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

[The Dan]

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.

No you are wrong there.

The rate at which the power increases is at a lower amount. Yes it makes greater power, but the HP/sec generated is less.

Look at the angle on the graph. Power climbs at a steeper angle when torque is greatest. Then when torque falls off, the power is still increasing but because there is less torque to overcome the resistance against it (rpm is a resistance to torque), the angle at which power is increasing becomes less.

In other posts I see comments like power being work over time but that is also completely wrong. The very definition of torque is work over time.

Power is then calculated from that torque.

[The Dan]

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.

Power DOES NOT determine torque. It is the other way around.

I'm going to leave this from here on. We are just going to keep going in circles. I'm totally with Birds on this one, he's got it completely right 100%

The rest of you are correct with your gearing etc. they use it to overcome the lack of torque in different rev ranges.

A diesel engine still one Lemans 24 hr last time and broke the records.....used less fuel than the rest and never revved over about 5000rpm from memory.

Fastest acceleration from lowest + widest torque range and it pwned all these other cars that can rev to infinity AND make much more hp

[The Dan]

One more thing

Drag cars - go and talk to any of the top door slammers and ask them when they change gears. Peak power or peak torque? You can even hear that they aren't peak revving them AFTER they get them off the line. It almost sounds like they shift early. Because the greatest RATE of acceleration is peak torque, therefore they change before it falls off so they remain in their maximum acceleration mark.

[bigmikespec]

In other posts I see comments like power being work over time but that is also completely wrong. The very definition of torque is work over time.

Did you mean to say the definition of Work is the Force over a Distance?

[The Dan]

Did you mean to say the definition of Work is the Force over a Distance?

Yeah sorry that statement is completely wrong. I read his wrong and wrote mine wrong.

I'm not going to say the T word any more. It's now a bad word

[bigmikespec]

Yeah sorry that statement is completely wrong. I read his wrong and wrote mine wrong.

I'm not going to say the T word any more. It's now a bad word

Lol.. all good. I must have had a good sleep last night, amazed I even picked it up this time on a Friday

[PM-R33]

My head hurts

[Rolls]

One more thing

Drag cars - go and talk to any of the top door slammers and ask them when they change gears. Peak power or peak torque? You can even hear that they aren't peak revving them AFTER they get them off the line. It almost sounds like they shift early. Because the greatest RATE of acceleration is peak torque, therefore they change before it falls off so they remain in their maximum acceleration mark.

lower the diff ratio and shift higher and they would make more tractive torque and hence go faster, but I imagine there are much more complex reasons why they shift early, it certainly isn't because it is faster, see my example for evidence of why.

Power DOES NOT determine torque. It is the other way around.

I'm not really fussed which way people calculate it but it can go either way, you can calculate torque and work out horsepower and vice versa, they are both mathematical constructs and either is correct. Also on dynos you don't have to measure torque and work out horsepower, you can do the opposite quite easily and some actually do physically measure both and compare the difference, I can explain how if you'd like.

Edited October 29, 2010 by Rolls

[The Dan]

I can explain how if you'd like.

Yes please. As I understand it, dyno's can only measure torque and then calculate power from that

[r33_racer]

We had Victor Bray at our workshop a couple of weeks ago...he said in their doorslammers, they are making around the 3800hp mark. He reckons its way more then they can actually use and put down to the ground. In their case it would probably have more to do with keeping traction, atleast to some degree.

Edited October 29, 2010 by r33_racer

[FineLine]

We had Victor Bray at our workshop a couple of weeks ago...he said in their doorslammers, they are making around the 3800hp mark. He reckons its way more then they can actually use and put down to the ground. In their case it would probably have more to do with keeping traction, atleast to some degree.

Does it chirp 2nd.

[r33_racer]

70 pounds of boost out of a supercharger the size of our RB's....I think it manages lol

[jarrod83]

when i said power causes torque i meant the power at the engine, will determine the torque and the angular velocity at the wheels, so when comparing an acceleration at a speed you would compare the possible gears you could be in for that speed. Im pretty sure you are right about the torque causing power because if you applied a torque to a bolt and it didnt move, then youve applied a torque.. but no work or power has been done, power wouldn't exist without the torque. Though you are wrong about the power and work relationship, as work can be calculated quite a few ways.

Power 1W= 1J/s=work/sec

or Power x sec = Work

Watt=work/sec=1kg.m^2/s^3

work =1J=1kg.m^2/s^2

Work =power(watt)*time(time)=1kg.m^2/s^3*s= 1kg.m^2/s^2

work=1J=1N.m=torque... work = torque.. rate of work = power..

1N=force=Mass x acceleration 1kg.m/s^2

work=force x distance=Fd

Work=Fd=m*a*d=maV/t=FV/t etc etc

there are alot of variables in determining whats best at the end of the day, maybe they dont like to rev there engines that much? maybe the gearbox loses with the extra torque and revvs add a considerable amount to the drivetrain loses, extra torque places extra tangential forces on the gears, especially considering the lower gears have a higher reduction ratio, requiring a smaller pinion to fit in the same sized box.. F=T/r=more force+ more friction due to higher revs.. and it doesnt pay to rev the engine out as long as you would if you were assuming there are no drive train loses.

[Rolls]

Yes please. As I understand it, dyno's can only measure torque and then calculate power from that

This guy explains it much better than me so I'll quote him

Okay, at last we're ready to talk about the metrics of torque and power, and about how dynamometers work. Contrary to what the urban myth says, there is no fundamental physical reason by which it is impossible to measure work or power directly on a dynamometer, i.e., without deriving the power through the torque measurement. Furthermore, even if it were true that the only way to measure power is by derivation through the measured torque, that fact would have no bearing on the significance of power.

In fact, measuring power on an inertial dynamometer is no more difficult than measuring torque, and is not predicated on the measurement of torque. Whenever the work performed goes to increase the kinetic energy of an object, power is the instantaneous rate of change of the kinetic energy. It is a trivial matter to calculate the kinetic energy of a moving drum if its rotational speed and moment of inertia are known. The rate of change of the kinetic energy can be determined in a manner essentially identical to the traditional method used to determine the angular acceleration of the drum, which is needed in order to calculate the torque.

In an inertial dynamometer, the engine is allowed to accelerate an inertial drum as quickly as it is able. Newer inertial dynamometers use an inertial accelerometer to give the measure of the angular acceleration continuously. With the traditional method, however, each time the drum rotates through a fixed number of degrees, an electrical pulse is generated which triggers the recording of the elapsed time. >From those data points, the average angular velocity can be computed for each of the individual time intervals between adjacent points, by dividing the fixed angular distance by each of the time intervals. Then an average acceleration is calculated for each adjacent pair of average angular velocity values, by dividing the difference in the adjacent average angular velocity values by the corresponding time difference. For each average acceleration value, the average torque over the corresponding time interval is then found by multiplying the average acceleration by the moment of inertia of the drum.

On an inertial dynamometer, the average rate of change of kinetic energy for each time interval can be determined using a method that is essentially identical to the traditional method used to determine the acceleration. The average kinetic energy for a given time interval can be calculated from the average angular velocity over the interval, using the formula: K.E. = ½M x V². (It is necessary to factor in a standard correction to account for the fact that the square of an average is not the same as the average of the squares.) If the difference between the average kinetic energies for two adjacent time intervals is divided by the corresponding time difference, the result will be the average power for that time interval. I want to emphasize that this is not in any way an impractical, far-fetched approach. It is for all intents and purposes identical to the ubiquitous, traditional method used to determine the acceleration and the torque on an inertial dynamometer.

It is also possible to measure power independently of torque on an inertial dynamometer that is equipped with an accelerometer. A computational technique known as numerical integration can be used to derive the angular velocity at many closely spaced points, from the accelerometer readings. The kinetic energy can then be calculated at each of those points. The average rate of change of the kinetic energy between each of those points can then be calculated the same as with a traditional inertial dynamometer, by dividing adjacent pairs of kinetic energy values by the corresponding time differences.

But even without that it is quite easy to see how both can be calculated, if you know how much something weighs and how fast it is spinning and how long it took to spin that fast, it is a fairly simple equation to work out work divided by the time and hence power.

Edited October 29, 2010 by Rolls

[jarrod83]

maybe you's are thinking about it the wrong way?? we have clearly proven that if you gear down the maximum power to match the speed of the maximum torque, that the maximum power will have more torque?? correct?.

If you look at the torque that occurs at the maximum torque though, no matter how you gear it, you can not gear it so that the maximum torque will have more torque than the torque of the maximum power at the maximum powers speed!?

simple enough.. so it stands to reason that at the speed or rpm's that the maximum power occurs.. that the car will be geared to accelerate at its maximum FOR THAT SPEED. so the more time you can get your car to stay accelerating around the maximum power, or through the maximum average power range, the highest average acceleration it will have. No matter what you do, youll never be able to out accelerate the maximum power with maximum torque through the maximum powers power range.

the same can be said for all 5 gears that when your passing through the rev range of the maximum power, your car will be accelerating at the maximum possible rate FOR THAT SPEED.. for all 5 gears

now it can be clearly seen that power is very important at accelerating a car verse its speed.. if your car can have the maximum average power through the range of speeds you spend the most time in, then your car will be accelerating at the maximum average rate it can be.. the only gear where maximum torque plays a significant role over the maximum average power is the first gear.

from this its simple enough to see that yes, your car will accelerate faster where you have your peak torque for that gear... but your car will accelerate on average, faster when the rev range it spends the most time in has the maximum average power... this is why cars that have nice big fat power curves are generally alot quicker than cars that have narrow power bands or lots of torque early and not enough in the top end resulting in bad average power or lots of torque too late giving a very narrow power band and not very much usable average power. You can tune for peak torque or maximum average torque, but without looking at the power curve or estimating average power of the torque curve you have no way of knowing how well it will perform to looking at the area under the power curve.

now if you had a average car and tuned it two ways... first way sacrificing maximum and average power for a higher peak torque.. then sacrificing the peak torque for a higher average power and compared the accelerations to their top speed or even the first 4 gears. If you compared the acceleration versus time, you would see that the maximum torque car would have higher acceleration peaks but spends considerably more time with a lower acceleration, which verse time isnt very effecient. If you looked at the maximum average power car, it wouldnt have the same peaks, but it would spend alot more of the time with a higher acceleration especially in the higher revs where the car spends alot more of the time, its acceleration curve would be alot more effecient with alot more area under the curve.

If you add to the fact that when your trying to get the maximum power to the ground.. its better to have less peak torque and more average power so you dont spin the tyres as easy

now honestly i cant make it any easier than that... if you dont understand it, then you simply dont understand physics.. if you really want to argue though.. prove the car can out accelerate the speed it has at its peak power at with its peak torque.. or better yet.. work = power x time.. if the car is doing more work at the higher power range, work goes towards kinetic energy, over coming friction and over coming drag.. which both cars have to do.. where is the extra work that the higher powered car is producing going towards.. if its not going towards increasing its kinetic energy? which causes it to accelerate faster than the lower powered car. Plus once you understand power as a rate of work, youll understand that just quoting torque(work) is useless, it would be like me saying look i need you to lift these 40 boxes onto that shelf.. which is the amount of work... your thinking, yeh i can do that.. then i say do it in 1 second.. which is the rate of work, 40boxes/sec.. can you still do it? In the same manner quoting work(torque) without the time its done in or the rpm is meaningless, once you add either time or the rpm..then you have the rate of work.. i.e power!!!! Last of all, if you actually sat down with the two torque curves, one with higher torque, one with more average power... you have the torque for every rpm range in 4th gear, you can then work out the torque for the other 4 gears, with the diff ratio you can determine the torque at the wheels, from the torque at the wheels you can compare wheres the best place to change the gear for both examples. Once you have narrowed down when you are changing the gears you have the torque versus speed from 0 to 260 or so, from there you can calculate the force the tyres are applying on the ground based on the tyre size. With the mass of the car you can determine the accleration for each force versus speed. you can then use numerical intergration to determine the required timed to get between each speed with the accelerations between the two points. you now sum the times and have the time taken to get to each speed... determine how far the car goes with the time between each point.. you now have torque vs speed vs distance vs time vs acceleration...

Edited October 29, 2010 by jarrod83

[Rolls]

and we do a full circle and come back to my original point that power is what is important and torque can basically be disregarded...

lol

good post btw, it makes perfect sense to me and basically mirrors what Im thinking in my head, dunno how everyone else is going, I spent the last two days thinking about this bs in every spare second (no exageration) to try and understand it properly, the joys of being a geek you can't leave anything until you understand it 100%.

the simplest example for everyone that is still confused is the CVT example, one with lots of torque, one with less torque but more hp, if geared to always be at peak hp the car with more hp will ALWAYS accelerate faster to any given speed. The reason being is the one with more hp means it makes torque at higher rpm hence it can use lower gear ratios at equivalent speeds of the other engine, lower ratios mean more multiplication and more tractive torque at the wheels hence faster.

I might still do the graph in excel to prove this tomorrow if I can be assed.

Edited October 29, 2010 by Rolls

[STATUS]

this is where its @

engine dynod first then into the shop on the dynapack.... no bullshit "roller derived tractive effort" figures... just look at the torque (cold hard real torque).... i can tell you now nothing compares to driving something with this much horsepower and torque... and this is a daily driver, that returns 10-14l/100km

[PM-R33]

XR6 Turbo or a blown LS2?

Edited October 31, 2010 by PM-R33

[tricstar]

if thats in 4th gear whats up with the speed ? no where near 200km at 6000rpm in 4th?

yeah ok thought about that, could be low ratio diffs etc ??

Edited October 31, 2010 by tricstar

[Rolls]

lol 300hp at 2000rpm, that is absolutely ridiculous, specs on motor?

supercharged v8?

Edited October 31, 2010 by Rolls