craig1410
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| posted on 29/7/06 at 08:01 PM |
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Power versus Torque - Good explanation here.
Hi,
At the risk of starting another endless thread on this subject I thought I'd share the following description of power versus torque. It's
one of the best I've come across.
Personally, I've always been comfortable with the whoel Torque v Power relationship but then Physics was my favourite subject at high school.
This article may help those who are struggling to visualise the concept to gain a better understanding.
Enjoy!
http://www.seatcupra.net/knowledge_base/knowledge_base/power_and_torque.html
Craig.
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cossey
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| posted on 29/7/06 at 09:40 PM |
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i dont get that he says that it is torque at the wheels that determines everything but that is effectively a measure of instaneous power
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cidersurfer
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| posted on 29/7/06 at 09:40 PM |
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Lots of useful theory can be found on the site below too...
http://www.pumaracing.co.uk/
shimming solid lifters is a job for a friend...
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craig1410
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| posted on 30/7/06 at 12:38 AM |
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Cossey,
This is probably one of those things you either get or you don't but I'll try to help...
Torque is essentially rotational force and where your tyre comes into contact with the road it is turned into normal (or linear) force. This is the
force that pushes your car along the road at either a steady speed or if the force exceeds the drag (rolling and aero) then it will accelerate. So the
author of this article is correct in as much as, at any point in time, torque is what is important. However, as he explains (well I think) power is
the ability to provide a given amount of torque at a particular rate and having more power allows a lower overall gear ratio to be used when
delivering the torque which results in greater forwards force.
That's as far as I'm going to go in explaining this subject. As I said, you either get it or you don't. 
Cheers,
Craig.
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Johnmor
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| posted on 30/7/06 at 08:00 AM |
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I like it!
Makes things clear, helps to explain that tuning an engine to huge amounts of BHP at high revs does not solve many of the problems if the torque
remains low..
What you need is a large volume high torque engine that revs freely to high revs, this also has to be as light as a feather and cheap as chips.
Sorted!
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MikeR
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| posted on 30/7/06 at 10:03 AM |
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I always had a really simplistic view of this......
torque is the cars ability to turn the wheel.
Power (bhp) is the cars ability to increase the engine revs.
Then multiply the two together to get how fast the car goes.
Ok, so its wrong, but it sort of makes sense in a simplistic view.
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cossey
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| posted on 30/7/06 at 10:05 AM |
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quote:
Originally posted by craig1410
Cossey,
This is probably one of those things you either get or you don't but I'll try to help...
Torque is essentially rotational force and where your tyre comes into contact with the road it is turned into normal (or linear) force. This is the
force that pushes your car along the road at either a steady speed or if the force exceeds the drag (rolling and aero) then it will accelerate. So the
author of this article is correct in as much as, at any point in time, torque is what is important. However, as he explains (well I think) power is
the ability to provide a given amount of torque at a particular rate and having more power allows a lower overall gear ratio to be used when
delivering the torque which results in greater forwards force.
That's as far as I'm going to go in explaining this subject. As I said, you either get it or you don't.
Cheers,
Craig.
i meant that his explaination doesnt make sense rather than i dont get how it works.
as soon as you bring any form of rotational speed into the argument torque becomes power.
i had a really long answer written out but it seems to have disappeared but the gist of it is that for a larger acceleration you want a large change
in velocity which means a large change in kinetic energy so a lot of power.
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craig1410
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| posted on 30/7/06 at 11:06 AM |
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Chris,
I don't have time right now for a long answer but remember as well that smaller engines tend to be "peaky" and only produce useable
torque over a much narrower band of RPM compared to a large engine. Hence the often used term, "Power Band".
Cheers,
Craig.
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DIY Si
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| posted on 30/7/06 at 11:31 AM |
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I believe Mr Mason has put it in a sensible way. NOt sure I agree totally with Craig though, as smaller engines tend to rev higher so the power band
in rpm terms is wider. Ie, my blackbird enigne has a power band of 4.5k-9.5k (ish) rpm. Not many cars enigne have a power band 5k wide. In terms of a
percentage of usable rpm, this may be different as it is roughly 40% of the rev range, but a car engine may have a 3-4k ower band, but only rev to 7,
thus giving 50% ish of the revs. Hope that makes some kind of sense.
Also bigger engines can have different gearing to take advantage of the higher power levels, so it's difficult to compare in a
"real" way.
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craig1410
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| posted on 30/7/06 at 01:11 PM |
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Si,
I was talking about torque not power although I may have misled by calling this power band. I was referring to the earlier comment that most cars have
a flat torque curve but I don't believe this to be the case with bike engines and other small cc engines.
[Perhaps to be more accurate, I should have said that "highly developed" engines tend to be peaky on torque because the high level of
tuning can only usually be maintained over a narrow band of RPM. Of course this usually means bike engines because they are small cc but need to
develop relatively high power.]
Sorry for confusion,
Craig.
[Edited on 30/7/2006 by craig1410]
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John Bonnett
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| posted on 30/7/06 at 02:25 PM |
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Power sells cars, torque wins races!
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DIY Si
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| posted on 30/7/06 at 03:03 PM |
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Craig, Fair point, now I fully see what you meant I do agree with you. I've only seen a few engines that have flat(ish) torque curves and even
fewer that have a wide, flat torque curve. Having said this, according to one power/torque graph I've seen for a bird engine it is quite flat,
varying by only 10lbft between 6k-10k. Even so, only flat for 30% of the curve. Even bigger engines tend to have nicely curved graphs, but depends a
lot on tuning levels.
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DIY Si
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| posted on 30/7/06 at 03:14 PM |
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Drop below 6k?! Only if I'm driving like a big girl! Or hit traffic.
BUt yes, above 6k it is fairly flat, with a the nice steep power curve that goes with it.
[Edited on 30/7/06 by DIY Si]
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craig1410
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| posted on 30/7/06 at 04:01 PM |
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quote:
Originally posted by John Bonnett
Power sells cars, torque wins races!
That will explain why you always hear the Formula 1 guys boasting about how many lb.f.ft or NM their engines are producing eh? 
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