SteA

I know what you all mean about torque, but everyone is interested in rate of work, the actual turning force on its own is meaningless without being able to quantify its actual effect on acceleration. A tractor could have 500 ftlb of torque and take 50 secs to reach 60 mph.

Peak torque and bhp are often misleading, as mentioned, on the RR graphs its the areas under the graphs that are of most interest... especially bhp as that actually represents work which is what we are interested in

Here is a passage taken from another discussion board which kind of highlights the above points:

http://forum.parkers.co.uk/topic.asp?TOPIC_ID=23060

__________________________________________________ _____________

Lots of people ramble on about power and torque, saying things like "My Skoda has more torque than a Porsche Boxster". But what does that actually mean? Is it faster than a Boxster? People say "Torque wins races, bhp sells cars". But what are they going on about?

A Torque is essentially just a turning force, a force x a distance. For example if you have a spanner that is 0.5m long, and you put a force of 5N at the end to undo a nut, it is a torque of 2.5NM.

Power and torque are often torqued (see what I did there? ) about as if they're totally different entities, but are in fact very closely related.

The golden rule is Power = Torque x Angular Velocity

So if we look at car engines, Power in bhp = ( Torque in lb-ft x rpm ) / 5252

So how do all of these relate to a car's performance? Is it really all about the amount of torque that the engine produces? Do we get in a nice new turbo diesel supermini and find it's faster than a Porsche? Well kids, let's have a nice example....

2 cars are travelling at 50mph. The road wheels are going at about 75 rad/sec (about 717rpm).


One is a dirty diesel. The engine is at 2000rpm and is walloping out 250lb-ft of torque.

The other is a petrol. The engine is at 4500rpm and only pushing 120lb-ft of torque.


Now there's 2 ways to work out which will have the most acceleration:

1. Torque and Gearing:

A gear ratio is simply the number of turns the engine does for every turn the road wheels do.
The gear ratio of the diesel is 2000:717, or 2.8:1
The gear ratio of the petrol is 4500:717, or 6.3:1

To work out the torque we have at the wheels (the force that will actually accelerate the car) we simply multiply by the gear ratio. Which is why gearboxes are sometimes called Torque Multipliers.


So the torque at the wheels for the diesel is 250x2.8=700lb-ft
And for the petrol is 120x6.3=756lb-ft

And hey presto, the petrol has less than half the engine torque, but has more torque at the wheels to accelerate.


The "other" way to do it......

2. Power!

The power at the engine will be equal to the power at the wheels (there will be a percentage loss in the transmission but it'll be the same in each case so we'll ignore it) due to conservation of energy.

Power at diesel engine is (250x2000)/5252 = 95hp
Power at petrol engine is (120x4500)/5252 = 103hp


So, the power at the wheels is equal to the power at the engine. You just need to divide by the rpm of the wheels (and x by 5252) to work out the torque.

Torque at diesel car's wheels is (95/717)x5252 = 696lb-ft
Torque at petrol car's wheels is (103/717)x5252 = 754lb-ft

Those numbers are slightly different to the ones above because of rounding numbers along the way.



So, torque is important, but it's the torque at the wheels that determines how fast you can accelerate. To maximise torque at the wheels you need to maximise power at the engine, not torque.


So what does this mean?


Well, if you stay in 1 gear going all the way up through the rev range, the biggest kick in the back will come when the engine is at peak torque. This will be when the VTech or turbo kicks in. This is simply because the gear ratio is constant, so more torque at engine = more torque at wheels.


But, if you want to accelerate from a certain speed, say to overtake someone. Which gear do you select? Do you select one that puts the engine at the most possible torque or one that puts it at the most power?

The answer is power. The gear where the engine is producing the most torque will more than likely not be the lowest possible gear. So in a lower gear, although the engine is producing less torque, more is getting to the wheels because of the ratio. It then follows that the most torque at the wheels, will be when the engine is producing the most power.


The main advantage of having a torquey engine is that it will acclerate well from low engine speeds. In the above example, the diesel accelerates almost as well from 2000rpm as the petrol does from 4500. This simply means you can get good acceleration without revving the nuts off the car. The best acceleration from any speed will always be when the engine is producing as much power as possible.

MarkFitz

Hi Doc

Torque is a measure of how hard the car kicks you up the ***.
Power is a measure of how quickly you're *** is getting kicked.

Cheers