have a maximum power of 10.000

As most people will know, gasoline engines have lower torque, but use higher rpm. Because of this, the engine can produce just as much power as a diesel engine.

So this "power" value is a far better value to compare engines with.
And not suprisingly this "power" value is the amount of HORSEPOWER of an engine.
And that is why HORSEPOWER is usually used to compare engines.
Yes and no. It says something about the maximum amount of power, true. And at what rpm that power is available. But an engine that produces twice the amount of torque at some rpm, will deliver twice the amount of power at that rpm. Simple as that.

There is an exception to all this though: Towing.
If you need to move a heavy load, then you start from 0 rpm. You cannot compensate for lower torque by using higher rpm, because you don't move fast yet.
So to get a large load moving, a high torque value is important.

(Although you can compensate for that with different gearbox ratio,s but lets not make this too complicated)
So, what is the difference between having a car towing something with a total combined weight of, say, 2000 kg, and having a car that weighs 2000 kg by itself? There is none.

But the gearbox ratio is indeed important. Because you get twice the amount of power at half the rpm from a diesel engine.
There is a different torque value that would actually be interesting: The torque on the wheel axis.
The rpm value doesn't play a role here anymore, because the rpm for accelerating from 0 - 100 km/h is the same between cars. (as long as the wheels have the same size)
Nonsense. Only when they would be the same weight and accelerate at the same speed.

You then also take the gearbox and other factors into account.
Yes.

Unfortunately those numbers are almost never included in the specs of a car.
Exactly.

I have found that many people do not truly understand what torque and HP means. So I advice everybody here to read the following page:
http://www.squadra-tuning.com/English/EN_what_is_torque_and_performance.htm

It explains clearly what it means, and what the graphs you see in the specs of cars tell you.
Well, I think I have a pretty good grasp of the things involved. But thanks anyway. ;)
 
Thank you, ShootMyMonkey. You explained it much better than I did. I agree completely.
 
Powderkeg said:
In 1971 Oldsmobile was producing naturally aspirated (Carburator) gasoline engines that produced 365HP and 500 ft./lbs of torque. HP vs. Torque is a matter of tuning, not a matter of fuel or induction type.
Hum, I'm pretty certain that example of yours isn't as much a matter of tuning, but rather one of cubic inches... ;) That's most certainly a HUGE cylinder volume engine, it'd be rather surprising if it didn't have relatively high performance. Then again, producing giant heavy (and thirsty) clunkers was pretty much the only way detroit knew how to gain high performance anyway, so... :LOL:

Rest assured, a turbocharged diesel with the same volume's likely to have 2x the torque of your oldsmobile cast iron stove, easily. Heck, I read over ten years ago about a D.A.F. trailer tractor engine that delivered 2000Nm of torque, the only gas powered engine I know of that comes close is the Bugatti Veyron's W16 quad-turbocharged monster at what, 1100-something?

Now, I wonder what Volvo's 600HP truck engine's delivering in torque, certainly a LOT more than 2000Nm... :p
 
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