Is Power (rwkw) Figure Everything? Why Are People So Hung Up On Kw Figures

[Rolls]

It would be a hell of a lot simpler to just say it has x kw by 1300rpm, saying it has 400nm between 1300 and 5200rpm doesn't mean anything, no one is going to get their calculator out and figure out how much power that is.

They should just quote power spread, anything else is pretty much useless information to the customer.

[joeyjoejoejuniorshabadoo]

yes phil, that is impressive, but what adam was saying was than normally a car that has peak torque by 1300rpm will have nothing by 4000 odd

[zebra]

It would be a hell of a lot simpler to just say it has x kw by 1300rpm, saying it has 400nm between 1300 and 5200rpm doesn't mean anything, no one is going to get their calculator out and figure out how much power that is.

They should just quote power spread, anything else is pretty much useless information to the customer.

I see what you are saying and it does make sense, but most people want to see a torque figure and if they see that figure starts from near idle and goes all the way to 5000odd rpm they can see that the car will pull easily and effortlessly up hills etc.

Well thats the way i look at it anyhow.

take my twin Turbo Soarer as an example, makes similar power and torque to the BMW but torque doesnt peak until 4800rpm, and peak power is something like 6400

I would do almost anything to have a torque curve teh same as the BMW.

Especially when over taking trucks and stuff when going up the snowy's like i did last weekend, I needed a boot full of 2nd gear (auto trans) to get the thing accelerating hard up the hill.

[scotty nm35]

I see what you are saying and it does make sense, but most people want to see a torque figure and if they see that figure starts from near idle and goes all the way to 5000odd rpm they can see that the car will pull easily and effortlessly up hills etc.

Well thats the way i look at it anyhow.

take my twin Turbo Soarer as an example, makes similar power and torque to the BMW but torque doesnt peak until 4800rpm, and peak power is something like 6400

I would do almost anything to have a torque curve teh same as the BMW.

Especially when over taking trucks and stuff when going up the snowy's like i did last weekend, I needed a boot full of 2nd gear (auto trans) to get the thing accelerating hard up the hill.

You need a matched torque converter, one of few benefits of running an auto.

3000 stall FTW.

[zebra]

it flares to 3200rpm on the road anyway, the factory stalls are quite loose in these on the brake I can get 2400-2500rpm.

I dont like big stallies on street cars, makes them annoying to drive to work with.

[PM-R33]

It would be a hell of a lot simpler to just say it has x kw by 1300rpm, saying it has 400nm between 1300 and 5200rpm doesn't mean anything, no one is going to get their calculator out and figure out how much power that is.

They should just quote power spread, anything else is pretty much useless information to the customer.

See I think that is the complete opposite.

Lets say they qouted the power at 1300rpm, it works out to be 73kw. It doesn't really sound that impressive to the average person. Also how do you qoute the power from 1300rpm to 5200rpm easily? Given that the torque remains flat and constant, you could use a function:

Power = (294.92x)/5252 between 1300-5200rpm

Obviously that is out of the question.

You could qoute power figures at every 1000rpm. Once again that is out of the question.

The easiest and best way is to qoute the torque.

Every one knows 400Nm is a lot. It is easy to say, "it holds it's peak torque of 400Nm from 1300-5200 and revs to 7000rpm." Straight away that small sentence tells me a hell of a lot about the engines behaviour. The only unknown in that case is how quickly the torque drops off from 5200-7000rpm. This is easily fixed by qouting "the peak kw figure of 225kw."

I really doubt you could think of a easier way to explain the engine behaviour.

yes phil, that is impressive, but what adam was saying was than normally a car that has peak torque by 1300rpm will have nothing by 4000 odd

Oh yeah no doubt, but by qouting the peak kw figure at 7000rpm it straight away lets you know it doesn't fall off to next to nothing like a diesel engine would.

So in essence just to sum up what I think.

I believe that qouting a low rpm torque figure in the case of selling a performance vehicle is very helpful. It lets you know how much power it will make down low and how easily it will "pull". Most people would know that once torque hits it's peak figure it will either hold for a while, or gradually drop down. It doesn't really spike/drop/spike/drop kind of thing. Finally qouting the peak kw figure at redline will let you know how much of a drop there is. If the peak kw figure is good at redline, you know it mustn't drop off that badly.

This combined information is the easiest way to explain an engines behaviour (obviously without pulling out dyno graphs) to the general public and also gives a lot of information to the people that understand a lot about it.

Edited October 28, 2010 by PM-R33

[Rolls]

See I think that is the complete opposite.

Lets say they qouted the power at 1300rpm, it works out to be 73kw. It doesn't really sound that impressive to the average person. Also how do you qoute the power from 1300rpm to 5200rpm easily? Given that the torque remains flat and constant, you could use a function:

Power = (294.92x)/5252 between 1300-5200rpm

Obviously that is out of the question.

You could qoute power figures at every 1000rpm. Once again that is out of the question.

The engine makes 100kw at 1500rpm and increases linearly to 225kw by 7000rpm... no more a mouthful than quoting torque and power, not to mention that everyone knows what power feels like.

Every one knows 400Nm is a lot. It is easy to say, "it holds it's peak torque of 400Nm from 1300-5200 and revs to 7000rpm." Straight away that small sentence tells me a hell of a lot about the engines behaviour. The only unknown in that case is how quickly the torque drops off from 5200-7000rpm. This is easily fixed by qouting "the peak kw figure of 225kw."

I really doubt you could think of a easier way to explain the engine behaviour.

I actually think that almost no one knows that 400Nm feels like or is, the figures are always quoted at different rpm (different engines and cars) so the figure soon becomes without meaning, it could mean 100kw or it could mean 500kw depending on where it is. Honestly I think quoting torque is nothing more than a very confusing marketing ploy.

[bigmikespec]

Agree with Rolls... the best part is when you explain to Joe Average that you can have 100000Nm of torque and not even turn a wheel; the look on their face is great!

Edited October 28, 2010 by bigmikespec

[The Dan]

I think you are missing the point. Quoted torque @ rpm is the only way anyone has any idea of how the car is going to drive without actually driving it. Everyone has those figures, it's a standard. If a manufacturer didn't quote them they would never sell. That's why they quote them.

You can't quote power at 1300rpm because it's not important. Torque is what tells you how quickly the engine overcomes the weight of the car + any other resistance against it's acceleration. The power it makes then makes absolutely no difference to how well it will accelerate

[TiTAN]

The engine makes 100kw at 1500rpm and increases linearly to 225kw by 7000rpm... no more a mouthful than quoting torque and power, not to mention that everyone knows what power feels like.

saying my car has 100kw @ 1500 isnt as impressive as my car has 400nm @ 1500.

end of the day these cars arent being sold to us, they are being sold to people that will rarely ever rev higher than 3000rpm and could care less what the power figure is as long as it has a fancy badge, nice seats, and a comfortable ride.

[Rolls]

You can't quote power at 1300rpm because it's not important. Torque is what tells you how quickly the engine overcomes the weight of the car + any other resistance against it's acceleration. The power it makes then makes absolutely no difference to how well it will accelerate

Of course it is important, if you only make 5kw at 1300rpm then you won't go anywhere. Power implies that torque is being made, if it wasn't then it wouldn't make the power!

My point is torque is far more confusing for the average joe (look at this thread lol) so power gives a more meaningful number to the person

You are talking as if torque and power are two completely different things, they aren't they are directly related, however talking NM is confusing as you need to multiply it by rpm to figure out what the actual power will be at the wheels.

Look at this for example, lets say you are making 100kw down low at 2000rpm, everyone in the world knows that means it will be great for towing and moving heavy cars, if you say it makes 400nm down low well how much power is that, does anyone even know without using a calculator? And just to show how misleading it is, what if you make 800nm at 100rpm, sounds impressive but it translates to bugger all power and it wont actually be good for towing or accelerating.

Just to prove my point further tell me does 400nm at 2000rpm mean a car will accelerate (or tow) better than a car that makes 100kw at 2000rpm? No calculators I want to know if you can actually tell the difference, because if you can't then it is just a useless figure.

saying my car has 100kw @ 1500 isnt as impressive as my car has 400nm @ 1500.

and that just goes to show how dumb the average person is that they are selling cars to, the 400nm doesn't mean anything to the average person, it is just a big number, at least you can translate 100kw to how fast it actually will be.

Edited October 28, 2010 by Rolls

[scotty nm35]

Not only that, I have been on many different dynos and my torque output varies wildly...

[Rolls]

Not only that, I have been on many different dynos and my torque output varies wildly...

that just comes down to them not knowing diff ratio/gear ratio/exact tyre size and deformation etc, if they plugged all the correct numbers into the vehicle they would be able to figure it out better, most the time they dont bother which is why you get readings of 1200nm with 100kw. Also why a lot of places tune in kph not rpm as it means they don't have to plug all those numbers in.

[The Dan]

No not really. Towing has more to do with torque than power.

Torwue and Power ARE two individual things. Yes they have a relationship but RPM works against torque. So the car making 400nm of torque at 2000rpm will accelerate quicker than the one making 400nm of torque at 6000rpm.

At the same time, though, the first car will have power fall over fairly early because RPM will kill it. The second car will continue to pull at the same RATE of acceleration long after the first guy has changed gears.

So which car is more usable on the street? Well both are intended for different purposes. Then we could start talking about VCT. VCT was designed for the sole purpose of widening the torque band over the rev range. This means the car accelerates at a greater rate for a longer period.

Torque is applied in a very different way.

And there is no multiplication required to work out Nm to actual torque. I think you might be referring to N which is short for Newtons of tractive effort.

[Rolls]

No not really. Towing has more to do with torque than power.

Torwue and Power ARE two individual things. Yes they have a relationship but RPM works against torque. So the car making 400nm of torque at 2000rpm will accelerate quicker than the one making 400nm of torque at 6000rpm.

At the same time, though, the first car will have power fall over fairly early because RPM will kill it. The second car will continue to pull at the same RATE of acceleration long after the first guy has changed gears.

400nm @ 2000rpm = 83kw 400nm @ 6000rpm = 250kw, the second engine will absolutely BLITZ the first one in every single way, it simply will not be faster, sure maybe at the low revs but that was not my point and but you had to do the calculation to know this, if they had simply stated the power down low it would be much simpler to work out what is going on.

Power tells you how fast you accelerate, torque does not, as far as 99% of people are concerned you never need to know the torque figure, you just need to know the power spread (which is indicative of the torque spread).

Even if you only make 200nm @ 6000rpm (125kw) it will be faster than the vehicle making 400nm @ 2000rpm, this is why torque figures are confusing and very hard to really get any meaningful value out of them.

The reason you seem to think torque is important is you are used to seeing big torque numbers associated with engines that make lots of power down low, then they fall over up top and vice versa however this is not always the case, and in the examples where it is not the case the assumption that big torque numbers down low will mean a faster car is wrong (see above example). This is why all you need to know is power spread as this tells the entire story. Either way more torque means more power, so if torque is more important then it infers power is just as important, hence why I don't agree with your statement.

Note everything I said above was assuming gearing was the same, yes you can make up for lack of power by changing the gearing, however this results in top speed falling over very quickly, examples are trucks that make bugger all power but seem to accelerate fine, however with their low gearing they will almost never see above 100kph, if you were to apply the low gearing to a similar car making more power it would absolutely blitz the equivalently geared vehicle making less power and more torque. Just look what happens when you put a 4.4 or greater diff ratio in a skyline, not only is it stupidly fast it will also tow anything you want with ease.

And there is no multiplication required to work out Nm to actual torque. I think you might be referring to N which is short for Newtons of tractive effort.

there is a multiplication to calculate power and that is what you need to know to work out acceleration as 100kw will always accelerate the same as 100kw no matter the rpm, 100nm however can result in WILDY different accelerations depending on the revs.

Edited October 28, 2010 by Rolls

[The Dan]

You're talking in complete opposites.

Torque is how quickly it will accelerate...not power

[Josh_TypeX]

So your saying a turbo diesel VW golf that makes 320Nm at 1750RPM will accelerate quicker than a standard GTST making 184kW at 6000RPM? With the golf at 1750RPM and GTST at 6000RPM.

[Rolls]

You're talking in complete opposites.

Torque is how quickly it will accelerate...not power

This just shows a complete misunderstanding of how torque and power work. I will explain it in very simple terms.

Power essentially = how much work you can do in 1 second.

Torque = is the force you are applying at that instant * distance, being as distance is constant just think of it as the instantaneous force.

This is a bit of an inexact analogy, but it works for our purposes.

Think of it this way, I can throw a 1kg object instantaneously, this is torque.

But I can throw 20 of these 1kg objects every second, this is power.

I can move 20kg every second, this is my acceleration.

Now I increase my torque 10x and I can throw a 10kg object now but I can now only throw 1 of them every second, my power has dropped by 20x

now I can only move 10kg every second, whilst my torque has increased 10x my acceleration has dropped by a factor of 2.

Notice how power and acceleration are directly related however torque and acceleration are not?

Another way to look at it is the instantaneous force is the power of the cylinder explosion (this isn't true but it works for the analogy), the amount of power is this force x how many explosions we get every second.

So you can see that even if we drastically drop the power of the explosion if we increase the number of them enough (rpm) then we make far more overall force and hence accelerate faster.

Hence torque is not how fast we accelerate, torque x rpm = power is how fast we accelerate.

So your saying a turbo diesel VW golf that makes 320Nm at 1750RPM will accelerate quicker than a standard GTST making 184kW at 6000RPM? With the golf at 1750RPM and GTST at 6000RPM.

yes and it is quite obviously wrong.

edit: If my great example somehow hasn't convinced you read one of the many explanations on google

http://www.w8ji.com/rotating_mass_acceleration.htm

Horsepower is a function of RPM and Torque, just like watts are volts times amperes. Horsepower is an ability to do work, but doing actual work requires time. Torque is pressure, and since it does not include speed it is not a very useful measure of system power or the ability to accelerate or move weight. Despite what we hear, crankshaft torque is not directly related to moving something off the line or pulling a heavy load. Up at the engine, it is really all about horsepower. The horsepower (torque at a certain RPM) is eventually converted through gears and other mechanical devices to a new torque value at a different RPM. Eventually all we care about is the rotational pressure on the contact patch of our tires that thrusts our car forward. A 800 lb/ft torque at 2000 RPM engine does not accelerate a vehicle as well as a 400 lb/ft engine at 5000 RPM, because horsepower is a product of torque and RPM. The higher RPM engine can be geared to provide more forward pressure at the wheels, in short the higher RPM engine in this example has more horsepower.

Edited October 28, 2010 by Rolls

[Josh_TypeX]

yes and it is quite obviously wrong.

Yeah I know. I was just demonstrating the ridiculousness of his claim with a practical example.

[The Dan]

You all need to read this:

http://www.4x4abc.com/jeep101/torque.html

I don't have time to sit here and explain it all