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torque and power 

 

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Posted: Sun Jul 24, 2005 1:29 pm 
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How does this formula take into account gear changes, driver error, friction, wind drag, power curve (not just peak power figures) and the many other variables ?

 

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Posted: Sun Jul 24, 2005 3:36 pm 
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It is impossible to take all variables into account. But the distinction between torque and power have been explained .

Then stockstandard said the magic words ..which I have believed for a long time now...."NOW THOSE POWER CURVES DON’T SEEM SO USELESS AFTERALL!"



This has been my belief for a while now..but no one seems to listen.

Why is it that in a 1/4 mile race that 2 manual fords..one with 141 rwkws and one with 170 rwkws both ran 14.6 1/4 miles?

Because the power curves were similar,even though the other had an extra 29 rwkws somewhere further down near terminal speed.

 

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Posted: Sun Jul 24, 2005 4:03 pm 
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As it says above, it doesnt - and it doesnt need to

The guys who worked this forumula out decades ago found that when they looked at the cars and the mph they were doing that all those other factors have a minor influence over mph. A small diff ratio change, say 3.08 to 3.45, wont change mph much (you increase torque to the wheels, but decrease the time applied for in each gear by a proportional amount). Normal variation between similar drivers once again doest have a huge impact - but I would say that if you are a slow shifter then your mph will be reduced. So while all these other factors will change mph by a small amount, it is trivial compared to what would happen if you change the power or the weight.

Because of this it was easy to come up with the formula. All these other things change ET in a big way though, so it is really hard to work out ET with any accuracy.

Its not a perfect formula, but it is a REALLY GOOD approximation. Bottom line is if two similar cars (similar weight) line up at the drags, the car that has the most power will consistantly pull higher mph passes - regardless of ET's, the gear ratios (withing reason), torque curve, etc. If you are pulling higher mph but slower ETs then your problem lies in those other areas, not the power department.

 

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Posted: Sun Jul 24, 2005 5:13 pm 
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Gaz wrote:
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?


just having a quick think this make sense..

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


Hit the nail on the head right there. Why do you see so many Gen 3's in the 12 second zone now? when a stocker will pull a low 14 or high 13. Its all about the average torque across the rev range.

Its no use saying that one car has X amount of torque at a certain RPM or Y amount of horsepower at a certain RPM. What about the rest of the rev range. If you want to run hard and your peak torque isn't that great then you can compensate by holding your peak torque over as wide a range as possible. That's why so many Gen 3's are in the low 12's now with nothing but a cam and exhaust change. Their cams are designed for as wide a rev range as possible.

What about pro stock engines and their narrow power bands? That all comes down to gearing. They run pretty tall slicks, short diff gears and a very close ratio 5 speed to keep them within their torque curve. Apart from turbo rotaries, they are the only engine I know of that makes peak torque up near the rev limit. Outside of their torque curves they are gutless wonders. If you don't shift spot on they nose over and literally stop accelerating because of how steep the rise and drop off of the torque curve is.

What about the real world? I'll tell you tomorrow. Till next time keep pondering about this.
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Posted: Sun Jul 24, 2005 5:26 pm 
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some more info on the other factors

Image

I dont have the rest of the paper though so I dont know what they find in the second order theory...

 

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Posted: Sun Jul 24, 2005 9:51 pm 
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So at the end of the day, you agree that the engine size makes no difference?

 

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Posted: Sun Jul 24, 2005 11:14 pm 
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South wrote:
So at the end of the day, you agree that the engine size makes no difference?


my first post...

stockstandard wrote:
So how does engine capacity figure into this?
Bigger cubic capacity engines generally produce more torque earlier, hence make power earlier, but to make peak power the engine has to be able to sustain its torque curve throughout the rev range. Just because a bigger capacity engine has more torque, that doesnt mean it will go any faster.

 

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Posted: Mon Jul 25, 2005 12:49 am 
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So how many small capacity engines at the moment have a top speed of over 200mph?

 

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Posted: Mon Jul 25, 2005 1:17 am 
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200mph - how many large capacity engines can crack that?

Im not saying that big capacity isnt an advantage. Its a lot easier to get power out of an engine if you have cubes on your side. In most cases, more capacity means more power, but this is not always true.

Take a 3L quad over head cam 4V v6 and cram in every bit of techno-gadgetry you find in all the high end cars and youll make good power, more than the average pushrod 5L v8 anyway. Take the same tech and apply it to a 5.0 and youll get more power again.

my point is, you cant predict power from cubic capacity alone, yet it is power that determines how fast the car will be (still asuming the same weight). You therefore cannot predict how fast a car will be based on its CI.

 

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Posted: Mon Jul 25, 2005 1:57 am 
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Gearing has a lot to do with what makes powerful cars fast.
You can compensate for a lack of torque through gearing, but you cannot compensate for a lack of power.

If you can make your car rev harder (and hence produce more power), then you can translate this into more torque at the rear wheels by simply fitting a shorter diff ratio.
Make your car rev 20% harder while sustaining similar levels of torque, and gear the car 20% shorter and theoretically it will also be 20% faster!

If you were not to fit the shorter diff ratio in this case then the car would accelerate almost identically to a stocker until a certain point (the point at which the stocker has to change gears). From there on the stocker immediately reduces the amount of torque it is transmitting to the wheels, whilst the modified vehicle continues accelerating as before.

That is a highly simplified example. Usually if we modify our cars we pickup midrange torque as well as extending the powerband upwards.

Now for the explanation of why increasing peak torque isn't particularly useful. Again it has to do with gearing.
Say you have two cars that both produce the same power at 5000rpm, the upper scope of their rev range. One car has been modified so that it has its bottom end to peak torque increased, which occurs at 3000rpm - whilst the power curve levels off again to stock figures above this.
The two cars race. First gear the car with higher torque gets away quicker, but from then on its even as each gear change only dips to around 3500 meaning this peak torque has no effetc from then on.

If the car were to have an increase in peak power, then this would have been useful in every gear.

The bloke who owns the torquey car says stuff this and gears his car taller so he ca rev his first gera out longer and make use of the extra torque. Unfortunately for him by doing this the rear wheels are not seeing anymore torque than the stocker, and now after 3000rpm the car is slower!

Torque does NOT mean bottom end 'grunt'. Increasing torque will make your car faster, increasing bottom end or peak torque may not. You want to increase torque in your useable power band! This will also increase your peak power!

So to make yor car faster! Increase power by increasing revs, or increase torque in the usable powerband!

That's my take 8-)

 

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Posted: Mon Jul 25, 2005 12:39 pm 
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Couldn't have said it any better my self. I was about to say this anyway but you bea me to it. Do you know why peak torque occurs that low in the rev range? How often is a factory car required to pull harder than 3000rpm? Not often at all if anything. On average, people rarely use above 20% throttle whilst driving and our cars are built to reflect that.
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Posted: Mon Jul 25, 2005 1:07 pm 
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stockstandard wrote:
some more info on the other factors

Image

I dont have the rest of the paper though so I dont know what they find in the second order theory...


I thought i left all this crap behind last semester :( :lol:
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Posted: Mon Jul 25, 2005 7:08 pm 
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So to re-iterate.

Mod your car for:

More torque at higher RPM with gearing to suit withought adversely affecting low down torque. Will = a faster car.

You can sacrifice a little low down torque for the above since it only affects the start of your start 0-400m time. Although it will directly affect your everyday low RPM driveability.

Other factors. Changing to a T5 = more gears keep car within power band, allows for narrower higher torque band and lower gearing overall in 1st 3 gears = more torque at wheels. Will make more difference on modded cars with increased torque at high revs.

Changing to lower diff ratio = similar except less of an effect.

So seeing what ppl do with their cam / exhaust / chip / diff mods we're pretty spot on in making our cars go faster. Ie more torque at higher revs.

This also explains why some of the 'seat of the pants' "yeah she goes heaps faster" mods don't always translate into faster 400m times since on the street you generally drive at lower RPM than a full power 400m run. So an increase in low down torque might not make too much of a difference on the strip.

Interesting thread.

 

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Posted: Mon Jul 25, 2005 8:08 pm 
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One other interesting point that can be made about drag racing...

Notice how in my post above when the physics teacher said that torque is good for the start of a 1/4 and power is good for the rest, he added that this assumes the tires were hooking up. By saying this he avoided some of the other complexities of drag racing.

A 1/4 mile pass can be broken down into two parts - the traction limited part at the start, and the power limited portion that follows. In my car for example, I cant apply all the engines torque of the line because I will just spin the wheels and go nowhere - I am traction limited. If I further increase low end torque without increasing traction, I will go no faster because I will have to use less throttle so I get off the line cleanly.

Instead, if I leave the engine alone and increase traction with some fatter rubber or an LSD, I can reduce the traction limited portion of my 1/4 run, and my ET will drop.

Its all pretty obvious when you think about it, but it helps you picture how all the parts of the car have to work together to get you down the end fast.

 

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 Post subject: way to much torque
Posted: Mon Jul 25, 2005 8:16 pm 
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stockstandard wrote:
One other interesting point that can be made about drag racing...

Notice how in my post above when the physics teacher said that torque is good for the start of a 1/4 and power is good for the rest, he added that this assumes the tires were hooking up. By saying this he avoided some of the other complexities of drag racing.

A 1/4 mile pass can be broken down into two parts - the traction limited part at the start, and the power limited portion that follows. In my car for example, I cant apply all the engines torque of the line because I will just spin the wheels and go nowhere - I am traction limited. If I further increase low end torque without increasing traction, I will go no faster because I will have to use less throttle so I get off the line cleanly.

Instead, if I leave the engine alone and increase traction with some fatter rubber or an LSD, I can reduce the traction limited portion of my 1/4 run, and my ET will drop.

Its all pretty obvious when you think about it, but it helps you picture how all the parts of the car have to work together to get you down the end fast.


hi

just thought i would throw my two bobs worth in this convo

sorry if i missed something earlier cause i havent read the whole posting

but the the emphasis is all on torque ??????

remember one thing torque drops of as engine speed (RPM ) increase ??? WHY :wink: ???

now my little question since we on about drag racing and so forth is
do u change gears down the quarter mile at max torque ??? or do you change gears at max horsepower??? and when your in top gear what should you be crossing the line at max torque or max horsepower ???


just my few questions and im sure someone from St Albans can answer the question PMSFL :wink:

cya

 

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