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

[jarrod83]

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.

Damnit this happens every time i take so long to type my point that someone has already made it by the time i say it , good point tho.. but you can actually shift a fair but after peak power and still be making more wheel torque.. considering the great change in gear ratios from first -> second -> third, significantly reducing the torque, you can see in my excel charts that everytime my car changes gear at 6750, the torque to the wheels takes a huge swan dive

[Birds]

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.

I hope this answers your original question: enjoy your brick wall of acceleration when you change from 7000rpm and it leaves you at 6000rpm, above the peak torque band. Yes staying in a lower gear can yield you faster acceleration than the next gear because of the total tractive effort at the wheels (sometimes) being higher, but when you hit that next gear you are well away from the torque band and suddenly your advantage is lost.

I have done many timed 0-100 runs, many drag races and shifting at redline or peak power never yields the fastest times. Staying in the torque band does.

[Harey]

+1 Simpletool

I find it hard to read Birds that you suggest to change gear just after peak torque but then say to ignore the next gear?? If there is not next gear what are you changing to?? If you only have one gear then you want to rev it out as high as possible.

You should change at 8,300rpm not 8,000rpm as you would be right at the peak of the torque curve in the next gear.

[simpletool]

I hope this answers your original question: enjoy your brick wall of acceleration when you change from 7000rpm and it leaves you at 6000rpm, above the peak torque band. Yes staying in a lower gear can yield you faster acceleration than the next gear because of the total tractive effort at the wheels (sometimes) being higher, but when you hit that next gear you are well away from the torque band and suddenly your advantage is lost.

I have done many timed 0-100 runs, many drag races and shifting at redline or peak power never yields the fastest times. Staying in the torque band does.

It does answer it ..............in a way. But it gets a D- at best.

The correct answer is: When the total area under the curve is highest. Get each gear, select shift point based on maximising total area for each gear taking note of effect of shift points on area for gear before and after. This would need to be done by iteration.

Edited October 29, 2010 by simpletool

[Birds]

+1 Simpletool

I find it hard to read Birds that you suggest to change gear just after peak torque but then say to ignore the next gear?? If there is not next gear what are you changing to?? If you only have one gear then you want to rev it out as high as possible.

You should change at 8,300rpm not 8,000rpm as you would be right at the peak of the torque curve in the next gear.

That is if 8300rpm will drop you into peak torque for the next gear, then yes. I am not familiar with the B16 gearbox, it is just an example. What these guys are saying is that peak horsepower is where you want to accelerate to, regardless of where the next gear will drop you to in the rev range. My example above about hitting a brick wall in the hypothetical engine shows this is not always the case. And I said before, changing gear was just an example in the graph to demonstrate the importance of being in the torque band. When I said ignore gears, I was saying ignore the next gear when trying to contemplate that peak torque produces the fastest acceleration in a gear.

It does answer it ..............in a way. But it gets a D- at best.

Yeah righto...no need for that.

[Rolls]

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. .

pretty much what you said

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.

The whole point of peak hp means it wont drop off more, you will always want to shift after peak power even if in the next gear you are above peak torque.

I hope this answers your original question: enjoy your brick wall of acceleration when you change from 7000rpm and it leaves you at 6000rpm, above the peak torque band. Yes staying in a lower gear can yield you faster acceleration than the next gear because of the total tractive effort at the wheels (sometimes) being higher, but when you hit that next gear you are well away from the torque band and suddenly your advantage is lost.

I have done many timed 0-100 runs, many drag races and shifting at redline or peak power never yields the fastest times. Staying in the torque band does.

Maybe with your gearing yes, but if you geared your vehicle to have max hp at that 100kph mark you would be faster, the only reason you would have been faster is due to less than optimal gearing, it is not because shifting at peak torque is faster.

enjoy your brick wall of acceleration when you change from 7000rpm and it leaves you at 6000rpm, above the peak torque band.

Ok lets look at an example (that I wrote before) and show that even if you shift and it lands you above peak torque you will still be making more tractive effort 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 at 7920rpm we have 3.96*200 = 792nm.

Notice how in the second example we shift after peak power, end up past peak torque but still make more tractive effort at the wheels? This is because by shifting above peak torque we can take advantage of lower gearing and do the same speed.

Edited October 29, 2010 by Rolls

[Birds]

The correct answer is: When the total area under the curve is highest. Get each gear, select shift point based on maximising total area for each gear taking note of effect of shift points on area for gear before and after. This would need to be done by iteration.

And which curve is that one? The torque curve? I suspected as such...

Cheers!

[Rolls]

And which curve is that one? The torque curve? I suspected as such...

Cheers!

the tractive torque curve, which is based on shifting at peak hp, not peak torque.

[Birds]

The torque curve is based on...the torque. Leave HP out of it. Calculated measurement based on the former. Nothing more.

[simpletool]

No, the power curve. Power....power....POWAHHHHHH.

I have no idea what the area under the torque curve would equal...if anything in terms of physics?

[Birds]

It would represent the area where the engine is producing it's strongest force.

Agree to disagree? This has stretched on way too long, rotary thread is evidence of my ability to argue a point into the ground but I don't want to go there again as this is pretty bad as is.

[jarrod83]

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

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

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

so power has nothing to do with acceleration?

P=wheel Torque x wheel angular velocity

Torque = F/r, angular velocity = rads/sec, wheel velocity = angular velocity x radius of wheel

P=F/r x angular velocity

P=F x wheel velocity

F = ma

P=ma x wheel velocity

wellah rate of acceleration for a car, that has a variable gearbox, so power and rate of acceleration is very closely related to velocity. Please find me the acceleration rate with respect to wheel speed from the torque alone. You can find the acceleration due to torque at the known measured dyno torque, but each torque point is only accurate for the fixed engine rpm and fix wheel speed, although the same can be argued for the power curve with a fixed gearbox but the maximum acceleration using the maximum power and velocity will be higher for a given speed than the maximum acceleration using the torque curve for that speed, therefore a gear ratio exists for that speed.. that a higher wheel torque is possible with the correct gear ratio for the maximum power than any possible peak torque/gear ratio... although cars dont have variable gearboxes they do have multiple gears.

[Birds]

That's all well and good man, but I wasn't talking about calculating acceleration. I was talking about what causes acceleration. And given torque has a nice key moment there in your equation...

[simpletool]

I cannot agree to disagree on an objective point. That is silly. I can agree to say we are arguing 2 different points.

You are arguing what is the fastest accelerating point in a given gear. This we all agree with, it's at maximum torque.

However maximising the area under the torque curve is as useful as redrawing a graph with a longer scale (ie. which would be like changing the final drive ratio). hey presto I accelerate faster! We ALL know this. This is your point. Yes, you are right.

But if you had the same torque at higher revs, then you could have an even shorter drive ratio, and hey presto.....even faster!! This is basic.

When it comes to the OP question of why are people hung up on power rather than torque. Well that has also been answered. More power is better, if it doesn't ruin flexibility. Having the same torque but spreading over a larger rev range is better. This isn't about torque, it's actually about power.

My power drops off after 6000rpm (mostly because the tuner was too lazy to advance the timing above MBT). If I had of increased the timing to make a bit more high-end torque and got peak power at 6400rpm then I would have got more power. But it wouldn't be any faster below 6000rpm. It would just be a little bit faster between 6000rpm and 6400rpm. HOWEVER, I could reduce my gear ratio a bit, then I have more effective torque at the wheels but my engine torque has not changed and neither has my peak speed in each gear.

I'm talking in circles here. I can't see how you could possibly disagree with what I am saying.

OK, you win. I agree to disagree. Except I don't know what I am disagreeing with but I am tired and the donut deal below has got me. A war of attrition.

[Rolls]

I'm talking in circles here. I can't see how you could possibly disagree with what I am saying.

OK, you win. I agree to disagree. Except I don't know what I am disagreeing with but I am tired and the donut deal below has got me. A war of attrition.

lol well at least I agree with you

The torque curve is based on...the torque. Leave HP out of it. Calculated measurement based on the former. Nothing more.

but you can decrease the gearing due to more hp and hence make more tractive torque by using revs past peak torque, hence the area under the curve is greater. You obviously has not read my example I showed multiple times in here.

Edited October 29, 2010 by Rolls

[Birds]

And I can't see how you could disagree with what I'm saying, that's just it. I think you are right though, in that we are arguing different points and somehow I got pulled into this debate in the first place when all I sought out to do was settle it

There's no winning here, the point of agreeing to disagree is that we can't settle something here and it's close to the weekend where alcohol is needed to forget about discussions like these

[Rolls]

Just read my bloody example lol, it clearly shows how by revving further (to peak power) and using lower gearing produces more tractive torque and yet you go exactly the same speed so it is a fair comparison.

[jarrod83]

yes but torque is meaningless without rpm.. to make the same wheel torque at 100 times the wheel speed is impossible... its simply not the way the world works.. combustion engines make it alot harder to understand the power/torque relationship, if you consider an electric engine with a fixed power applying work to an input shaft. They apply a fixed power, therefore a fix amount of work to the input shaft, yes the motor will be working also at a fixed torque and rpm. But the amount of work it can do is determined by the power the motor is making. The motor then runs through a gearbox, the amount of torque it can supply to the output shaft is directly related to the gear ratio and speed of the output shaft. The faster the output shaft has to rotate, the less torque it can apply to the shaft due to the increased velocity ratio. Although the motor is now outputting less torque.. it is still producing the same output power and work neglecting energy losses.

P=TxRPM

from the peak power you can output any torque and any rpm.. but not without proportionally effecting the other, this is simply because the electric engine can only produce so much work, even if you had multiple gears as soon as you gear up to increase the wheel speed you are doing so by sacrificing torque.

Combustion engines are exactly the same except there power is a variable with engine rpm.. however the total amount of work the car can do that goes directly towards accelerating the car occurs while the power is the highest, at this power range you can output any torque/rpm.. in the case of a 5 speed gearbox you have fixed gear ratios... whatever way you look at it its easy to see that the torque output to the wheels is directly related to rpm of the wheels, the amount of torque for the given wheel rpm to determined by the amount of power the engine makes at the current engine rpm. Its also very easy to see since Work = Power x time, that more work is applied to the car while the power is at its highest... its also the reason why the highest average power will give you the fastest average acceleration.

Comparing torque at a fix gear ratio over a range of rpms is meaningless without taking into consideration the amount of work that torque is doing to the car.. ie the power at that point.

[bigmikespec]

Agree to disagree? This has stretched on way too long, rotary thread is evidence of my ability to argue a point... until everyone gets sick of reading my long winded posts and lets me have my bottle!

Fixed!

[jarrod83]

and i forgot to mention that obviously the torque for the given power causes acceleration.. but as i clearly pointed out, what determines the torque? the power. dynos may calculate their power from torque.. but power is still the measure the amount of work being done.. the work is what adds to the kinetic energy causing an increase in speed and overcomes the forces of drag and friction. Therefore to compare the peak torque a engine produces at 2000 rpm in 4th at 60 kmph to the lower torque it produces a 5000 rpm at 142 kmph, without taking into consideration the work the engine is actually producing and the increase of wheel speed. It kind of similar to how everyone compares the speed/cost of a bike and a car without taking into consideration how much more a car weighs. If you dropped the wheel speed of the engine at 5000 rpm down to 60kmph instead of 142kmph then it will have a proportional increase in torque, so for a given amount of work a lot more torque would be produced. Similarly, if you dropped the weight of the car to the bikes weight, for a given amount of work, it would produce a lot more acceleration.