Torque vs KW

[JB]

Guys, I am trying to understand the correct description of Torque and KW.

In a very raw form I understand that Torque is about low down grunt, the more Torque, the more pulling capacity etc.

And I believe that KW is raw power.

However what is the exact description for them both, and how do they actually differ from each other, and consequently join together with each other?

[Slymeat]

My understanding is Torque is twisting force, and is calculated using rpm and kw's. And horse power is a measure of pulling force against a constant. Kw's are the same as horse power only a different constant, one rates against a horse standard the other againsty Watts of power. This is thinking back a few years though.

[Trumpy]

Simon from Nizpro has put together a very good explanation of the above question. Have a look his website.

[JB]

Thanks Trumpy, That makes sense, but what I also want to understand is how can some vehicles have a large amount of Torque, but only have a small amount of horsepower and some have large HP and small amount of torque? Does anyone know the answer to that?

for those who are interested in the explaination to my initial question here it is:

HORSEPOWER V TORQUE

For some people it’s all about horsepower, while others say it’s torque that accelerates a car. But of course you can’t have one without the other. Indeed, I have never seen a car without much power win anything, however I have seen races won without stacks of torque. Why? Gearboxes and diff ratios make torque, they don’t make power. EG. A Yamaha FZ1000 motorcycle.

It is torque that usually breaks things and in the case of the XR6 Turbo, culprits include autos/manual transmissions/clutches and as some have found out; CON RODS.

RPM also has its problems. Increased piston wear is probably the biggest, although most other engine components will suffer. But if we go back to our original example of 800 ft-lbs, we’ve already increased torque by around 250% over standard to achieve the 1035hp and have increased the rpm by only 17%.

Alternatively, achieving the same power at 1000rpm lower (5800rpm) we have an increase of 270%, and this can’t go on forever. Therefore an increase in rpm has to come. However, if we increase rpm by 1000, we are only looking at an increase of approximately 17%, and this is a long way from the 270% torque increase.

See Rule One: NO MAGIC.

Having dyno-tuned a huge diversity of engines over the past 15 or so years, an odd calculation came to me one day and I have been using it ever since. You see, people often talk about good engines making around 100hp per litre of capacity which is fair, however this figure really doesn’t apply to turbo or super high RPM engines.

After looking at the many engines I’ve tuned over the years, I concluded that another good way of looking at power per litre was to change this to torque per litre, per absolute boost pressure by RPM. Seems pretty close and works well on both naturally aspirated and ‘boosted’ engines.

Examples below:

• Yamaha FZR1000: 80 ft-lb x 1-litre x 1 atmosphere x 10,500rpm divided by 5252 = 160 hp.

• 3-litre F1 engine: 80 ft-lb x 3-litres x 1 atmosphere x 19,000rpm divided by 5252 = 870hp.

• 5-litre V8 Supercar: 80 ft-lb x 5litres x 1 atmosphere x 7,500rpm divided by 5252 = 571hp (pretty close contrary to popular belief!)

• Nizpro BA: 80 ft-lbs x 4-litres x 2.5 Bar Absolute (23 psi or 1.5 Bar boosted pressure) x 6800rpm divided by 5252 = 1035hp.

N.B. Absolute Pressure. Remember that we live on a planet that has 14.7 psi or 1-Bar of pressure at sea level. (Remember the Absolute Pressure number as this will come in handy in a later article on Inlet Manifolds and Exhausts).

Have a good look at all of the examples and you will see that all numbers are in fact quite close to fact.

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[poida97]

Yes, torque is a measure of rotational force. The metric unit is Newton-Metres. To give you an understanding of what that means, imagine an engine producing 300Nm of torque at a set rpm. If the flywheel had a radius of 1 metre, then the force that is produced at the rim of the flywheel is 300 newtons- (ie 300N*1m=300Nm).

Power is the product of force and velocity. In rotational physics (ie engines) it is the product of torque (rotational force) and rpm (rotational velocity). Power is never directly measured by a dyno. A dyno actually measures the torque being applied to the rollers, and CALCULATES the power by multiplying that figure by the corresponding rpm.

In short, torque and rpm is what is measured. Power is a function of both.

Some engines can have mountains of torque but little power simply because they do not rev hard. Examples include diesels and long-stroke petrol engines.

High-revving motorcyle engines and car engines like that found in the Honda S2000 have quite a lot of power but not a great deal of torque.

Performance-wise you will find that an engine with a good torque figure is needed to get a heavy vehicle going. They are also easier to drive because they don't have a powerband that is high in the rev range, and don't always need to be in the correct gear to get reasonable performance.

No doubt others will add to this.

Cheers,

Anthony

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[Dr Z]

There is a good layperson oriented article titled "Power versus Torque: Part 1" with a sub-title of "So which do you need more of to go fast - power or torque?" and you can find that HERE.

Part 2 of this series titled "Power versus Torque: Part 2" with a sub-title of "Torque is for acceleration and power is for top speed... or is it?" can be found HERE.

For those who do not have the time or inclination to read these 2 articles, here are some conclusions, as directly quoted in yellow:

* So, it is clear that you can compensate for a lack of torque if you have power.

* But you cannot compensate for a lack of power by gearing.

* So power is the critical determining factor for maximum acceleration, and torque is necessary for driveability.

Finally, if that isn't enough talk about torque, there are two articles (Part 1 and Part 2) titled "Constant Torque Performance" with a sub-title of "Constant torque - the driveline-friendly approach to performance..." where you can find Part 1 HERE and Part 2 HERE.

Safe and happy driving...Dr Z.

[JB]

Thanks Guys, extremely informative, thanks for your input!

[Blown BA]

There is a good layperson oriented article titled "Power versus Torque: Part 1" with a sub-title of "So which do you need more of to go fast - power or torque?" and you can find that HERE.

Part 2 of this series titled "Power versus Torque: Part 2" with a sub-title of "Torque is for acceleration and power is for top speed... or is it?" can be found HERE.

For those who do not have the time or inclination to read these 2 articles, here are some conclusions, as directly quoted in yellow:

* So, it is clear that you can compensate for a lack of torque if you have power.

* But you cannot compensate for a lack of power by gearing.

* So power is the critical determining factor for maximum acceleration, and torque is necessary for driveability.

Finally, if that isn't enough talk about torque, there are two articles (Part 1 and Part 2) titled "Constant Torque Performance" with a sub-title of "Constant torque - the driveline-friendly approach to performance..." where you can find Part 1 HERE and Part 2 HERE.

Safe and happy driving...Dr Z.

:lol: Trust Dr Z to find an answer

[mickq]

Easiest description I have heard is this:

Imagine you are on the 10th floor of a building. You have a rope going out the window down to the ground. You are wanting to lift things up to the 10th floor in a basket tied to the end of the rope.

The more torque you have, the more weight you can lift up in the basket. You might be slow, but you can grunt away and lift lots of weight in that basket.

The higher your horsepower, the faster you can pull the basket up towards you. If you dont have much torque, you can do it quickly as long as you arent lifting much weight.

If you have both high horsepower and torque, not only can you lift lots of weight, but you you can do it damn quickly.

If you look at some Japanese cars (like the Honda S2000 mentioned earlier) it has quite high horsepower but not much torque. It can accelerate quite quickly, but aim it up a hill, put 4 people in it, or have it tow something and the lack of pulling power (torque) effects it a great deal and its not nearly so fast.

The same applies to many motorbikes, but because motorbikes also have low weight, they can pull damn hard up hills despite only having say 120-185hp (for 2004 model 600-1000cc jap sports bikes). But stick 3 people on one (somehow!) and it will slow down. It doesnt have the raw pulling power to cope with that well.

Compare them to an XR6 Turbo. The XR6T is unique in that it makes massive torque at only 2000 revs. So it copes well up hills regardless of revs (as long as >2000rpm) and can carry 4 people without too much of a problem. Same applies to most V8's.

But despite this, if you put it in a situation where grunt isnt as important: such as just the driver in their car racing on a flat road, the little jap car with not much torque but low weight and ok hp might kick the more powerful XR6T. But come the first hill, or add a few passengers and suddenly torque is important and its a whole different story then.

There are some unusual examples of horsepower and torque.

For example steam engines are the only engines that can develop torque without revs (motion).

Edited 28/09/04 01:14 PM by mickq

[3tl]

Try this site http://science.howstuffworks.com/fpte4.htm