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

[Birds]

lol it's not. You have it the other way around. Power is the descriptive measurement calculated from torque and speed, torque is the name given to the forces acting. Engine speed is relevant to how fast the wheels are turning at a given moment, not their tractive force. It will help you calculate how fast a vehicle is moving, not how fast it accelerates. This *might* help to demonstrate how it works:

[simpletool]

No, I'm not wrong. We are talking about engine power and torque which is then factored by gearing. Not power or torque at the wheels (ie. not gearing effects, no spread between ratios)

Also the statements on that graph are almost all incorrect. Statements like "power continues to be made but not at the same rate" is incorrect. Power is made at a GREATER rate, just not increasing at the same rate. Shift points comment is incorrect. Changing at 8000rpm would be SLOWER than changing at 8300rpm as you are going into the next gear at a lower RPM. Whoever wrote that doesn't actually know what they are talking about. Sorry.

I will say the comment that the engine will accelerate quickest at peak torque is correct, but since we are talking about engines and not the wheels this is irrelevant for my previous point about CVT transmissions. These keep the engine at the same point and wheels accelerate, therefore changing the gearing ratio as they go.

anyway this guy has already debunked all the above myths on that graph - see here:

http://www.fjr1300.info/misc/torque-power.html

[simpletool]

P.S. This is getting good.

[zebra]

Just as a note,

Peak is generally made at the engines best volumetric efficiency (or very near too) hence modern turbo cars having a dead flat torque line

[Rolls]

Shift points comment is incorrect. Changing at 8000rpm would be SLOWER than changing at 8300rpm as you are going into the next gear at a lower RPM. Whoever wrote that doesn't actually know what they are talking about. Sorry.

Agreed.

Going by that graph lets say we want to be doing 60kph which has been magically worked out to be a wheel speed of 2000rpm, we want as much tractive torque at the wheels at this speed.

If we gear for peak torque at 7500rpm that requires a reduction of 3.75 so 3.75 * 210nm = 787nm of tractive torque at the wheels, if we gear for peak power which is at 8250rpm we require a reduction of 4.125 so 4.125* 200nm = 825nm of tractive torque at the wheels, hence the car accelerates faster if we gear for peak power, not peak torque.

However in both scenarios the car will still accelerate at its peak at 7500rpm (peak torque) but in the second example we wont be quite at the speed we want to be at, so we are trading off speed for extra tractive torque at the wheels.

Now if we have gear ratios of say 3.75:1 for first and 3.6:1 for second (same ratio gap) lets say we shift at peak torque of 7500rpm, we are making 3.75 * 210nm =787nm at the wheels, now when we shift into 2nd we drop to 7200rpm, now we are making 210nm * 3.6:1 = 756nm of tractive torque.

Now if we geared for maximum hp keeping the speeds the same we use 4.125:1 and 3.96:1 for second (very very close ratios)

Now if we shifted at peak hp after the torque has rolled off slightly at 8250rpm we have 4.125*200nm = 825nm and when we shift into second we have 3.96*200 = 792nm.

Now both cars are going the same speed as we have geared them to match speeds, one is just shifting at peak torque one is shifting at peak hp, the one geared to shift for peak hp goes faster, now you could go why dont we gear the other vehicle the same so it makes more torque, the problem here is it would be going at a slower speed as the trade off.

So by gearing for peak hp and shifting as late as possible we can maximise torque at the wheels, basically the longer you can stay in the lower gear the better, even if you shift well after peak power you are still making more tractive effort in first than second, hence why everyone shifts after peak power when racing and not before it.

edit: Good link.

Edited October 29, 2010 by Rolls

[Birds]

No, I'm not wrong. We are talking about engine power and torque which is then factored by gearing. Not power or torque at the wheels (ie. not gearing effects, no spread between ratios)

Also the statements on that graph are almost all incorrect. Statements like "power continues to be made but not at the same rate" is incorrect. Power is made at a GREATER rate, just not increasing at the same rate. Shift points comment is incorrect. Changing at 8000rpm would be SLOWER than changing at 8300rpm as you are going into the next gear at a lower RPM. Whoever wrote that doesn't actually know what they are talking about. Sorry.

I will say the comment that the engine will accelerate quickest at peak torque is correct, but since we are talking about engines and not the wheels this is irrelevant for my previous point about CVT transmissions. These keep the engine at the same point and wheels accelerate, therefore changing the gearing ratio as they go.

anyway this guy has already debunked all the above myths on that graph - see here:

http://www.fjr1300.info/misc/torque-power.html

Yes, you are wrong. Here's an hypothetical for you...an engine makes 100kw at 3000rpm, 200kw at 5000rpm and peak power of 205kw at 7000rpm...do you really think between 5000 and 7000rpm it is going to be accelerating any faster than between 3000 and 5000? Between 3000 and 5000 is where the major torque band is, hence the major increase in power and the most acceleration. After 5000 it is dropping off = hit a massive brick wall in acceleration, and it will actually accelerate slower than when the engine was between 3000 and 5000. Motorbikes are very good at demonstrating this due to their high revving nature as both peak torque, peak power, and redline are well spaced apart. My kingdom for a physics professor to come in here and explain this!

And the statement in bold, is what I was saying in the graph...you just read it semantically different, I was afraid this would happen.

[Rolls]

Yes, you are wrong. Here's an hypothetical for you...an engine makes 100kw at 3000rpm, 200kw at 5000rpm and peak power of 205kw at 7000rpm...do you really think between 5000 and 7000rpm it is going to be accelerating any faster than between 3000 and 5000? Between 3000 and 5000 is where the major torque band is, hence the major increase in power and the most acceleration

Yes you are right but you will be going SLOWER as in actual speed of the vehicle during this range, you are trading off speed for tractive effort at the wheels, if you geared the car to do the same speed but shifted at peak hp it would get there faster, but peak actual acceleration would be at peak torque still.

So yes your peak actual acceleration is at peak torque, but to get to a certain speed as fast as possible you will get there quickest by gearing to rev to peak power.

Edited October 29, 2010 by Rolls

[simpletool]

Torque is a force. Power is how the force acts over time. Faster revs = less time for the force to act.

So you have 10000Nm, but it's acts over 20 minutes (ie. 0.05 rpm). Any good? No, it's not. It's slow, it's not powerful. How this acts OVER TIME is dependent upon gearing (amongst flywheel mass, etc). We all care about time, trust me, we care a lot. It's just that time marches on regardless some people do not care so much. After all it'll keep going on and on and on......well actually we could travel faster close to the speed of light and slow it down, or we could get deeply involved in complex thought experiments and time seems to speed up (dammit, always when we are having fun...sheeshh I'm such a geek). Anyway the point is that torque is simply a description that can be used to describe engine characteristics. As I said before people do understand it, but as an absolute number it means much less than power. Sure I disagreed a little before with Rolls about power vs torque understanding, but I was just playing Devil's advocate and the subject matter was a little subjective (people's apparent understanding of the implications of torque, etc etc). But now I disagree because that graph is flat out wrong (objectively).

Again, lets look at yet ANOTHER scenario. Not punching....I've too tired after my workout of talking about punching...phew. Let's talk about the KERS system used by Porsche in the GT3 R. It uses a mass that is spinning at a very high rpm with very little torque. But it has a lot of power (albeit delivered electrically). But it is stored mechanically in a mass spinning with low torque at high rpm. Do they know that torque is more important than power? No they bloody don't because torque doesn't matter. The electrical circuit has been designed (like the gearing of a car) to take advantage of the high speed, low torque arrangement. Would it store more energy with lower speed and higher torque.....again....no it won't. Does the higher speed mean a lower percentage of frictional loss.....yeah....probably. If it had the same torque but at higher speed would it make more power...yes, yes, yes that is exactly how the system works! Is this relevant to the point above.....well...barely.

[Birds]

Here you go boys, go do some reading on torque and power, seriously, it will help you understand what is at work and what is simply a measurement calculated from the other:

http://en.wikipedia.org/wiki/Torque

is the tendency of a force to rotate an object about an axis

http://en.wikipedia.org/wiki/Acceleration

the acceleration of the body is proportional to the net force acting on it (Newton's second law)

It has NOTHING to do with power, power is a measurement derived from torque and engine speed. That's it.

[zebra]

That is just mechanical advantage, same with any electric motor.

Sure you could have a motor spining at 100000 rpm and making 2hp and have it geared to it does 80km/h in top gear at 100,000rpm.

It would go yes, most effective way of doing it no.

Oh and no-one uses KERS anymore because it was shit.

[Rolls]

Good quote from that article

The vehicle will have greatest acceleration at a given wheel speed when the gear selected results in the greatest rear wheel torque. If a gear is selected that puts the engine speed somewhat higher than the engine speed at which the engine torque peak occurs, that numerically lower gear will result in greater torque multiplication and the rear wheel torque will be greater even though the engine torque will be somewhat less than its maximum value.

[Birds]

So yes your peak actual acceleration is at peak torque

That's all I'm saying man, that a car is going to accelerate it's fastest at around peak torque.

[Rolls]

Here you go boys, go do some reading on torque and power, seriously, it will help you understand what is at work and what is simply a measurement calculated from the other:

Oh come on we have clearly done butt loads of reading like anyone else still in this discussion, no need to be patronising.

We agree with what you said that peak acceleration occurs at peak torque, but you still want to shift after peak power and gear to shift at peak power as it will be faster, see my examples why.

That's all I'm saying man, that a car is going to accelerate it's fastest at around peak torque.

yes but if you want to get to a certain speed as fast as possible (everyone does who is racing) you will get there faster by gearing to rev past peak power, not peak torque, this is where we are misunderstanding each other.

Edited October 29, 2010 by Rolls

[simpletool]

Yes, you are wrong. Here's an hypothetical for you...an engine makes 100kw at 3000rpm, 200kw at 5000rpm and peak power of 205kw at 7000rpm...do you really think between 5000 and 7000rpm it is going to be accelerating any faster than between 3000 and 5000? Between 3000 and 5000 is where the major torque band is, hence the major increase in power and the most acceleration. After 5000 it is dropping off = hit a massive brick wall in acceleration, and it will actually accelerate slower than when the engine was between 3000 and 5000. Motorbikes are very good at demonstrating this due to their high revving nature as both peak torque, peak power, and redline are well spaced apart. My kingdom for a physics professor to come in here and explain this!

A true hypothetical and yes it will accelerate faster at this point. But why aren't you a gear lower?

[Birds]

Oh come on we have clearly done butt loads of reading like anyone else still in this discussion, no need to be patronising.

We agree with what you said that peak acceleration occurs at peak torque, but you still want to shift after peak power and gear to shift at peak power as it will be faster, see my examples why.

yes but if you want to get to a certain speed as fast as possible (everyone does who is racing) you will get there faster by gearing to rev past peak power, not peak torque, this is where we are misunderstanding each other.

I don't mean to be patronising but I don't think some people have read the concepts in depth enough to grasp them, it doesn't matter how much reading you've done if you're not understanding how it works.

Really, can we argue with the statements that I quoted...right there is the very definition of torque...and right there is the basis for Newton's second law regarding acceleration...please argue with them. The reason you lot are getting so confused is because torque is so closely related to power.

[Rolls]

Really, can we argue with the statements that I quoted...right there is the very definition of torque...and right there is the basis for Newton's second law regarding acceleration...please argue with them.

Obviously no one disagrees with them? We gave different examples of how a car will accelerate fastest to a speed if geared for peak power and not peak torque due to being able to use a lower gear, hence more torque multiplication, peak acceleration will still be at the peak torque figure, but total time taken to get to the speed will be less due to being able to use a lower gear ratio as you are revving to peak power.

Do you get what we are describing and why?

[Birds]

Yes I am now aware of what you were trying to say, but I was talking about acceleration in a given gear, and the points throughout the rev range where it will accelerate fastest for that particular gear - it is around the area of peak torque, that's all I'm saying. Agreed? No need to bring the next gear into it, I only stated a gear shift on the graph as an example of why it is important to be in the peak torque band in the next gear...for if you end up above the peak torque band in the next gear then you technically aren't accelerating as fast as you could be, even though the differences aren't huge.

[jarrod83]

i think there probably is no correct answer for this one.. as elite said, its very dependent on the gearing, the peak torque will give you the maximum acceleration for that rpm for any gear ratio, the tricky thing with power is if you looked at the two cars acceleration over a range of speeds the car with the most power would have more acceleration at some speeds, theoretically the highest average power should give you the highest average acceleration even though it may have less peak torque.

If you were to compare the torque/power curves of two cars... the car with the most acceleration at a particular speed would constantly change over the course of a quarter mile. If you look at the excel pictures i drew up it shows that although the car with more power doesnt accelerate as hard in a particular gear.. it can use the lowered ratio gear more effectively for longer before it needs to change into the next gear, which is where the power vs speed part of considering a cars acceleration comes into play.

The torque curve is still an important aspect, and it would depend on your definition of a better/faster car on whether peak torque or peak/average power is faster, im sure we can all agree that early torque is more important for acceleration/response, whereas late torque is more important for top speed etc.. if you were after the fastest accelerating car say from 3500 to 7000 rpm, comparing to identical gearing setups, the car with the maximum average power through this rev range will have the fastest average accelerating car through this rev range regardless of peak torques, average torque curves or peak power, which is why i say as useful as torque is, average power is more useful..you dont need rpm references for average power quotes. Also any car with a descent peak power, aslong as its in a wide enough power range to keep a descent average power through the gears, it wouldnt matter where in the rpm range it lied the car would still be fast, whether it is 50, 1000, 7000, 200000 rpm. Can you say the same for torque? I would say that people that read torque curves interept them with respect to the rpm, they probably look for good midrange torque that hopefully stays nice and flat to the top end like PMR-33 was mentioning, that torque curve would sitll give you a very high average power. But the peak power figure would give you a better idea of the average power than the peak torque without a rpm reference.

At the end of the day you can argue all you want with facts and opinions, all you really need are the laws of physics to back up your claims. If anyone can find me two torque/power curves of skylines one with more average power from 4000-7000 and one with more peak torque down early, ill be happy to determine the acceleration versus time and speed, and distance versus time, i can even include the drag forces if you want with an appropriate drag force coefficient for a sports car

[simpletool]

I'm not confused at all. Please fine a single thing I had said that is not true. Feel free to use at much maths as you wish.

I'm still waiting for an answer to an extension of your hypothetical..... "why aren't you in a gear lower?"

That questions is EXACTLY my point. You keep talking about absolute torque or power according to physics without ever looking at the real world, which includes MULTIPLE gears. If I wasn't at work literally trying to finished off selling scientific equipment to university professors I'd draw some graphs to prove you wrong mathematically with area throughout the gear range. You could even cut them out and weigh them...would that be proof enough?? Anyway.....back to teh question. Please be so polite as to answer it and give reason.

So...Here's an hypothetical for you...an engine makes 100kw at 3000rpm, 200kw at 5000rpm and peak power of 205kw at 7000rpm...do you really think between 5000 and 7000rpm it is going to be accelerating any faster than between 3000 and 5000? Between 3000 and 5000 is where the major torque band is, hence the major increase in power and the most acceleration. After 5000 it is dropping off = hit a massive brick wall in acceleration, and it will actually accelerate slower than when the engine was between 3000 and 5000. Motorbikes are very good at demonstrating this due to their high revving nature as both peak torque, peak power, and redline are well spaced apart.

Part A.

Now...remember we are on a motorbike. You have 6 gears to choose from.......you can rev to 7000rpm, but you are running between 3000rpm and 5000rpm due to maximum torque being here.............why aren't you in a lower gear?

Part B.

Assume you are in 1st gear (so you cannot change to another gear)......where should you change gear.....bearing in mind that by changing up a gear you both drop into a lower point of the rev rang and de-multiply your torque (at the wheels)

All this aside from the fact that this motorbike has some seriously unrealistic breathing issues up high.

Edited October 29, 2010 by simpletool

[zebra]

Every Drag racer I have spoken too, all say the same thing, that on the up-shift you want the revs to fall back to peak torque, which is why a lot of us are saying that knowing the torque curve of an engine is more important.

and we all know that drag racing is all about acceleration,

I know you guys are saying that the mechanical advantage increases torque at the wheels, but depending on how quick it drops off may mean that peak engine torque still gives peak tractive effort at the wheels.

I really wish i had more time to be able to get right in depth like a lot of you are.