HP versus weight? - Chevelle Tech
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post #1 of 12 (permalink) Old May 11th, 07, 5:53 PM Thread Starter
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Patrick
 
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HP versus weight?

Was wondering if there is any formula out there that can calculate (or come close) to figuring out weight reduction as it relates to horsepower. Ie, if I were to take 50 or 100lbs of weight off my car, about how much more hp would the car "seem" like it has? Or maybe a better way of asking is this: Your car weighing 3200 lbs ets at exactly 12.00 secs in the 1/4 mile. Now if that same car, only this time weighing 100 lbs more (3300 lbs)(everything else being equal but the weight) ets at exactly 12.00 secs, how much extra horsepower would you need to make the heavier car et the same as the first? Does lets say an extra 100 lbs make a 400 hp motor perform like maybe a 380 hp motor?

Patrick
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post #2 of 12 (permalink) Old May 11th, 07, 6:07 PM
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Re: HP versus weight?

Try here. http://www.slowgt.com/Calc1.htm

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post #3 of 12 (permalink) Old May 11th, 07, 7:07 PM
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Re: HP versus weight?

in the magazines, they used to say that 100 pounds is worth a tenth in the 1/4.
of course, 100 pounds is a lot less % weight loss on a 4000 pound car than it is on a 3000 pound car, so will probably be worth a little less at the track. but, either way, 100 pounds is 100 pounds..

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post #4 of 12 (permalink) Old May 11th, 07, 7:41 PM
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Re: HP versus weight?

Quote:
Originally Posted by furball8994 View Post
I don't know about you guys, but I found the equation there for ET based on weight and hp, to be quite optomistic. In other words, it shows cars being a good bit quicker than they are in the real world.

A couple of years ago, I generated an equation based on 15 to 20 real world sportsman gasoline drag cars. My equation is "the cube root of (weight divided by hp), then all that multiplied by 6.1", which will give you the approximate best ET. Of course it is impossible to be absolutely exact for a specific car, but it does work out to be a fairly close approximation for reference and planning.

If you use that equation, it shows that a 400hp, 3200 lb car should be able to run about 12.2 in the quarter. If the car's weight is reduced to 3100 lbs, the ET should be 12.07, or a .13 second improvement.
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post #5 of 12 (permalink) Old May 11th, 07, 8:09 PM
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Re: HP versus weight?

Quote:
Originally Posted by 540 RAT View Post
I don't know about you guys, but I found the equation there for ET based on weight and hp, to be quite optomistic. In other words, it shows cars being a good bit quicker than they are in the real world.

A couple of years ago, I generated an equation based on 15 to 20 real world sportsman gasoline drag cars. My equation is "the cube root of (weight divided by hp), then all that multiplied by 6.1", which will give you the approximate best ET. Of course it is impossible to be absolutely exact for a specific car, but it does work out to be a fairly close approximation for reference and planning.

If you use that equation, it shows that a 400hp, 3200 lb car should be able to run about 12.2 in the quarter. If the car's weight is reduced to 3100 lbs, the ET should be 12.07, or a .13 second improvement.
I agree its a little optomistic. I think its based on a "perfect" sealevel run.
The interesting thing is, As in "real world" The faster the car, The less gain you see from the 100# loss.

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post #6 of 12 (permalink) Old May 11th, 07, 8:16 PM
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Re: HP versus weight?

A mag did an article years ago called "Caddy Hack" where they took a DeVille and chop sawed it to see just what loss of weight would show up like on the strip!!

I think they cut on the poor old Cad from 5500+ pounds down to 2800 or so pounds (??) and when I mean cut, chop saw they CUT!!

I wanna say from 17.+ seconds down to 13.+ seconds in the quarter!!

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post #7 of 12 (permalink) Old May 11th, 07, 8:38 PM
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Re: HP versus weight?

Quote:
Originally Posted by 540 RAT View Post
I don't know about you guys, but I found the equation there for ET based on weight and hp, to be quite optomistic.
That depends on the car and how it's setup. The mph is right on for my car and the et is actually ~.2 soft.
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post #8 of 12 (permalink) Old May 11th, 07, 11:45 PM
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Re: HP versus weight?

I don't think its exactly 100 # to 0.1 second. If I remember right is something like to the 1/3 power square root. Here is what I get for going from 3200, to 3100 to 3000#

11.77

11.65 - change of 0.12 per 100#

11.52 - change of 0.13 per additional 100#

http://www.virtualengine2000.com/Calculator.htm

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post #9 of 12 (permalink) Old May 12th, 07, 1:09 AM
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Re: HP versus weight?

Quote:
Originally Posted by pdq67 View Post
A mag did an article years ago called "Caddy Hack" where they took a DeVille and chop sawed it to see just what loss of weight would show up like on the strip!!

I think they cut on the poor old Cad from 5500+ pounds down to 2800 or so pounds (??) and when I mean cut, chop saw they CUT!!

I wanna say from 17.+ seconds down to 13.+ seconds in the quarter!!

pdq67
that was Hot Rod, back in the late 80's.
they started off with a 500 powered luxu cruiser, and ended up with a 500 powered frame with a seat and a steering wheel.
i think they were gonna do it again a couple of years ago, but no track wanted the liability..

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-Hank Hill
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post #10 of 12 (permalink) Old May 12th, 07, 1:11 AM
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Re: HP versus weight?

and who can forget the Chevette that Magnante built with a bone stock junkyard Caddie 500/TH400 crammed into it..
i'm thinking it went into the 11's.

"Detroit hasn't felt any real pride since George Bush went to Japan and vomited on their auto executives, I tell you whut"

-Hank Hill
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post #11 of 12 (permalink) Old May 12th, 07, 8:08 AM
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Re: HP versus weight?

Every combination is gonna be different. In my Malibu, I have a 100 lb ballast bar in the rear bumper. I've taken it in and out several times and not seen any difference in ET.

This past winter I did aftermarket disc brakes, lighter front wheels/tires and a smaller fuel cell all of which took 100 lbs off the car with over half of that being unsprung and rotating weight. I saw zero difference on the ET slip.
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post #12 of 12 (permalink) Old May 12th, 07, 8:14 AM
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Re: HP versus weight?

I think, now this is just my opinion but it seems the faster my car has gone, the theories don't hold. Maybe in the average street car 100 lbs is a tenth, but in my car now 100 won't get me a tenth.

Ray

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