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How much weight loss = 1 (theoretical) HP?

Is there an equation if all things stay the same on your car?
How much weightloss = 1 HP? Is this even true?
For instance if I removed 50 pounds from my car and changed nothing else would I gain the performance advantage of + or - 1 HP? Am I insane?
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Old 05-22-2003, 06:21 AM
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If your car weighs 2300 lbs, and has presently 215 hp, for example, then you have 10.75 lbs for each hp to lug about.
Therefore, it would seem that in this case removing 10.75 lbs from the overall weight OR adding 1 hp to the engine output would have roughly the same effect on performance.
Lighter cars will handle better, assuming same COG, tires, suspension settings, etc.. so removing weight should be BETTER than adding hp at the track...
How does that sound???
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Old 05-22-2003, 06:37 AM
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You will gain, but it's better to think in percentage as opposed to actual values. For instance, 10% power gain can be seen (not exactly but roughly) the same as a 10% weight loss. So if the car had 300 hp and was 3000lbs for instance, a weight loss of 300lbs (which is significant) would be about the same as a 30 hp gain. If you want to think of it more accurately, use proportions (which is how power to weight ratios are figured out) like 3000lbs/300hp would essentially (assuming perfect world variables like gearing) be the same as 2000lbs/200hp. But the thing about wieght loss is that acceleration is just one by product. Deceleration, quicker handling reaction, less stress on brakes for a given amount of deceleration are some of the other advantages.
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Old 05-22-2003, 06:37 AM
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In addition, weight is never really equivelant to hp. Take two cars with the same power to weight ratio, the same power curves, and aerodynamics- one is lighter with lower hp, the other is heavy but big hp. The lower weight car will most likely pull ahead, due to its lesser inertia (it can put power to the ground better). The higher hp car will catch and surpass the lower hp car when speeds get up to the point where wind drag become significant.

Brad
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And then the lighter car will out-corner it.
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Playing around with cartest, using their 911SC profile, I had to reduce the test weight by 180lbs to achieve a 1/4 mile time equivalent to adding 10hp and 10lb-ft, with everything else held equal and allowing the software to figure out the power curve based on the changed peak numbers.

So that's 18lbs to lose for each horsepower.

When you increase horsepower in the real world, the peak rpms and shape of the curve always changes unless you increase displacement or have a constant pressure supercharger or something. So a more realistic number from is probably 1hp per 10-15lbs lost.
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Old 05-22-2003, 07:17 AM
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Not all weight is the same. 1# from the flywheel is more significant than 1# from the tires which is more significant than 1# from the rear bumper which is more significant than 1# from the front bumper which is more significant than 1# from the seats.

Just remember this mantra from the Church of Light weight "Lighter is Better"
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Old 05-22-2003, 11:47 AM
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An old drag racing saying is:

"For every 100lbs of dead weight you lose you gain a 10th of a second in the quater mile."

The time increases if its reciprocating weight ie. wheels, flywheel, driveshaft.
Old 05-22-2003, 12:23 PM
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Not all weight is created equal, and none of this is linear.

Seems like you could come up with a set of functions to calculate theoretical HP gains by removing weight from different areas.

I also wonder - should we be thinking about the gain in HP, or the gain in Torque?
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Old 05-22-2003, 01:23 PM
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1/10th per 100, huh? Here's what Cartest says for the hypothetical 911SC:

2950lb total weight = 15.1sec in the 1/4 mile
-100lbs = 14.9
-200lbs = 14.8
-300lbs = 14.6
-400lbs = 14.4
-500lbs = 14.2
-600lbs = 14.0
-700lbs = 13.8
-800lbs = 13.8
-900lbs = 13.6
-1000lbs = 13.5
-1100lbs = 13.3
-1200lbs = 13.1
-1300lbs = 12.9
-1400lbs = 12.7
-1500lbs = 12.5
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I have an '86 911 and would like to drop weight. I removed the spare and jack/tools (50-60lbs). I also tried to reduce battery weight (20lbs). I even removed the headlight wash and filled and painted the front bumper. I know front end weight only.

I would like to lose more from the rear and middle. The exhaust and seats seem to have some potential for saving. The seats are in need of recovering so replacements would help save weight (recaro without motor). I was told the exhaust can save 50-60 lbs. Can I knock off another 150 lbs to get down to 2500 lbs total?

I even thought of totally removing the sunroof (it doesn't work). Where can I save weight without losing protection (fiberglass would not be as strong for the street).
Old 05-22-2003, 03:18 PM
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Old 05-22-2003, 03:18 PM
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