AnthonyJ

OK I have a question which I hope some of you techie boffins can explain to me. I've never managed to get a sensible explanation of the difference between power and torque when it comes to driving a car. People always talk about "low end torque" and "maximum power", but how do the two actually relate - and what affect do they have on the drivability of the car?
It seems that torque relates to "ability to accelerate" (and is therefore mass related), but power relates more to top speed (& is therefore related to aerodynamics & transmission losses etc). Obviously the two are connected, but there's more to it than that because some engines produce comparitively more power than others but very similar toque. For example - the M3 Evo develops over 320bhp, but only(!) 260lbft of torque. How would this be different, say to my WRX which produces much less power but similar torque?! At what points in a drive does one become more important than the other - i.e. when does the fact that the Beemer has 40bhp more help him?

I'd appreciate it if anyone could explain the differences. I know "boy racers" out there who use the phrases, but when quizzed, none of them can actually relate what it means in real life!! In my own mind, I've formed the opinion that torque seems to matter more than bhp, but I'd like to know what the truth is!

Thanks in advance!

Maf

I'm not a true tech-head so I'm sure I will be corrected, but here goes.

Torque is the accelerative turning force of the engine. Yes it matters most for acceleration. But it varies over the rev range.

BHP is actually proportional to the torque produced multiplies by the revs its produced at. So if a car produces its maximum torque higher in the rev range (even if that max torque is slightly lower) it will have higher BHP. Yes BHP equates to maximum speed more.

Read a quote once "torque moves cars, BHP sells them".

iaindean

In laymans terms, torque is what makes the car more driveable (ie it doesn't need to scream its (VTEC) head off to get going.

The lower the torque peak (generally) the better, because it means (most times) the power band is larger and more accessable.

As a general rule I stick by:
if a car has similar torque to bhp, then it is 'torquey' (more driveable) ie
Subaru Impreza Turbo 2000 Turbo (UK) has 208bhp (allegedly) and 201 lb ft torque: a very driveable car with accessable torque (peaks @ 4000rpm(and has about 70% of max torque from 3000rpm approx)) and power (peaks @ 56000rpm).

A bad example is the Honda Integra Type-R
With power of 190bhp (peaking @ 8000rpm approx) and torque of 150 lb ft (peaking @ 6000 rpm approx) means it needs to be thrashed to go fast, and you will always be working the box!

[This message has been edited by iaindean (edited 26-03-99).]

clarence

I think we should look closer at the torque curves instead of the quoted max torque @ so and so rpm figures.
Example: the twin-turbo EJ20 engines in the Legacy GT-B/RSK. This engine produces 280ps/6500rpm and 35kg-m/5000rpm. The raw figures point out an extremely peaky engine, but looking at the torque curves, it produces over 25kg-m of torque at 2000rpm. That is more than the peak torque produced by the double VANOS BMW 2.5 straight-six at 3500rpm.
A turbo car can have loads of torque and peaks at mid-range (eg. Toyota 2JZ-GTE produces 46kg-m/3600rpm), but one must not forget that there is turbo lag.

MarkL

Can anyone be more specific and (much) more technical about the effects and relationship between power and torque? An explanation based on physics principals would be welcome.

I've noticed several people (not just on this thread) stating that power is proportional to torque and revs. While this sounds reasonable in theory, examining power-torque curves from rolling roads seems to disprove this. The power and torque curves are clearly not directly proportional to each other because the curves have different shape, and peak power and peak torque do not appear at the same revs, which would be the case if they were directly proportional.

Hopefully I'm missing something obvious, but I would also appreciate a good technical explanation.

Maf

Power is not "proportional" to torque.

Power = torque "multiplied" by revs.

That is why power curve goes up as torque stays flat, and peak power is higher than peak torque. Past a certain level of revs the engine just cant give any more, torque drops of dratically & then so does power.

Benny Boy

Does that still work with Truck engines where you might get 500lb/ft of torque and 200bhp at say 1,800 rpm?

Yours,
Mildly interested of Berkshire

MarkL

Maf,

Since you can express a mathematical relationship between power and torque then then are by definition "proportional" to each other. The relationship simply isn't linear.

On a more positive note, I've taken another look at the rolling road graphs and it's making sense to me now. 'Power = torque x revs x c', where c is a conversion constant, seems to give consistent results. So you're right about the relationship.

As for what effect power and torque have on the car, I'll make some opinion based on physics principals rather than personal knowledge. If anyone knows better, please post as I'd be very interested.

Torque is the turning force, which relates to the force with which the wheels can drive the car forward. 'Force = mass x acceleration' so, since the mass of the car is constant, the amount of torque produced is a direct measure of ability to accelerate the car.

High power can be produced simply by revving to high revs and do not relate directly to ability to drive the car forwards as torque does, so power figures seem pretty meaningless.

There is an advantage to having a high-revving engine in that you can hold onto gears for longer. A shorter gear ratio means that more of the torque generated by the engine is exerted as force at the wheels. Of course this is dependant on the engine producing enough torque at the higher revs for it to be worth staying in the lower gear rather than changing up.

What is really important is the torque curve rather than the figure at any specific point. A higher torque figure is better, but you need to look at the torque across the working rev-range to get a proper picture of the performance.

Imran

MarkL

It seems that as simple beings, we are always trying to sum-things-up, in a single sentence or in the case of car performance, with a single measure of power, or speed, or torque etc.

So, how about area under the torque curve?
On reading your posting, it seemed that this could be a very representative performance measure, as it takes into account the revability of the engine, the torque plateau and the peak torque figure!



[This message has been edited by Imran (edited 30-03-99).]

AnthonyJ

Cool idea! Maybe you should weight the calculation to make torque at lower revs "worth" more.

OK, I'm beginning to see how this all fits together now (I hope). Maximum power output is fairly irrelevant as a statistic by itself, but when combined with a torque figure, it can help you to extrapolate what the torque curve is like. So, in the case of the M3 Evo engine, although it doesn't peak more torque than the STi, from the maximum power output we could assum that the torque may not fall off as sharply as it does on the STi (which would seem to be correct given that it has those two extra pots to play with). This might also explain what happens with the Prodrive kit - although the peak power may not be much more than a standard turbo (because the torque drops away faster), it is probably quicker than a standard car because the torque through the midrange (3.5k - 5.5k) is higher.

Does this make sense? It all seems to hang together quite well. Any other comments? BTW, I tried looking at the graphs on globalnet to see what all this means in real life but the connection just timed out - are they still there?!

Maf

Mark L

Don't want to be a maths grad bore here but actually the definition of proportional is:

Variable A = Variable B X Constant

Revs aren;t constant, so power is not proportional to torque.

Not that any of this matters, cos I've just stacked my car so I'm as big a pillock as you are gonna get anyway.

brumster

I think this thread has pretty much answered the original question, however when I saw the title I was suprised no-one had made the age-old comparison of "riding a bicycle", which always seems to crop up when I see this question. People seem to relate to it quite well.

Max.Power : the highest rate at which you can peddle.

Max.Torque : the highest amount of "twist" your legs can put into the pedals.

So, for example, you could have two quick cyclists. Both are very fit but in different ways. One is capable of peddalling very quickly for long amounts of time, but is quite skinny. The other has massive tree-trunk legs, quite a big chap, lots of stamina, but can't pedal particularly quickly.

Chap 1 can ge quick by using his high rate of pedalling to accelerate and go faster, provided he stays in the right gear. But catch him in too high a gear, and he has trouble. Put him on a hill, and he's stuffed. His legs produce more "power". Chap 1 is a Honda Integra Type-R, or a Civic VTEC, or a Rover K-series, or whatever.

Chap 2 can pedal more effectively in a higher gear so, provided he's not asked to pedal too quickly and he's appropriately geared to make use of all that "grunt", he can waltz up a hill no problem. His legs can produce more "torque". Chap 2 is any sort of V8, a diesel 300 TDi Discovery, a Jaguar V12, a Ford 3.0 Capri.

The analogy with an Impreza is kinda more difficult - it's more like the weedy guy at times, but as soon as he's peddaling more than 3000rpm his legs inflate!!!

I could go over the techie details of car engines as opposed to an "analogy" but everyone in this thread has already covered these in detail.

Laters,
Dan

RobAnderson

Dave Walker (technical guru at CCC mag) defined the difference between power and torque thus:
"Torque is measured because it is a twisting force.You can't measure bhp-it doesn't exist.Brake horsepower is simply a rate of doing work.An assumption is made that if you lift 33000 lb by 12 in. you have done the amount of work that an average horse could achieve in one day.Therefore to arrive at bhp we measure the torque(weight) and then the engine rpm(distance) and calculate the horsepower from there.Since the torque is measured on an engine 'brake',we call it brake horsepower.
As a very rough instant guide,at 5250 rpm the torque and horsepower are exactly the same."
Succinct and to the point.

andymac

So presumably all the plots of BHP vs RPM etc are effectively torque * RPM * constant vs RPM ? i.e., BHP is a derived figure whereas torque is a measurement. How is torque measured both at the wheels and at the engine, i.e., before transmission losses ?

Thanks

Andy

brumster

For most accurate results, you would put the engine on an engine dyno (the dynometer is connected directly to the crank) vs. a rolling road, which measures power at the wheels.

Having said that, most modern rolling road dynos can extrapolate the power at crank by measuring power under load, then measuring the loss through transmission (gearbox, diffs, etc) by "freewheeling" the car on the dyno and seeing how much power is takes to stop it.

...if I remember rightly!

Another interesting little note - as a general rule, cars are most economical (under load) at peak torque since they are burning the fuel/air mixture more economically. Pretty obvious, really.

RobA (or rather, Dave Walker)'s description is spot on in my opinion.

Laters,
Dan

RobScreene

I noticed this earlier, described quite well at:

jeff

unfortunately, torque and horsepower are never linear unless rotation and inertia are at a contant arrival of the mass(weight) of the reciprocating assembly. That is why a wankel engine(mazda rx7) can produce nearly a perfectly linear halving of torque and horsepower. Less rotating mass, a more perfect harmony of rotation and constancy of motion in the mass. whereas a Piston engine has to overcome many reciprocating obstacles (so to speak) and the disharmony of overcoming internal rotating friction. The pistons connected to a rod and twisting on a crankshaft produces 3 types of reciprocating obstacles. Frictional, rotational and inertial forces. (they oppose the others work) And this is only the beginning. TO answer your question, about the relation of torque and horsepower, unfortunately its not an easy answer..alot of variables. torque..twist or thrust. horsepower, how fast you can send the car down the road. hope it helps some.
jeff