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

[fatz]

mother of god

lap time-explains why you are a shit driver.......... unless you drive an r35 gtr

1/4 mile time- =shows how much power you have

the rest is just a wank

no maths, no engineering.

pure fact

[Birds]

Edit: epic post here, but it should explain it all:

Woah...sorry guys I did not mean to spur this on, this is heading for the rotary thread! Many are saying the same thing and mistaking it for argument. Phil, I had seen many a motoring journalist and advertisement bragging "and xxxnm of torque, available from just 1500rpm" I believe it to be pretty misleading as a selling point because they do not quote where the torque falls off and anything can happen after that 1500rpm. They really need to quote a spread, be it torque or power. They are also emphasizing torque because bragging about power in engines like turbo diesels does not look good on paper - so instead they turn to big figures characteristic of the diesel engine. And I would hate to see consumers start quoting this peak torque at such low rpm stuff. Just my opinion though.

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.

The Skyline wins because even with less engine torque than the Golf at a lowly RPM, the short gearing gives it more torque at the wheels, which is what is responsible for the vehicle accelerating. Two Skylines, all things equal, except one has a Golf engine in it: if accelerating from a lowly RPM e.g. 1500rpm, the Golf engine will accelerate faster than the Skyline engine...until torque drops off in the mid to upper range, as is characteristic of a diesel. Reality is that you can launch the Skyline engine at 3500rpm, well within it's torque band and suddenly the early developed torque of the diesel engine does not have an advantage. The purpose of high stall torque converters in autos is to have the engine sitting within the ideal torque band prior to launch. In a manual you shift at a point that will drop you into the torque band for the next gear...anything higher and you will accelerate slower because you are running out of torque.

As Newton said, give me a lever long enough and I will move anything (something along those lines...and it was Newton wasn't it?). Power is simply a measure of an engine's torque at a given engine speed, nothing more. It's no more or less worthy to quote this and RPM than torque at the same RPM. Only, being a single number and frequently quoted, it's easier for the lamen to compare power figures and derive what they want from it. This is where all your arguments are stemming from. Now silence! Birds has spoken!

If everyone rode a high revving sports motorbike on weekends and a drove a diesel car during the week, understanding of the topic would be more widespread.

[Rolls]

Yeah the reason why even lower torque motors is faster is that if you want to do say 60kph and one motor makes 300nm @ 1200rpm and one makes 175nm @ 2400rpm the one making 175nm needs to be geared down twice as much to do the same speed, hence that 175nm turns into 350nm at the wheels.

So the higher the rpm the torque is made at the more you can gear reduce it to make even more wheel torque than a motor that makes less. Or you could just go it makes more power and realise that this takes into account that it must make the torque at a higher rpm, much simpler really.

So what elite racing was saying was technically correct, but the motor making less torque at higher rpm HAS to be geared differently to do the same speed, otherwise you can't compare them as one is going faster than the other, and by gearing it differently it makes more torque at the wheels even though the engine torque is less.

[PM-R33]

BIRDS it wasn't Newton, it was Archimedes

Went along the lines of "Give me a long enough lever and fulcrum and I can move the world".

Something like that any way.

[jarrod83]

Heres an interesting spin on our dilemma, if you had a variable gearbox that could hold the engine at a constant rpm while the car accelerates, the fastest acceleration would occur while the RPM’s were held at the maximum power not at the maximum torque. Also whenever you quote maximum torque with an rpm, you are indirectly quoting the power.

I got bored and decided to do a torque verse speed comparison of a stock torque curve and a torque curve that’s roughly been offset forward 2500 RPM to give two torque curves of similar average area. One has more top end power, would be good to do something similar to two actual torque curves of skylines that have similar peak torques but considerably different peak powers. The final diff reduction has not been included in the torque calculation and assumed to be 1:1, except for the second car B data point, that has been changed it to a 1.3:1 ratio

Although its not the most ideal comparison, it does highlight the fact that the rpm range of the peak torque can have a considerable difference to the overall average acceleration of the car due to the torque. Both curves have the same amount

of average torque, but the graph that has the most average power has considerably more average torque over the range of the first few gears.

I wouldn’t mind working out the acceleration of the car versus speed and the time required to reach each speed.

[zebra]

Mmm this thread is too complicated for a Friday lol

[Randon_I_Am]

^ +1

beer oclock yet?

[The Dan]

Try to think of it this way:

Two cars exactly the same in every way except one makes 400Nm of torque at 2000rpm and the other makes 400Nm of torque at 6000rpm. Apply an equal braking force of 1000N to each car pushing against the front of it.

The car making 400Nm of torque at 2000rpm will not only start moving first but it will accelerate at a greater rate (eg. M/sec) until the other car can produce the equivalent torque.

Forget gearing and mechanical advantage. Forget about what is going to happen when the first car reaches 5000rpm.

Torque is the engine's ability to overcome the resistance against it.

Dyno's measure torque. They do not measure power. Power is calculated from torque.

It is more important to know how much torque you are making and when in the rev range then it is to know how much power you are making when in the rev range.

[Birds]

BIRDS it wasn't Newton, it was Archimedes

Went along the lines of "Give me a long enough lever and fulcrum and I can move the world".

Something like that any way.

Oops...I confess to never taking physics lol, I had to learn everything I know about it from internet forums

It's all about teh fulcrum

Heres an interesting spin on our dilemma, if you had a variable gearbox that could hold the engine at a constant rpm while the car accelerates, the fastest acceleration would occur while the RPM’s were held at the maximum power not at the maximum torque. Also whenever you quote maximum torque with an rpm, you are indirectly quoting the power.

I was in a friends CVT Lancer Aspire a couple of weeks ago, and whilst I would like to test it again to confirm this...from memory, in fully automatic mode when he planted the pedal it would sit in the upper midrange and never actually got near redline unless you were in manualised auto mode. Also I don't fully understand the workings of a CVT (maybe more RPM is better for it?) so it's difficult to comment honestly on it, but certainly in the situation of a manual gearbox, when the drivetrain is statically connected to the engine, it will always accelerate fastest around it's peak torque.

[Rolls]

Try to think of it this way:

Two cars exactly the same in every way except one makes 400Nm of torque at 2000rpm and the other makes 400Nm of torque at 6000rpm. Apply an equal braking force of 1000N to each car pushing against the front of it.

The car making 400Nm of torque at 2000rpm will not only start moving first but it will accelerate at a greater rate (eg. M/sec) until the other car can produce the equivalent torque.

Forget gearing and mechanical advantage. Forget about what is going to happen when the first car reaches 5000rpm.

Torque is the engine's ability to overcome the resistance against it.

Dyno's measure torque. They do not measure power. Power is calculated from torque.

It is more important to know how much torque you are making and when in the rev range then it is to know how much power you are making when in the rev range.

I get what you are saying Elite Racing and it is correct but I say that isn't a fair comparison as they are doing different speeds, lets say we have the two cars in your example, lets say we want to be doing 2000rpm at the wheels which is 60kph (I have no idea how fast it actually is), due to the fact that car 2 has 400nm at 6000rpm (far more horsepower) it means we need to gear reduce it 3x so it actually makes 1200nm at the wheels at 2000rpm wheel speed while car A only makes 400nm. Now you might go this is cheating youve put lower gearing on car B, but lower gearing is required to make them do the same speed, if you put the lower gearing on car A as well yes it would make the same torque but you would be limited to a max speed of 666rpm vs 2000rpm which means they cant technically be compared, at the same speed with appropriate gearing the higher hp (makes torque at higher rpm) will go faster every time.

The fact it has more horsepower translates to it having more tractive torque at the wheels.

Another example is lets say the two cars have CVTs that will allow it to be held at the perfect RPM all the time.

Now it becomes more obvious because every single speed you pick the second car will ALWAYS make more tractive effort at the wheels than car A, this maximum tractive effort for the speed [/b ]chossen is at peak horse power, not peak torque, however the actual maximum actual tractive effort will occur at peak torque, but that will mean you are going slower than your chosen speed.

Example of how car B will make more tractive effort at peak horsepower than peak torque.

Lets say we pick the example of car B and 60kph (2000rpm wheel speed) once again.

If we choose peak torque @ 6000rpm that means it needs 3x gear reduction and it makes 1200nm at 2k wheel speed, lets say we pick peak horsepower at 8000rpm with only 320nm, now we need a 4x gear reduction to make 2k wheel speed, that means 1280nm at the wheels if we gear for maximum hp and not max torque, hence at any chosen speed if the car is geared for max hp it will go faster than at max torque.

Now why is this important? Because all cars on the street are geared appropriately to take advantage of their horsepower, if all cars had identical gearboxes and they never changed then yes you would only want to look at torque figures and compare a car entirely on this, but this isn't the case. If you put a CVT in every car so they could always be at the correct rpm they would go fastest if they sat at peak HP not peak torque. I can draw a graph in excel to demonstrate the difference if required, this is why HP is important and tells the story more so than torque, ASSUMING the cars have been correctly geared (which is the case 99% of the time).

actually I am going to draw the example in excel to show that cars always accelerate the fastest (at that speed) at peak hp, and that two cars with the same torque but one with more hp, the higher hp car will always accelerate faster, bbl in an hour.

Note when i was saying assuming gearing was equal in previous posts I meant equal speeds not identical actual gear ratios, this is where I think we both differed and why I was arguing your points, I apologise.

I was in a friends CVT Lancer Aspire a couple of weeks ago, and whilst I would like to test it again to confirm this...from memory, in fully automatic mode when he planted the pedal it would sit in the upper midrange and never actually got near redline unless you were in manualised auto mode. Also I don't fully understand the workings of a CVT (maybe more RPM is better for it?) so it's difficult to comment honestly on it, but certainly in the situation of a manual gearbox, when the drivetrain is statically connected to the engine, it will always accelerate fastest around it's peak torque.

They sit there so they don't blow up by sitting at redline for a whole minute. What you said is correct they will accelerate at their fastest (for the entire time) at peak torque, but they will accelerate the fastest at a specific speed at peak hp. Reason is peak hp is always above peak torque and hence can use greater gear reduction.

Edited October 28, 2010 by Rolls

[The Dan]

I get what you are saying Elite Racing and it is correct but I say that isn't a fair comparison as they are doing different speeds, lets say we have the two cars in your example, lets say we want to be doing 2000rpm at the wheels which is 60kph (I have no idea how fast it actually is), due to the fact that car 2 has 400nm at 6000rpm (far more horsepower) it means we need to gear reduce it 3x so it actually makes 1200nm at the wheels at 2000rpm wheel speed while car A only makes 400nm. Now you might go this is cheating youve put lower gearing on car B, but lower gearing is required to make them do the same speed, if you put the lower gearing on car A as well yes it would make the same torque but you would be limited to a max speed of 666rpm vs 2000rpm which means they cant technically be compared, at the same speed with appropriate gearing the higher hp (makes torque at higher rpm) will go faster every time.

The fact it has more horsepower translates to it having more tractive torque at the wheels.

Another example is lets say the two cars have CVTs that will allow it to be held at the perfect RPM all the time.

Now it becomes more obvious because every single speed you pick the second car will ALWAYS make more tractive effort at the wheels than car A, this maximum tractive effort for the speed [/b ]chossen is at peak horse power, not peak torque, however the actual maximum actual tractive effort will occur at peak torque, but that will mean you are going slower than your chosen speed.

Example of how car B will make more tractive effort at peak horsepower than peak torque.

Lets say we pick the example of car B and 60kph (2000rpm wheel speed) once again.

If we choose peak torque @ 6000rpm that means it needs 3x gear reduction and it makes 1200nm at 2k wheel speed, lets say we pick peak horsepower at 8000rpm with only 320nm, now we need a 4x gear reduction to make 2k wheel speed, that means 1280nm at the wheels if we gear for maximum hp and not max torque, hence at any chosen speed if the car is geared for max hp it will go faster than at max torque.

Now why is this important? Because all cars on the street are geared appropriately to take advantage of their horsepower, if all cars had identical gearboxes and they never changed then yes you would only want to look at torque figures and compare a car entirely on this, but this isn't the case. If you put a CVT in every car so they could always be at the correct rpm they would go fastest if they sat at peak HP not peak torque. I can draw a graph in excel to demonstrate the difference if required.

But you're talking about mechanical advantages again. I am talking about RATE OF ACCELERATION. If you were to measure how fast a car accelerates in M/s/s (meters per second per second) the greatest rate of acceleration occurs at peak torque. Yes the engine makes more horsepower at the higher rpm but the rate of acceleration declines as it meets wind resistance and rolling resistance etc.

CVT transmissions hold the engine at 80-90% peak torque constantly until they reach top gear. Once they are at final ratio they continue through the rev range until redline - (this was in answer to the lancer guy btw)

[Rolls]

But you're talking about mechanical advantages again. I am talking about RATE OF ACCELERATION. If you were to measure how fast a car accelerates in M/s/s (meters per second per second) the greatest rate of acceleration occurs at peak torque. Yes the engine makes more horsepower at the higher rpm but the rate of acceleration declines as it meets wind resistance and rolling resistance etc.

Yes and no, there are two ways to look at it. Lets say we want to do 60kph like in my examples, you will have the greatest RATE OF ACCELERATION AT 60kph at peak hp in that case, your peak actual acceleration (the most you ever accelerated) would be at peak torque yes BUT that would be at 50kph, not the 60kph you wanted to do, however if you used a CVT so both cars had the perfect gear ratio at all times, they would both accelerate the fastest at peak hp always.

So yes I am talking about mechanical advantage, but the fact that a car has more hp means it can use more mechanical advantage to do the same speed, it makes more sense for the speed to be the same.

You wouldnt compare a diesel car with a 4.4 diff to a skyline with a 4.4 diff, the diesel would make it to about 80kph and run out of gears, and the skyline would keep going. You would gear them to do the same speed (so the comparison is fair) and in real life this is almost always the case, cars are geared to do the same speeds so I think as we are discussing this in regards to real world cars it is the best way to show that peak hp is more important than peak torque with a realistic example.

For example your two cars with no gears how the one with peak torque earlier pushes earlier and overcomes the weight earlier yes, but the one with it later would push the weight to a much faster speed, obviously a faster speed means more energy which is what more hp translates to.

Edited October 29, 2010 by Rolls

[The Dan]

Yes that is absolutely correct but your original argument is that torque does not tell you how the car will drive. I would have to say that hp is exactly the same because both rely on mechanical advantages to make them accelerate how the manufacturer wanted.

But power is always calculated FROM torque so my argument is that torque is a very valid value to have included in the specs of any car. Torque is what you feel pushing you in the back. Early torque means it will be punchier from lower revs. Late torque means it makes more hp but it's laggier off the line

[Rolls]

Yes that is absolutely correct but your original argument is that torque does not tell you how the car will drive. I would have to say that hp is exactly the same because both rely on mechanical advantages to make them accelerate how the manufacturer wanted.

But power is always calculated FROM torque so my argument is that torque is a very valid value to have included in the specs of any car. Torque is what you feel pushing you in the back. Early torque means it will be punchier from lower revs. Late torque means it makes more hp but it's laggier off the line

Fair enough, I guess I gave you less credit than I should have, that and I misunderstood what you said by gearing being equal.

I'm still going to stand by that hp is the most important, because if you have a huge amount of it at high revs, you can make use of high mechanical advantage giving more tractive torque at the wheels, and as you said this is what you feel and the more hp, the more you get pushed back.

I guess I gave the torque figures less credit than needed, but I still feel hp gives a bigger picture of how the car will drive. Either way I spent too long thinking about this, but I now have a much better idea of how it all works.

[Brezza]

mother of god

lap time-explains why you are a shit driver.......... unless you drive an r35 gtr

1/4 mile time- =shows how much power you have

the rest is just a wank

no maths, no engineering.

pure fact

i gotta agree with this bloke he puts it nice and simple.

there are a few blokes who go to track days with 300+kw but still manage to have guys with cars close to the 200kw mark lap faster than them. on a track its not all about power, its about suspension, brakes and driver skills and experience.

the thing that i look for when i do power upgrades is how hard my car accelerates in the midrange, on the point on the graph around 4000rpm where the graph goes from 150kw up to 280 within 1500rpm is where i wanna see the largest jump in power, it will peak around peak torque, max KW might be up around 7800rpm but really how usable is it?

i think that some people get a little bit to technical about how powerful their car is, and how it compares with others? yours makes 10more kw than mine but my torque is higher blah blah.

ideally a fast car should be able to provide strong acceleration in midrange with as minimal lag as possible. simple as that

[Rolls]

When they are very similar you need to get technical if you want to come to an accurate conclusion though.

People designing $100k bmws certainly don't just wing it!

[Birds]

They sit there so they don't blow up by sitting at redline for a whole minute. What you said is correct they will accelerate at their fastest (for the entire time) at peak torque, but they will accelerate the fastest at a specific speed at peak hp. Reason is peak hp is always above peak torque and hence can use greater gear reduction.

Peak HP is not the point where an engine will accelerate the fastest. Peak HP is always after peak torque because torque is multiplied by the engine speed to calculate power...so even after torque has started to drop off, if you increase engine speed then you keep producing more power because you are making up for the loss in torque with more engine speed - it will just produce more power at a less linear relate because of less torque. Peak HP occurs when the torque has dropped off to the point where giving the engine more RPM does not produce anymore power. Simple dyno graphs will explain this.

People are giving power too much credit here...it is a measurement of torque at a given RPM. Nothing more. The name given to the tractive force at work is torque. It's just that simple, but has been way overcomplicated in this thread. The origin of the term horsepower was based on a horse/donkey pulling a certain weight at a certain speed, not how fast the horse was accelerating to that speed.

[simpletool]

^ what Rolls said +1

HP is factored by gearing to make torque at the wheels. This is what matters.

However this is only at a single instant - the ability to maximize this throughout the rev range used is also important. This would be area under the curve.

Most people understand torque vs power already but I want to use a punching analogy.

Torque is like how hard you can punch. Revs is how many times you can punch per second. Power is both put together.

If you are a medium sized guy who can punch medium hard and medium fast you are quite handy. Probably just as handy as a large guy who punches hard but not as fast. And if you are a little guy who punches lightly, but very fast you would still be fine (as long as you can punch hard enough to stun).

In the end you are all having the same effect, even though you punch with different strengths (torque).

OK, maybe that didn't help anyone. But I like to talk about punching sometimes. ahh.....that's better.

[Rolls]

Kind of like my explosion = torque, number of explosions * torque = power explanation, sort of helps

Peak HP is not the point where an engine will accelerate the fastest. Peak HP is always after peak torque because torque is multiplied by the engine speed to calculate power...so even after torque has started to drop off, if you increase engine speed then you keep producing more power because you are making up for the loss in torque with more engine speed -

I mentioned this, but if you gear for a specific speed it will make more tractive torque if you are at the rpm of peak hp, not the rpm of peak torque, however when you are on the way to the speed you want to be at, you will reach peak tractive torque at the ground, however you won't quite be at the speed you want to be.

[simpletool]

Well Birds you are right and wrong. Peak torque is where an engine will accelerate fastest IN A GIVEN FIXED GEAR RATIO. If you use a CVT it will accelerate fastest at peak power. It's simply a limitation of the fixed gear ratios that peak torque is the maximum acceleration point in typical cars.

As Rolls points out, power is really the only important (instantaneous) factor in acceleration. Everything else can be factored by gearing. Torque is accepted as a descriptor as it has been traditionally used (with power) to describe engine flexibility at midrange rev levels.