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torque and power 

 

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Posted: Sat Jul 23, 2005 2:13 am 
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ok, can i throw a real world one into this debate????

my i6 wagon has:

148rwkw
3.27 diff gears
t5
approx 1700kg
me driving = 14.9 @ 90.86mph

my old xr8 has:

146rwkw (same dyno, same day)
3.89 diff gears
auto
approx 1600kg
me driving = 15.0 @ 94mph


this whole thread was started because my mph in the wagon was considered low, which is a fair comment (btw ignore my original reply in other thread about ratio's, was very tired, have re thought now, lol)

im not saying anyone is right or wrong, just very interested in the answers coming forward, truely.

im not sure of the torque figures of my old xr8 that is why i havent mentioned either.

in case anyone asks, i change at 5200-5400 in the wagon, and i was shifting at 5600-5700 in the xr8, dunno if it matters, just thought i would throw that in.

*can of worms now open, lol*

 

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Posted: Sat Jul 23, 2005 2:26 am 
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your xr8 seems to be more consistant with the power and mph that I am used to. I also have 146rwkw in my falcon (confirmed by 3 dynos) and do a high 94mph pass. MPH decreases as weight increases, which is why I asked how much your wagon weighs. It might be right on the money, but just seemed a little high to me.

The ET comes down to the different ways the cars launch and gear ratios. It would seem that the T5 3.27 combo is working better than the auto 3.9. Probably because even with 3.9's the t5 has a better 1st gear ratio.

what were the 60' times like for each

 

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Posted: Sat Jul 23, 2005 2:37 am 
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i think you might be on the money, the xr8 60' was a 2.39, while the wagon was 2.15, heaps better.

 

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Posted: Sat Jul 23, 2005 2:59 am 
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2.15 is a mighty fine 60' for the wagon

 

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Posted: Sat Jul 23, 2005 3:24 am 
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ladies please! This is 4 pages long and ive been away about 6 hours!

Here it is, for anyone missing from their physics classes.

Torque is a rotational or shear force, with unit newton meters.

Power is the amount of torque available instantaneously at a finite time, measured in joules/second, or watts (W)
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Posted: Sat Jul 23, 2005 3:30 am 
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peak torque shouldnt really have much to do with the time a car will run?

to get a true indication of which car is quicker would it not be better to look at the average torque provided over the rev range that will be used?

ie.

2 cars in a race both use the rev range 3000-6000 rpm

for purposes each car is identical, weight, diff, gears, efficiency etc. if you can think of it.. it is the same.. the faries did it. and if you question it, you willb e zapped to neverland ranch

now, each engine might have a peak torque as a different rpm, and 1 might have a higher torque peak than another, but it might be a quick peak in torque.. the other might be smoother..

so to decide what would be faster would you not take the average torque over the rev ange and see which had the higher average?

the car with a more peaky curve would pull away at the peak, but drop back after its peak, while the other car has a more constant torque and therefore provides a smoother pulling..

just having a qucik think this make sense..

the average torque over the usable rev range... :shock:

 

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Posted: Sat Jul 23, 2005 4:20 am 
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Yes, if both cars use the same rev range, but no - average torque means crap (at the engine at least). Its average power that counts!!!

Take two identical cars and run them down the 1/4. Shift car A so that its rpm band maximises average torque, and shift B other to maximise average power. CAR B WILL WIN.

WHY?
Because A will be shifting at about 4000 rpm, and car B at about 5200 rpm. Although A is averaging more torque at the engine, car B is averaging more at the wheels because it holds gears for longer.

 

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Posted: Sat Jul 23, 2005 4:48 am 
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Torque is the force that turns your wheels. The more you have, the harder your driveline will try to rotate your wheels. It really is as simple as that.

If a 10 litre V8 and a 2 litre straight 4 had the exact same torque curve, they would both go just as fast as each other assuming all other things equal, and don't give me that 'its not the real world' rubbish...

Also, in this day and age, Torque x RPM / 9549 would be more apt as it refers to kilowatts and newton metres.

Having torque down low makes a car drivable, after all, 500nm @ 2,000rpm is 104kw. It's the same as driving around at 7,000rpm in a pulsar or something. Having torque high up makes that same car fast. 500nm @ 6,000rpm is 314kw - now we're talking!!! :twisted:

In an ideal world, the torque curve of the engine would be flat. This would mean that power builds in a totally linear way, and peak power would actually mean something. Unfortunately this is far from the case.

 

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Posted: Sat Jul 23, 2005 12:08 pm 
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This is some funny stuff. I think we need to paint a line on the road. One side for people who think they know what they are talking about, the otherside, for people who DO know what they are talking about.

 

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Posted: Sat Jul 23, 2005 2:10 pm 
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Ride: ford au wagon 4.0 na

Power: 170 rwkw

Location: Merrylands
NSW, Australia

Guys if you have read this thread and gone to the website which explains it ,you will realise what is right and stop carrying on.


http://vettenet.org/torquehp.html



Peak torque figures are quoted and then peak horsepower.

If peak torque in 2 cars occurs at 4000 rpm at 400nm and one has 150rwkws and the other has 170rwkws at 5300 rpm ,the car with more horsepower will win. Peak horsepower is simply a way to say that.."past 4000 rpm where peak torque occurs, the 170 rwkw engine will have a higher value of torque than the 150rwkw car.

Its all about the terminology.

I like the useless definition of power...."if I am in front of you ..I will beat you."Easy!!

 

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Posted: Sat Jul 23, 2005 2:18 pm 
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Allow me to tie into the debate with some real world experiences. At work I have ran more than a couple of comparisons between two identical trucks on our chassis dyno to get to the bottom of low power complaints. Normally one driver will say I cant keep up with the sister truck on the hills. Our first test is to tell the customers to swap drivers between trucks and if the problem persists with the truck we than run a dyno comparison. These comparisons are usually performed for major fleets whos information is more reliable and trucks specs identical. When I say identical I mean same engine with same ratings, same gearbox with same ratios, same diffs and ratios as well as same tyres, same kilometres travelled same service history even the same oils being run in the driveline all this in exactly the same chassis and cab, as identical as you can get. The trucks are ran on the dyno within 10 minutes of each other to keep conditions as close as possible. These trucks will generally perform within a few horsepower of each other throughout the rev range measured at 100 rpm intervals. Yet on the road one performs better than the other. So thats real world. Maths will tell you these two trucks will perform the same but in the real world they dont. Im not taking anyones side here just sharing my experiences. Im a diesel mechanic of nearly 10 years and I dont pretend to fully understand power. Torque is easier to understand but people still refer to power without fully understanding it. I suppose power sounds more impressive.

 

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Posted: Sat Jul 23, 2005 4:21 pm 
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THE POST TO END ALL POSTS!

Its obvious that nobody wants to listed to little old me so I spent 5 minute on google search and here are the results. IF YOU ARE ACTUALLY INTERESTED IN LEARNING SOMETHING FROM THIS THREAD INSTEAD THEN READ THIS. No matter how many websites I look at on this topic they all say the same thing… Are we all wrong?

First, power, torque and drag racing. This is actually a high school physics students question and a physics teacher’s answer.

Quote:
QUESTION:
It is to my understanding that Torque is pulling power and horsepower is the ability to keep moving to eventually reach top speed. There has been a lot of controversy about torque and horsepower and how they relate to automobile performance. What is more important for drag racing and why??


ANSWER:
In order to make things clearer I suggest to look at another situation first, i.e. the lifting of a heavy weight. When lifting such a weight one needs to apply a "force" that exceeds gravitation. As you will surely know, there are 'machines' such as a block and tackle that reduce the force needed. However, the distance you have to pull is accordingly longer when such a device is used (the golden rule of mechanics). Therefore physics has introduced the term "work". In order to lift a certain weight to a certain height one needs to do a certain amount of "work". "Power" is work divided by time.

Back to drag racing:
The engine produces a certain maximum "torque" (that can be compared to the term force in the case of a turning wheel). This torque is transformed by the gearing (block and tackle) and transmitted to the wheels. The car is able to pull the weight if the pulling force exceeds the (sticky) friction force of the weight. Surely, a good contact of the tyres to the ground is essential (the main problem in drag racing!).

While the weight is being dragged, the car has to "fight" against the friction of the weight. In this situation the power of the engine is what counts and power is measured in Watt, kilowatt or PS. In a way, power is the ability of maintaining a force over the racing distance and doing it fast.

In short: "Torque" (in Newton Meter) is important for the start of the race, "power" (in W or PS) for the rest. In both cases a good contact to the ground is needed.



Or another site:
Quote:
I'd like to think that torque is an intuitively easier concept to understand. If that were true, though, then more people would understand the relationship between torque, horsepower, and vehicle acceleration. In reality, none of it is intuitive. If it were, Newton wouldn't be considered the Really Great Guy that he is.
The classic mistake is to conclude that the fastest way down, let's say, a 1/4 mile drag strip is to keep the engine RPM at the torque peak (or as close as possible). The technique is usually stated as "shift just after the torque peak", or "shift N RPM above the torque peak so you are N RPM below the torque peak in the next gear when you finish the shift".
……
OK, so what about power? As has been noted by a previous contributor, Power (hp) = Torque (ft-lb) * RPM / 5252. Note that power is also force * velocity, specifically:
Power (hp) = Force (lb) * Velocity (MPH) / 374
That's net horsepower, which is engine power minus losses like transmission and tire friction. The force is the sum of the longitudinal forces at the contact patches of the two driven tires.
Hmmm... P = F * V ...rearrange to get F = P / V ...
that means that you get the maximum force pushing the car if you maximize your *Power* at any given velocity. This gives us another useful rule:
Shift to maximize engine POWER, not engine torque!
This is *exactly* the same as saying "shift to maximize transmission output torque". But it's a little easier to apply.


NOW THOSE POWER CURVES DON’T SEEM SO USELESS AFTERALL!

 

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Posted: Sat Jul 23, 2005 4:23 pm 
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Now for the relationship between power and ¼ mile mph

Found the following empirical formulas (which means it was found by comparing the attributes of real cars and their actual drag times)
Quote:
MPH = 230 (hp/weight)^(1/3)
ET = 6.269 (weight/hp)^(1/3)

In his journal article, Professor Fox lists the following key variables that affect ET and MPH.
• vehicle weight
• engine power, and the details of the torque or horsepower curve
• coefficient of friction of the tires on the track
• aerodynamic drag
• moment of inertia and frictional loss of moving parts
• drivetrain gearing
• shifting mechanisms
• location of the vehicle's vertical and horizontal center of gravity
• driver skill
Fox notes that the first two variables, weight and power, are the primary influence on MPH making it easy to use a constant power approximation to determine a formulaic relationship. It is the variability of the remaining variables and their influence that makes it much harder to determine ET based soley on vehicle weight and engine power

What this is sayings is that although there are many factors that affect the actual mph you get down the drag strip, the power to weight ratio is such a dominating attribute that you can all but forget about everything else, even the power/torque curve (constant power approximation).

This empirical formula has been backed up in theory by:
Image

Heavy s**t for a car forum ey?

anyway, forget all that and apply it to our cars. If I use the constant value 270 (from the above forumula) instead of the 230 found empirically (probably based on dragsters from some time ago now) I get some pretty dam close values

My car, assuming 1650kg (base model EF + 100kg for driver and fuel) comes out to 93mph vs 95 actual. Yobbos wagon assuming 1700kg and a couple more kw comes out to 92.2mph. Working backwards from his actual mph puts his power between 138 and 145rwkw (assuming that this formula may be about 2mph shy of what time is actually done based on my car – not exactly a conclusive but it has dynoed within 1rwkw across 3 dynos).

So after all that I have finally proven myself wrong and yobbos dyno which I thought seemed high for his mph is probably quite reasonable given the mph and weight of the vehicle.

 

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Posted: Sat Jul 23, 2005 6:27 pm 
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ouch, my head hurts, lol.

definatly an interesting read, with (in the end) a result that i can agree with, to try and bring some closure i will go and get my car weighed at a weighbridge near home, i have never actually weighed it but i have been told a wagon is around 1650-1700kg before the driver gets in, and dont forget im a fat bastard too, lol. stockstandard, if you can get yours weighed on a weighbridge aswell it would be interesting to put some real figures into the equation and see how close we get to each others dyno results.

cheers Rob.

 

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Posted: Sun Jul 24, 2005 12:05 am 
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I think i know which side of the line stockstandard is on :)

 

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