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

[Rolls]

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

If you can't explain why I am wrong and counter my many many examples then I'm just going to take it as you don't understand it like 99% of people in the world.

edit: I just skim read that article and it is blatantly wrong.

here is where he gets it totally wrong

Fine. Take your non turbo car (turbo lag muddles the results) to its torque peak in first gear, and punch it. Notice the belt in the back? Now take it to the power peak, and punch it. Notice that the belt in the back is a bit weaker? Fine. Can we go on, now? :-)

It is the change in acceleration (acceleration of the acceleration) that you feel as a punch, not torque. Example if an electric motor, it makes the same power at all revs and hence has zero punch, it doesn't feel 'torquey' though.

You can see where he gets confused by providing an completely opposite example of what he said above here

For an extreme example of this, I'll leave carland for a moment, and describe a waterwheel I got to watch awhile ago. This was a pretty massive wheel (built a couple of hundred years ago), rotating lazily on a shaft which was connected to the works inside a flour mill. Working some things out from what the people in the mill said, I was able to determine that the wheel typically generated about 2600(!) foot pounds of torque. I had clocked its speed, and determined that it was rotating at about 12 rpm. If we hooked that wheel to, say, the drivewheels of a car, that car would go from zero to twelve rpm in a flash, and the waterwheel would hardly notice :-).On the other hand, twelve rpm of the drivewheels is around one mph for the average car, and, in order to go faster, we'd need to gear it up. To get to 60 mph would require gearing the wheel up enough so that it would be effectively making a little over 43 foot pounds of torque at the output, which is not only a relatively small amount, it's less than what the average car would need in order to actually get to 60. Applying the conversion formula gives us the facts on this. Twelve times twenty six hundred, over five thousand two hundred fifty two gives us:

6 HP.

Oops. Now we see the rest of the story. While it's clearly true that the water wheel can exert a *bunch* of force, its *power* (ability to do work over time) is severely limited.

Edited October 28, 2010 by Rolls

[The Dan]

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.

No, I'm not saying that.

If the Skyline makes 320Nm at 6000 rpm and the VW makes 320Nm at 1750 then the rate of acceleration for both vehicles will be identical when making that much torque as long as both cars are geared identically and they weigh the same

[Rolls]

No, I'm not saying that.

If the Skyline makes 320Nm at 6000 rpm and the VW makes 320Nm at 1750 then the rate of acceleration for both vehicles will be identical when making that much torque as long as both cars are geared identically and they weigh the same

and that is where you are completely wrong, the car making the greater power will go faster, simple reason being is it is doing more work.

The fact that you did not address any of my examples makes me wonder why I am bothering.

I can exagerate the example further if you want and make the skyline rev to 20,000rpm and continue to make 320nm at that 20,000rpm, it is now making 670hp and you honestly think they will accelerate the same?

Edited October 28, 2010 by Rolls

[zebra]

Rolls - your on paper explanation works, but Torque is the instantaneous force on the drive wheels, the more torque you have acting on those drive wheels the faster they will be rotated. Thus accelerating the vehicle.

Comes down to gearing as well I know.

Think of your throwing bricks example - Say all you can throw is a 1KG brick with the amount of torque your arm produces, now double that torque but keep the brick at 1KG, the brick will fly faster and further.

[The Dan]

and that is where you are completely wrong, the car making the greater power will go faster, simple reason being is it is doing more work.

The fact that you did not address any of my examples makes me wonder why I am bothering.

Seriously pull your head in mate. I am not arguing with you on this because I have work to do. This is the last post I make until I HAVE TIME TO DO SO.

It's not because I can't argue to your so called valid points, I don't have the time to.

[Rolls]

Seriously pull your head in mate. I am not arguing with you on this because I have work to do. This is the last post I make until I HAVE TIME TO DO SO.

It's not because I can't argue to your so called valid points, I don't have the time to.

No need to get aggressive, also how on earth can you expect me to take you seriously when you've essentially gone 'you are wrong, read this one link that is also wrong, but I'm not going to explain why or counter any of your points and explanations'.

Rolls - your on paper explanation works, but Torque is the instantaneous force on the drive wheels, the more torque you have acting on those drive wheels the faster they will be rotated. Thus accelerating the vehicle.

Comes down to gearing as well I know.

Think of your throwing bricks example - Say all you can throw is a 1KG brick with the amount of torque your arm produces, now double that torque but keep the brick at 1KG, the brick will fly faster and further.

You are confused, if you increase the torque you also increase the power, that is why it goes faster, you cant increase the torque and keep the power and rpm the same, it cant be done. My point was torque alone does not explain how fast something accelerates, power can assuming gearing is the same.

See example above why torque doesnt = acceleration and power does. if what you said was true then the 600hp car would accelerate the same as the 150hp one, everyone here knows that isn't the case.

Edited October 28, 2010 by Rolls

[Josh_TypeX]

No, I'm not saying that.

If the Skyline makes 320Nm at 6000 rpm and the VW makes 320Nm at 1750 then the rate of acceleration for both vehicles will be identical when making that much torque as long as both cars are geared identically and they weigh the same

Well actually the Skyline will only be producing 293Nm, however the skyline at 6000RPM will accelerate MUCH faster than the golf at 1750RPM. Any one who has driven these cars will concur.

And the gearing/weight difference is not the cause of this. Place the golf engine in a skyline and repeat the test. Same weight and gearing, yet the 6000RPM 293Nm skyline will still accelerate much faster than the 320Nm 1750 RPM golf engine powered skyline.

[Rolls]

Yeah technically you really need horsepower x time to work out how fast a vehicle will accelerate, but you assume that the power will be roughly constant so just the horsepower figure is all that is needed, knowing torque alone will not tell you though.

There is a reason things like those ET calculators ask for horsepower and not torque.

[simpletool]

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.

Agree.

Both Rolls and PM-R33 are not wrong. However PM-R33 is more correct.

Torque at given rpm is important. People should understand torque simply because they have been exposed to comparative figures from manufacturers for ages.

If you want to say people don't understand torque, then people don't understand power either. Since these two are related by revolutions per minute and gearing, to say people don't understand torque is to say people don't understand power. Indeed as you have stated quite correctly, they are proportionately related.

To illustrate get a passenger in a RX8 (with 177kw), and then in a BA falcon (182kw) and asked them which has more power. Even with a huge weight penalty I bet my middle nut that most will say the falcon. People understand torque MORE than power in most cases.

Although admittedly they are feeling the effects of power factored by gearing to create torque at the wheels.

In any case it's still area under the curve, which is energy. Your gearing is how quickly you use this energy. This then gives the power/torque you feel.

Edited October 28, 2010 by simpletool

[dano4127]

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.

Well yes the golf would accelerate quicker off the line and the be reeled in and left for dead by the skyrice

I raced my 2.0L MKV Golf TDI against my friends 2.5L XR5 3 times last night had him till 80kph each squirt

Golf: 103kW @ 4000rpm | 320Nm @ 1750rpm

XR5: 166kW @ 6000rpm | 320Nm @ 4000rpm

[simpletool]

Yeah technically you really need horsepower x time to work out how fast a vehicle will accelerate, but you assume that the power will be roughly constant so just the horsepower figure is all that is needed, knowing torque alone will not tell you though.

There is a reason things like those ET calculators ask for horsepower and not torque.

Correct, and correct. Area under the curve would give energy for each gear would be the best (and very accurate) way to use those calculators.

[J - Rad]

You nerds have ruined this thread, hope your happy.

[PM-R33]

Some really good (and confusing/contradicting) points are being made by everyone.

The shame about this argument is, that it is a very complex matter and add to the fact, there is more than one way to think about it.

It is quite evident that a lot of us are saying the same thing, however we are thinking about it in a different way and explaining it in different ways.

Personally in my mind, I can picture an engines behaviour a lot better with torque figures and then just a peak kw figure. Some people might not use that method of thought and prefer to use a different method. Is any on us wrong? No not really. It is just we have different ways of thinking about it.

Power is a function of torque and RPM. It is the torque that you are increasing when doing modifications on an engine where gearing is remaining constant. If you have a stock GTST for example and up the boost, you are increasing the torque. The RPM points are remaining the same and therefore the power increase is due to the torque increase.

[Josh_TypeX]

Well yes the golf would accelerate quicker off the line and the be reeled in and left for dead by the skyrice

I raced my 2.0L MKV Golf TDI against my friends 2.5L XR5 3 times last night had him till 80kph each squirt

Golf: 103kW @ 4000rpm | 320Nm @ 1750rpm

XR5: 166kW @ 6000rpm | 320Nm @ 4000rpm

Yes, but I was meaning instantaneously. Not taking into consideration the time required to get to the respective engine speeds.

[jarrod83]

ok i think i know where elite is going wrong, hopefully i can better explain this without offending anyone. People often draw vast conclusions from half vast data, meaning people often assume a car has one gear and often get confused the information that a dyno sheet actually provides, from the posted website this sentence is correct.

"First of all, from a driver's perspective, torque, to use the vernacular, RULES :-). Any given car, in any given gear, will accelerate at a rate that *exactly* matches its torque curve (allowing for increased air and rolling resistance as speeds climb). Another way of saying this is that a car will accelerate hardest at its torque peak in any given gear, and will not accelerate as hard below that peak, or above it. Torque is the only thing that a driver feels, and horsepower is just sort of an esoteric measurement in that context"

yes the maximum torque produced at a engines rpm for that gear will produce the fastest acceleration FOR THAT GEAR, which i stated im pretty sure in my first post. But once you start comparing the rev ranges the car is in as it goes through all its gears the difference in acceleration becomes blatantly obvious, so yes in 4th when ur car has 400Nm of torque at 1500 rpm at a speed of 43 kmph, with your 1:1 drive train ratio you will have 400N.m of torque at the wheels. you will accelerate faster than my car throughout my entire 4th gear rev range assuming my car only has 350Nm of torque at 4900rpm, but while your in 4th at 43... im in 1st at 43.. hello im at 4900 RPM, so my 350Nm of torque through my first gear reduction ratio of 3.592:1 becomes 350x3.592=1257.2Nm, i now have triple the acceleration of your car.. so you decide to drop your car back to first gear and upto 4900RPM where my car has more torque than your car.. i win again. The only time your car will have more acceleration than my car is when we are taking off, and yes my car will never have more acceleration than that early burst you get in first gear.. but after my torque curve passes yours at say 3500RPM my car will have more torque to the wheels at the end of first and the remaining 4 gears.. as each time i change gear my car drops back to 4000+ rpm, so from 30-260kmph my car will have more acceleration. If you think about it logically... besides a gtr who the hell can get there power down in first gear?

I know this point is exeggerated somewhat, but im just trying to display why maximum power decides the maximum "average" acceleration more than the maximum torque, too many people read a dyno and assume there car only has one gear, or dont truely understand the power torque relation, or how it changes once the variable of a gearbox is introduced. If your chasing the biggest push into the back of your seat you can possibly get.. then go after the maximum torque you can get, if your after the fastest accelerating car through the gears then you require the maximum average power between an appropriate rev range.

The only time early torque is a really a factor, is for towing, where your require the early down low force for taking off

Edited October 28, 2010 by jarrod83

[zebra]

Yes, but I was meaning instantaneously. Not taking into consideration the time required to get to the respective engine speeds.

That is the whole point of acceleration though, like I have said earlier, we are limited to 5 or 6 gears, so we will never always be at peak power, having more torque (and therefore power) at low rpm means the engine can accelerate the car to its peak power quicker.

Edit - from a standing start, as I have said before, the car needs the grunt to accelerate the car.

So I'll will go back to my F1 engine point, while it would be very fast if you could keep it at high revs, it would be massivly slow in a 1700kg car tying to get it to those revs, having massively short gearing would help as Jarrod said above, but as soon as the revs dropped out of the power band in a full weight car, the thing would labour like crazy and more than likely stall.

[DCIEVE]

No, I'm not saying that.

If the Skyline makes 320Nm at 6000 rpm and the VW makes 320Nm at 1750 then the rate of acceleration for both vehicles will be identical when making that much torque as long as both cars are geared identically and they weigh the same

I'll illustrate using your example. If you want to compare acceleration then we assume the instantaneous wheel speeds are identical with one producing 320Nm at the crankshaft at 6000rpm and the other making 320Nm at the crankshaft at 1750rpm.

To produce 320Nm at 6000rpm engine 1 is making 201kW

To produce 320Nm at 1750 rpm engine 2 is making 59kW

Lets assume a wheel speed of 1000rpm (this speed is not important).

Engine 1 will produce 1919Nm at the wheels

Engine 2 will produce 561Nm at the wheels.

Given Engine 1 is producing 3.4 times the wheel torque (proportional to the difference in engine power) despite engine torque being identical it's immediately obvious which will accelerate at a greater rate.

Edited October 28, 2010 by DCIEVE

[zebra]

DCIEVE - you make a good point, but now we are talking about gearing (mechanical advantage) rather than the power/torque and engine produces.

take vehicle a an XB Falcon sedan using a 351cube v8 making say 350hp and having a 1:1 top gear, this is good enough to push that car to a low 12sec pass on the strip using 4.11:1 diff gears with a 3 speed auto.

Now change those diff gears to 2.75:1 the car will now run 14s down the strip because the engine cant get up into the revs fast enough in order to use the most power productive rpm range.

now same car that has 250hp and a low peak power rpm but has masses or torque between 1500-3000rpm

now using 4.11:1 diff gears the 250hp car will shoot straight past its peak torque and will not accelerate any where near as quick as the 350hp at high rpm.

Now change those gears back to 2.75:1 and the 250hp car will blow the doors off the 350hp car until the 350hp car can get up in the revs where it will start reeling the 250hp car in.

hope that makes sense and the thought of a 5.8L pushrod dinosaur v8 doesnt send you into a rage lol

[Rolls]

yes the maximum torque produced at a engines rpm for that gear will produce the fastest acceleration FOR THAT GEAR, which i stated im pretty sure in my first post. But once you start comparing the rev ranges the car is in as it goes through all its gears the difference in acceleration becomes blatantly obvious, so yes in 4th when ur car has 400Nm of torque at 1500 rpm at a speed of 43 kmph, with your 1:1 drive train ratio you will have 400N.m of torque at the wheels. you will accelerate faster than my car throughout my entire 4th gear rev range assuming my car only has 350Nm of torque at 4900rpm, but while your in 4th at 43... im in 1st at 43.. hello im at 4900 RPM, so my 350Nm of torque through my first gear reduction ratio of 3.592:1 becomes 350x3.592=1257.2Nm, i now have triple the acceleration of your car.. so you decide to drop your car back to first gear and upto 4900RPM where my car has more torque than your car.. i win again. The only time your car will have more acceleration than my car is when we are taking off, and yes my car will never have more acceleration than that early burst you get in first gear.. but after my torque curve passes yours at say 3500RPM my car will have more torque to the wheels at the end of first and the remaining 4 gears.. as each time i change gear my car drops back to 4000+ rpm, so from 30-260kmph my car will have more acceleration. If you think about it logically... besides a gtr who the hell can get there power down in first gear?

good explanation, makes better sense than what I was trying to say.

Edited October 28, 2010 by Rolls

[Josh_TypeX]

Zebra - DCIEVE's point is that acceleration is not determined by the torque produced at the crank, which is what it seems Elite was trying to say.