Power vs Torque

[911p]

I remember reading a thread a while back on power and torque, there were plenty of details on there, but I still don't understand properly how it all works. I understand it in ridiculously brief terms (i.e. Power = top end, Torque = acceleration), but there I may still be totally wrong!

Some people stated that the more powerful car would always win, if things like resistant forces were taken out of the equation. However, would this mean that a 420bhp car with 420ft/lbs of torque would lose to a car with 430bhp and 300ft/lbs of torque? I would greatly appreciate anyone explaining this topic to me in simpleton terms! Thanks

[williamp]

how does the saying go?

Understeer means you see what you hit,
oversteer means you dont see what you hit,
power means how fast you hit it,
torque means how much of it you take with you

[vonhosen]

911p said:

I remember reading a thread a while back on power and torque, there were plenty of details on there, but I still don't understand properly how it all works. I understand it in ridiculously brief terms (i.e. Power = top end, Torque = acceleration), but there I may still be totally wrong!

Some people stated that the more powerful car would always win, if things like resistant forces were taken out of the equation. However, would this mean that a 420bhp car with 420ft/lbs of torque would lose to a car with 430bhp and 300ft/lbs of torque? I would greatly appreciate anyone explaining this topic to me in simpleton terms! Thanks

I rather like the description that

Power = how fast the engine punches
Torque = how hard it punches

[SlipStream77]

There are a few articles on the web about this subject, try this one...

http://vettenet.org/torquehp.html

[kerosene]

don't worry lots of confusion here.

HP=Torque*RPM/5252 (this using lbft for torque, with Nm the number 5252 would be different)

Equally

Torque=(HP*5252)/RPM

Now lets pick an example engine that produces
200hp @ 3500 RPM
300hp @ 5500 RPM
350hp @ 6500 RPM

The equivalent torques can be calculated with the formula above.
300 lbft @ 3500 RPM
286 lbft @ 5500 RPM
282 lbft @ 6500 RPM

Little funny torque curve maybe as I pulled the numbers out of a hat. However it does show couple of things.

1st and the most important thing is that Really HP and torque are locked together if we exclude the multiplier it really comes down to torque=hp/rpm and vise versa hp=torque+rpm. You cannot have two engine that have same power curve but different torque curve. Just seeing torque curve has all the information for drawing the power curve and vice versa - both are shown for easier visualizing.

You cannot have engine that produces 400lbft and 200hp @ 3500 rpm - if it produces 200hp the torque MUST be 300lbft


Now we get to the typical claim that its not power that accelerates the car but torque. This is of course true but only partially - it is the the torque on the patch of the tire that touches the pavement that accelerates the car. As we can gear down the engine RPM down the HP really defines the torque we can have at the tire.

So at any given RPM torque and HP are locked together and really tell the same thing.

Now what people usually mean with high torque engine is that they have high torque available at a broad rpm range. But equally it could be said that such engine has high power curve over broad rpm range.
Small CC engines that produce very high power usually do it only on very high revs and on relatively narrow power band thus they can be inferior in real life situations - they need more gears and more shifting etc. over a broader power band vehicle.


Not sure if that was the clearest explanation. AT least I tried and my very 1st post on the forum ended up being a novel.

[911p]

Welcome to PH!

Thanks for all of the help on this topic everybody. I'm now going to use the 996 Turbo and an E92 M3 as the examples - please correct me if I'm talking rubbish. Figures may not completely correct, I haven't bothered to look them up.

996 Turbo: BHP = 420, FT/LBS = 410, Max RPM = 7000.
E92 M3: BHP = 420, FT/LBS = 300, Max RPM = 8500.

Because the M3 can rev significantly higher than the Turbo, it makes the same power as the Turbo from a much lower ft/lbs figure. If this is correct, what I don't get is how torque is significant at all. Would having the same power and different torque still mean that the given cars still perform the same?

Thanks!

[grumbledoak]

They are related, as said above.

But, in essense,
Torque = acceleration
Power = top speed

[i remember]

BHP = Pub talk

Torque wins races

[The Lukas]

Crumbs, the amount of threads we have on this subject is ridiculous! Power is the figure you want as it's the final output measurement. Torque is absolutly useless without RPM. Gearing is also very important. But in simple terms, power is the measurement of absolute force/work dispersed from the wheels/crank.

[The Lukas]

i remember said:

BHP = Pub talk

Torque wins races

Absolute rubbish.

[PhillipM]

i remember said:

BHP = Pub talk

Torque wins races

That's diesel owners deluded pub talk.

[911p]

I've gained some knowledge of this topic, but have still failed to grasp it fully. In a nutshell, would it be true to assume that;

BHP = the maximum amount of the given cars capability
Torque = how quickly/hard it'll get to its top capability

Hence, in some respects, Power = Speed, Torque = Acceleration?

Edited by 911p on Tuesday 1st June 00:53

[ArtVandelay]

911p said:

Double post

Edited by 911p on Tuesday 1st June 00:53

You can delete double posts you know or any post of your own for that matter, I think.

Next to edit is a little bin icon, well at least there is on PH Classic.

[911p]

ArtVandelay said:

911p said:

Double post

Edited by 911p on Tuesday 1st June 00:53

You can delete double posts you know or any post of your own for that matter, I think.

Next to edit is a little bin icon, well at least there is on PH Classic.

Don't know how the hell I missed that!

[ArtVandelay]

911p said:

ArtVandelay said:

911p said:

Double post

Edited by 911p on Tuesday 1st June 00:53

You can delete double posts you know or any post of your own for that matter, I think.

Next to edit is a little bin icon, well at least there is on PH Classic.

Don't know how the hell I missed that!

Happy to help

[Or888t]

Thanks for the tread OP much learnt
It's got me all though thinking into it now, if torque = acceleration, why does a car need to rev more to continue accelerating when rev'ing more will cause loss of torque thus , loss of acceleration.

it's all subject to gearing i surpose. no... i think i've just lost the plot and got

[Sticklebrick]

Another way to think about it is:

Torque is effectively a force, albeit it a rotational one - a moment. Hence the unit - Nm or lbft. A force exerted at a given distance from the centre of rotation. So it is this force exerted through a set of gearing to the wheels on then to the ground through the tyre contact patch. So this force can be geared up or down, and is always what is driving the vehicle no matter the speed.

Power is an expenditure of energy. Just like kwh that you buy your electricity in, just different units for different applications. Horsepower is the usual unit for power relating to cars but it is also quoted in kilowatts kW sometimes as well. This is just an energy exertion per time. A watt is a joule for a second.

So given a force can be expressed as an amount of work (or energy expended) over a certain distance, crudely the power is a measure of how much torque is available at a given speed. So power can indeed be a representation of top speed as the engine needs to able to exert the force at the high engine speeds to reach the top speed.


My apologies for anyone reading this, I know this is not a perfect explanation - I didn't want to start playing round in equations to much to explain all the relationships.

To the OP I hope this helps and makes sense.

Ryan

[grumbledoak]

Or888t said:

why does a car need to rev more

You need to increase the revs to increase the speed; the engine is directly connected to the wheels. You then change up a gear to get the torque back.

Clearer?

Edited by grumbledoak on Tuesday 1st June 01:15

[Or888t]

grumbledoak said:

Or888t said:

why does a car need to rev more

You need to increase the revs to increase the speed; the engine is directly connected to the wheels. You then change up a gear to get the torque back.

Clearer?

Edited by grumbledoak on Tuesday 1st June 01:15

Yea, thanks, was kinda thinking that in my head.
yea im getting it, basically to accelerate you need as much torque as possible, and the lesser amount of rev's the more torque, although it's a trade off between torque, and how fast the wheels are attualy going round.

Kinda why diesel seam like they have got potensial power, all the time due to high torque because of the low rev's, but not as fast as the can't rev high enough to spin the wheels that fast.

my god, im typing drivel but im starting to have a deeper understanding, im grasping the idea.

Edited by Or888t on Tuesday 1st June 01:42

[GravelBen]

kerosene said:

Good stuff.

Top first post! Welcome aboard.