What's the formula for HP savings with every lb taken off our car?
08-12-2001, 11:43 AM
#1
I know there's a formula to calculate HP saved / freed up by making out car lighter ... which is why people use a lighter flywheel or UDP or carbon fibre hood!
but what is it?
I'm contemplating the carbon fiber hood ... 25 lb savings ... is that really a big deal? I could just go on a diet!
but what is it?
I'm contemplating the carbon fiber hood ... 25 lb savings ... is that really a big deal? I could just go on a diet!
08-12-2001, 03:01 PM
#2
Member
Join Date: Oct 2000
Posts: 51
I suspect the formula that you are looking for is the one which shows the effect on ET of a change in HP or weight. You can play with it mathematically by holding the ET constant and watching the effect on HP of the weight reduction
The relationship between HP, weight and ET is as follows;
HP = (weight)/(ET/5.825)^3 ......that would be to the third power or cubed on the last term
As an example if one were to ask the question how much HP would it take to propel a 3440 lb car to a 14.05 ET, the above equation would net 245HP.
BTW, all of that has nothing to do with why people lighten flywheels. That has to do with overcoming rotating inertia and is a whole different phenomenon.
The relationship between HP, weight and ET is as follows;
HP = (weight)/(ET/5.825)^3 ......that would be to the third power or cubed on the last term
As an example if one were to ask the question how much HP would it take to propel a 3440 lb car to a 14.05 ET, the above equation would net 245HP.
BTW, all of that has nothing to do with why people lighten flywheels. That has to do with overcoming rotating inertia and is a whole different phenomenon.
08-12-2001, 03:07 PM
#3
hmmm ... wow ... more info than I was expecting but that's gr8! I was looking 4 the more gen'l formula ... like 1 lb = 2 hp savings etc ...
Originally posted by Gerry
I suspect the formula that you are looking for is the one which shows the effect on ET of a change in HP or weight. You can play with it mathematically by holding the ET constant and watching the effect on HP of the weight reduction
The relationship between HP, weight and ET is as follows;
HP = (weight)/(ET/5.825)^3 ......that would be to the third power or cubed on the last term
As an example if one were to ask the question how much HP would it take to propel a 3440 lb car to a 14.05 ET, the above equation would net 245HP.
BTW, all of that has nothing to do with why people lighten flywheels. That has to do with overcoming rotating inertia and is a whole different phenomenon.
I suspect the formula that you are looking for is the one which shows the effect on ET of a change in HP or weight. You can play with it mathematically by holding the ET constant and watching the effect on HP of the weight reduction
The relationship between HP, weight and ET is as follows;
HP = (weight)/(ET/5.825)^3 ......that would be to the third power or cubed on the last term
As an example if one were to ask the question how much HP would it take to propel a 3440 lb car to a 14.05 ET, the above equation would net 245HP.
BTW, all of that has nothing to do with why people lighten flywheels. That has to do with overcoming rotating inertia and is a whole different phenomenon.
08-12-2001, 03:08 PM
#4
Senior Member
Join Date: Jun 2001
Posts: 778
I've been told that for all intents and purposes, lightening your car by 100lbs. will shave .1 second off a 1/4 mile run. Now, losing 25 lbs off the front end will help to brign your car into better 50/50 weight balance and give it more neutral handling...
08-12-2001, 03:10 PM
#5
Hey Bud,
Is that a new ride? I thought u had a 3rd or 4th gen?
Is that a new ride? I thought u had a 3rd or 4th gen?
Originally posted by Ironlord
I've been told that for all intents and purposes, lightening your car by 100lbs. will shave .1 second off a 1/4 mile run. Now, losing 25 lbs off the front end will help to brign your car into better 50/50 weight balance and give it more neutral handling...
I've been told that for all intents and purposes, lightening your car by 100lbs. will shave .1 second off a 1/4 mile run. Now, losing 25 lbs off the front end will help to brign your car into better 50/50 weight balance and give it more neutral handling...
08-12-2001, 03:24 PM
#8
Originally posted by Gerry
Generalizations are not linear, which is why there is a cubed term in the formula, but if you knock 25 lbs off the example above, the HP changes by about 1.5
Generalizations are not linear, which is why there is a cubed term in the formula, but if you knock 25 lbs off the example above, the HP changes by about 1.5
08-12-2001, 03:34 PM
#9
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Join Date: Jun 2001
Posts: 778
Originally posted by ohboiya
that's f&cking it? I get a 1.5 hp savings for buying a $450 carbon fiber hood?
that's f&cking it? I get a 1.5 hp savings for buying a $450 carbon fiber hood?
And gerry, i know its not perfect, but everything i've been told is that its pretty d*mn good and really close to reality...
08-12-2001, 03:42 PM
#10
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Join Date: Oct 2000
Posts: 51
Yes, I realize the 0.1 sec per hundred lbs is a pretty good estimate. I was just trying to address his original question where he asked about a formula relating weight to HP. Being an Engineering nerd, I couldn't help but cough up the proper math. Its a personal weakness........
08-12-2001, 03:56 PM
#11
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Join Date: Jun 2001
Posts: 778
Originally posted by Gerry
Yes, I realize the 0.1 sec per hundred lbs is a pretty good estimate. I was just trying to address his original question where he asked about a formula relating weight to HP. Being an Engineering nerd, I couldn't help but cough up the proper math. Its a personal weakness........
Yes, I realize the 0.1 sec per hundred lbs is a pretty good estimate. I was just trying to address his original question where he asked about a formula relating weight to HP. Being an Engineering nerd, I couldn't help but cough up the proper math. Its a personal weakness........
08-12-2001, 04:25 PM
#12
Originally posted by Ironlord
I've been told that for all intents and purposes, lightening your car by 100lbs. will shave .1 second off a 1/4 mile run.
I've been told that for all intents and purposes, lightening your car by 100lbs. will shave .1 second off a 1/4 mile run.
08-12-2001, 05:12 PM
#14
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Join Date: Jan 2001
Posts: 3,628
are these actually accurate? theres a lot of things that take place in using that ET forumla. Auto/Manual is one variable, How tall/short the gearings are, the power band of the engine, BLAH BLAH. so how can you guys assume that this many lbs will take off this much on your ET? and you have to put into consideration of your Rims too... someone correct me if im going onto my little world of mine.
08-12-2001, 05:31 PM
#15
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Join Date: Oct 2000
Posts: 51
The relations are correct from a perspective of basic physics, and are just variations of the basic equations of mass and acceleration. They are from the "Auto Math Handbook" which is a well regarded reference by most serious car folks. In fact, that is the same basic equation that is going on inside a g-tech pro or similar device.
You are quite correct that there are other variables which will have effects and muddy the waters. But the equation is generally accurate for the case in question, where you are making one change (ie. weight) to a given car. They would be much less reliable if used to compare, say et's for two very different designs. It is certainly more accurate than most of the "general rules of thumb". Its not perfect, but useful.
You are quite correct that there are other variables which will have effects and muddy the waters. But the equation is generally accurate for the case in question, where you are making one change (ie. weight) to a given car. They would be much less reliable if used to compare, say et's for two very different designs. It is certainly more accurate than most of the "general rules of thumb". Its not perfect, but useful.
08-13-2001, 10:27 AM
#18
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Originally posted by Gerry
I suspect the formula that you are looking for is the one which shows the effect on ET of a change in HP or weight. You can play with it mathematically by holding the ET constant and watching the effect on HP of the weight reduction. . . .
I suspect the formula that you are looking for is the one which shows the effect on ET of a change in HP or weight. You can play with it mathematically by holding the ET constant and watching the effect on HP of the weight reduction. . . .
I've used the following formula for estimating 1/4 mile mph
mph = 225 * (hp/weight)^(1/3) (Formula 1)
Solving for hp required as a function of weight gives the following:
hp = weight * (mph/225)^3 (Formula 2)
So removing 25 lbs from a 245 HP car that originally weighs 3440 lbs (predicted to run 97.12 mph in the 1/4 using Formula 1) is about the same as increasing the HP by 1.7 or so. Or with the same HP, a trap speed of 97.34 mph instead of 97.12 mph. [edited for consistency]
One of the magazines published both of these formulas recently, and I think they used a number slightly different from my 225 in the one for trap speed, which might change the equivalent HP by a tenth or so. Like what's been said, it's approximate.
Norm
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