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torque

omega0684 - 10/9/09 at 09:29 PM

can someone please explain 'Engine Torque' to me please, i know that cars are measured as having brake horse power and 'Torque', but how is it measured and what is it referenced too? what are good torque figures and what are bad torque figures?

i've heard people say that diesels have more torque than petrol powered cars, but why, is it only in the event of having a turbo diesel etc?

cheers

[Edited on 10/9/09 by omega0684]

graememk - 10/9/09 at 09:33 PM

Graeme goes to google to find the answer.

i was going to copy and paste the results in order to make myself look cool and cleaver, but cant be arsed as i'm on the wine.

so heres a web link to look at.

http://www.streetrodstuff.com/Tech_Stuff/Tech_Terms_II.php

[Edited on 10/9/09 by graememk]

omega0684 - 10/9/09 at 09:46 PM

thats a very poor explanation IMO, i posted on here cuz i know i would get a thorough well explained answer

graememk - 10/9/09 at 09:46 PM

sorry i'm drunk....

Jasongray5 - 10/9/09 at 09:52 PM

Torque is the turning force of an engine. Its measurse in a weight x distance. For example ft/lb's so an engine with 80ft/lb's has a turning force of 1lb at a distance of 80ft?

A diagram would explain better, but I know what I mean??

smart51 - 10/9/09 at 09:53 PM

torque is a turning force. Think of a chinese burn. If you want to tighten a bolt, you apply more torque.

You're familiar with force = mass times acceleration. Torque = mass times rotary acceleration. given a rotary thing, lets say the roller on a dyno, of a certain mass plus a means of measuring the its speed against time, you can calculate the torque needed to accelerate that mass.

Now work done = force times distance. Torque is a turning force so work done = torque times turning. Power = work done per second so power = torque times revs.

more torque (at the wheels) = more acceleration (of the wheels), with the proviso that your tyres still grip the road. Too much torque and all you get is wheel spin. Torque to weight ratio governs your acceleration.

Now diesels don't rev as high as petrols. We've discussed that power = torque x revs. If a diesel makes the same torque as a petrol, it will make less power because it doesn't rev as high. In a given car, the standard fit diesel engine is either bigger in capacity or has a turbo or both compared with the petrol, to give similar power output (compare a 1.4i citroen to a 1.4tdi for exanmple - about 75 BHP each). This being the case, the turbo diesel will have more torque but the same power as the NA petrol.

This only really makes a difference in 1st gear, which will be a similar ratio. Ratios are wider spaced on the diesel to counter the lower revs. Torque at the wheel = torque at the flywheel / gear ratio. So an equally powered diesel may get the jump off the line compared with its petrol twin, but the petrol's longer legs will see it catch up and slowly overtake. Equal power means equal top speed, but the petrol will get there slightly sooner.

NigeEss - 10/9/09 at 10:03 PM

As mentioned, torque is a measure of force.
Someone here has a footnote that sums it up perfectly but alas I can't remember who. Sorry to that person

But to quote.............

Power/BHP is how fast you hit the wall....
Torque is how far you ake the wall with you !

Unquote..................

Steve Hignett - 10/9/09 at 10:09 PM

If you tighten a wheel with your 1/2 " ratchet and go really quick, that may be considered more Hp, the quicker you turn your arm, but the more leverage you apply (a breaker bar) the more torque you are exerting on that bolt...

brianthemagical - 10/9/09 at 10:28 PM

Torque is force times distance, hence to tighten a bolt up more, which we know means it's got more torque/been torqued to a higher value, you need a longer bar or more force. this equates to engines as they are simply a lever and a force. the leaver being the crank, with the big end a distance-half the stroke- form the mains. the force is the combustion process, via the piston/conrod. as we know a certain capacity uses a certain amount or air and fuel, thus a very similar power per revolution, in a long stroke engine this gives more torque simply due to the crank dimesions.
more power comes from the revs, with a shorter stroke engine being able to rev higher due to the lower piston speeds afforded by it's short distance.

power is a factor of the frequency of a force. more revs, more combustion, more fuel being used therefroe, within reason, we can assume more 'power' or working force is being converted, hence power being faster than torque.

think of it as a hammer. lots of little taps is more power, one big tap is more torque. both get the nail in, but if the nail needs a hit of strength 10 to move, and the lighter taps are only 5, then it'll never move.

Madinventions - 10/9/09 at 11:35 PM

Yep - agree with all the above.

It can sometimes help to 'visualise' the torque. For instance, my Mazda Diesel estate has 300Nm of torque: That means that if it had 1 metre radius wheels (it doesn't...) then it could push/lift/move 300N of weight (about 30Kg ish). Measure your arm, and work out what weight that's equivalent to you holding. My arm is approx 500mm from should to grip so it'd be like me holding a 60kg weight at full stretch. Hmmm...

As an aside, my Mojo has 110Nm of torque from it's mighty 1.25 Zetec SE with 75BHP. The Mazda is 300Nm of torque and 136BHP. The Mojo is about twice as quick 0-60, probably because it weighs 523Kg and the Mazda is >1500kg, but remember it's rolling weight your torque is pushing. I can push the Mojo with one hand, but the Mazda needs a lot more grunt!

Ed.

blakep82 - 10/9/09 at 11:56 PM

quote:
Originally posted by NigeEss
But to quote.............

Power/BHP is how fast you hit the wall....
Torque is how far you ake the wall with you !

Unquote..................

i thought it was the other way around?

ie more torque gives you better acceleration, and power is how well you keep the speed up if something tries to slow you down?

jeffw - 11/9/09 at 05:38 AM

The important thing to remember about torque and cars is that you cannot out accelerate the torque curve. The shape of the torque dictates how the car drives.

My RS4 had 530 ft/lb (718Nm) at 4000rpm. This is made up of a very sharp rise in torque at around 2750 where the turbos start to work upto the peak and then a steady decline down to the redline. The sharp rise in torque gives you the kick in the back. My A6 3.0Tdi has a completely different torque curve which peaks at 415 ft/lb (562 Nm) at 2000 rpm but stays within 10% of that until 4250rpm. This gives you the classic diesel flat curve.

The other thing to remember is that rolling roads only measure torque. The BHP or Wheel HP figure is a calculated number from the measured torque.

clairetoo - 11/9/09 at 05:59 AM

As Carroll Shelby once said
"Horsepower sells cars , but torque wins races"

alistairolsen - 11/9/09 at 07:00 AM

quote:
Originally posted by blakep82

quote:
Originally posted by NigeEss
But to quote.............

Power/BHP is how fast you hit the wall....
Torque is how far you ake the wall with you !

Unquote..................

i thought it was the other way around?

ie more torque gives you better acceleration, and power is how well you keep the speed up if something tries to slow you down?

All of these quotes, and the shelby one are all nutsack tbh.

Also remember that a single torque figure and power figure are fairly meaningless.

Take for instance a 1000cc 150bhp bike engine making 80ftlbs at 12krpm and a 2000cc 150bhp car engine making 160ftlbs at 6000rpm. Conventional wisdom would say that with the appropriate gear ratios they would produce exactly the same effect at the wheels, but the bike engine will have much less area under the torque (and hence power) curves making it much more peaky to drive and overall slower in use.

stevec - 11/9/09 at 07:18 AM

Torque is the distance you travel through a wall after you hit it.

Steve.

alistairolsen - 11/9/09 at 07:40 AM

quote:
Originally posted by stevec
Torque is the distance you travel through a wall after you hit it.

Steve.

no, thats momentum

stevec - 11/9/09 at 07:49 AM

to you as well.

alistairolsen - 11/9/09 at 08:12 AM

Sorry, just these bullshit explanations of torque and power really do me in cos people just repeat them parrot fashion with no clue as to whether theyre correct or not.

torque is turning force
rpm is speed
power is a function of the two.

Caroll Shelby was wrong, its area under power the curve that wins races. His point that peak power isnt everything is however true.

flak monkey - 11/9/09 at 08:23 AM

Simples

Exactly as Alistair says. The 2 are intrinsically linked.

If you want a basic explaination in terms of performance. Torque gives you acceleration and BHP gives you top speed. The rate of acceleration is linked to the torque curve.

alistairolsen - 11/9/09 at 08:37 AM

yes and no, its quasistatically linked to the torque at the wheels at some point in time for which you want the instantaneous acceleration, but overall its related to the bhp which indicates the rate at which work can be done.

also worth remembering when trying to make comparisons that only the torque and power at the wheels are of any great relevance in a direct comparison due to gear ratios etc.

02GF74 - 11/9/09 at 08:47 AM

quote:
Originally posted by alistairolsen
Sorry, just these bullshit explanations of torque and power really do me in cos people just repeat them parrot fashion with no clue as to whether theyre correct or not.

My parrot could torque.

smart51 - 11/9/09 at 09:44 AM

quote:
Originally posted by stevec
to you as well.

That's the best thing I've ever seen on a forum. The use of an emoticon as an insult.

blakep82 - 11/9/09 at 11:26 AM

quote:
Originally posted by smart51

quote:
Originally posted by stevec
to you as well.

That's the best thing I've ever seen on a forum. The use of an emoticon as an insult.

insult on a primary school level too

MikeR - 11/9/09 at 11:38 AM

My simplistic view (and whilst i appreciate before it wasn't 100% accurate now i'm not even sure if its 20% accurate) is ....

Torque is the cars ability to pull a weight,
HP is the cars ability to accelerate that weight.

So F1 car with sod all torque but lots of HP goes damned fast

Large truck with relatively little HP but lots of torque pulls lots of weight.

MikeRJ - 11/9/09 at 11:55 AM

quote:
Originally posted by flak monkey
If you want a basic explaination in terms of performance. Torque gives you acceleration and BHP gives you top speed. The rate of acceleration is linked to the torque curve.

As Alistair says, this is a bit of a misleading explanation. The thing that gives you acceleration is torque at the wheels, not the crank. The gear ratios between the engine and wheels (i.e. the torque multiplier) are just as important as the engine specifications.

This is something diseasal drivers don't seem to realise; they like to talk about the torque their engines produce, but these engines have a very limited RPM range so they have tall final drives (less torque multiplication). The torque at the wheels is therefore less impressive than the engine specs might suggest.

Even the "area under the curve" is only important because of the limitations of conventional transmissions. If reliable, wide range continuously variable transmissions were available the peakiness of the engine would be inconsequential, since it could just run at peak power and the transmission would always deliver optimal torque to the wheels. This is why hybrid technology is moving toward solutions with no mechanical linkage between engine and wheels, instead the engine drives a generator just like a diesel-electric locomotive. It means the engine can run at a constant RPM where it is tuned to be most efficient.

Every car forum I visit has had long, protracted threads on the difference between torque and BHP, and the same misleading (and some total BS) explanations get recycled again and again. I am honestly stumped as to why people find this such a difficult topic to grasp, it's really very basic physics, hardly even GSCE level stuff.

MautoK - 11/9/09 at 04:28 PM

The imperial unit of torque is 'ft lb'.
Tq is the product of force and distance.

It most definitely NOT ft/lb as is so often seen. Pet hate #1!

John.

Ninehigh - 12/9/09 at 07:07 AM

Torque is what women do all day, and you can spot the amateurs they have to stop to breathe (seriously I watched 2 girls blah at each other for 50 mins non-stop, and I mean NON stop).

My version of the layman's explanation would be this:
The higher the torque the better you accelerate at 1500-3000rpm, ther higher the bhp (the amount of power to break a horse ) the better you accelerate at 3000+rpm

Torque is nicer cos you're not screaming the crap out of your engine and you can laugh at Saxo boy and his mates as you glide past

Ivan - 12/9/09 at 07:40 AM

It's a common misconception that torque dictates acceleration - if that where true then turbo diesel cars with their massive torque would p1ss all over F1 and BEC cars with minimal torque. So torque dictates things like climbing or towing but not acceleration.

What dictates acceleration is rate of work, i.e. horsepower which for the same torque is lower at low revs and higher at high revs, which is why BEC's and F1's that rev high to get good horsepower from low torque accelerate like stink.

To belabour a point - what would accelerate faster - a Busa in a Locost or a VW turbo diesel in a locost presuming the BEC is weighted to match the deisel 7 - the one makes most probably 3 X the torque and the other 50% more HP. I'm willing to bet its the Bussa motored car.

Ninehigh - 12/9/09 at 07:43 AM

quote:
Originally posted by Ivan
To belabour a point - what would accelerate faster - a Busa in a Locost or a VW turbo diesel in a locost presuming the BEC is weighted to match the deisel 7 - the one makes most probably 3 X the torque and the other 50% more HP. I'm willing to bet its the Bussa motored car.

Most probably, but then if you limit both cars revs to the peak torque limit?

The Black Flash - 13/9/09 at 08:35 PM

quote:
Originally posted by Ivan
It's a common misconception that torque dictates acceleration - if that where true then turbo diesel cars with their massive torque would p1ss all over F1 and BEC cars with minimal torque. So torque dictates things like climbing or towing but not acceleration.

Quite wrong I'm afraid.
Torque at the wheels determines acceleration. Nothing else.

You are right that power is a better measure of performance, but that's because it takes rpm into account.

Edit: Someone had already explained it better.
A really good discussion of torque vs power here: http://www.vettenet.org/torquehp.html. Well worth reading.

[Edited on 13/9/09 by The Black Flash]

prawnabie - 13/9/09 at 08:43 PM

Im with the brick wall!

nigel.hillier - 14/9/09 at 03:11 AM

In simple terms - What it actually means to you as a driver (rather than a complicated mathmatical White Paper)

Torque is the only thing the engine produces. This varies during the RPM range.

At any given RPM more Torque is Better (more Power).

You as a driver you want Power/Kilowatts.

Power (Kilowatts/Horsepower) is simply the amount of Torque multiplied by the RPM of the engine divided by a Constant (5252 from memory this is a mathmatically generated number from last century)

Torque x RPM / Constant = Kilowatts

If you want to make more Power you must do one of 2 things (or both is even better)
1) Produce more torque at any given RPM
2) Rev your engine to higher RPM (so long as torque does not drop off significantly)

Example 1 - High reving engine (low torque)
You engine produces 100 units of Torque at 8,000 RPM = 152 kilowatts

Example 2 - Low reving engine (high torque)
You engine produces 1000 units of Torque at 800 RPM = 152 kilowatts

Both examples will deliver to you the same power. It is just how they get there that is different.

High torque low rpm is better for engine life but that is another discussion.

[Edited on 14/9/09 by nigel.hillier]

Ivan - 14/9/09 at 07:41 AM

quote:
Originally posted by The Black Flash
Quite wrong I'm afraid.
Torque at the wheels determines acceleration. Nothing else.

You are right that power is a better measure of performance, but that's because it takes rpm into account.

Quite right - but torque is a very poor measure of a car's potential performance whilst horsepower (which takes into account peak torque location and spread) is an excellent indicator - that's why it's dangerous to choose a motor for performance based purely on torque - if you did that there would be no BEC's and every racing car would be running low revving turbo diesels.

Your refference articles are good but tend to basically support the HP argument for assessing performance. (Given that it's all about gearing)

[Edited on 14/9/09 by Ivan]

Antnicuk - 14/9/09 at 05:07 PM

i think its explained above but i dont understand it, why does the bhp and torque curve cross at 5250 rpm on every car i have had dynoed,???

I have 350 torques and its more than enough it certainly takes the wall a ong way!!!

MikeRJ - 14/9/09 at 05:52 PM

quote:
Originally posted by Antnicuk
i think its explained above but i dont understand it, why does the bhp and torque curve cross at 5250 rpm on every car i have had dynoed,???

Because for power in bhp and torque in lbft:

Power = ( RPM * Torque ) / 5250

Rearranging gives:

RPM = ( Power / Torque ) * 5250

So, when Power(bhp)=Torque(ftlb) the right hand side of the equation equates to 5250.

simonk - 14/9/09 at 08:19 PM

Hi all

It was posted further up the thread but I also found this same torque / horsepower primer some years ago and it really is a good explanation. If you read it carefully it should only take 15 minutes and the writer has put it in 'car' language and as such it's very readable - solves and answers all the arguments and questions in this thread...

http://www.vettenet.org/torquehp.html.

Cheers
Simon

iti_uk - 14/9/09 at 08:41 PM

Torque isn't a Force!!! It's a moment!
M = F*d
(F being force applied, d is perpendicular distance from the rotational axis at which the force is applied)

The fact is, it's the shape of and area under the torque curve which is important. Peak numbers are only for impressing people with.

MikeRJ - 14/9/09 at 08:58 PM

quote:
Originally posted by iti_uk
The fact is, it's the shape of and area under the torque curve which is important. Peak numbers are only for impressing people with.

As I mentioned above, the only reason it is important is due to the limitations of the transmission system.

liam.mccaffrey - 15/9/09 at 09:21 AM

torque is rotational force (moment )-full stop end of story
power is a rate at which work is done due to force-full stop end of story

I didn't mean this to sound curt, sorry

[Edited on 15/9/09 by liam.mccaffrey]

flak monkey - 15/9/09 at 09:23 AM

quote:
Originally posted by liam.mccaffrey
torque is rotational force (moment )-full stop end of story
power is a rate at which work is done due to force-full stop end of story

Nicely put

02GF74 - 15/9/09 at 10:01 AM

quote:
Originally posted by nigel.hillier
Example 1 - High reving engine (low torque)
You engine produces 100 units of Torque at 8,000 RPM = 152 kilowatts

Example 2 - Low reving engine (high torque)
You engine produces 1000 units of Torque at 800 RPM = 152 kilowatts

Both examples will deliver to you the same power.

let's say you have these two engines in identical cars and you want to drive them up an increasingly steep hill, keeping rpm the nbad torque the same, if that is possible.

which one wil get further?
which one will get there faster?

alistairolsen - 15/9/09 at 10:10 AM

do you mean constant, or the same?

if you mean constant then you would use different gear ratios so that the torque and speed at the wheels was the same and hence both would be identical.

phelpsa - 18/9/09 at 11:49 AM

It's all to do with power, torque means fook all once gear ratios are involved.

Power is the amount of energy transferred in a certain time period. You apply more energy per second and you will accelerate faster.

Area under the power graph is how much energy is transferred as you rev the car from 0-the rev limit. This is what makes the difference, the more energy you can transfer in that period the more speed you will gain.