Writeup: How to determine approx. HP gains from removing weight!
Okay, want to know how to figure out how many lbs = hp? It's quite easy really! Follow these steps, and you'll be able to estimate "hp gains" from removing weight from your Focus!
Car Weight / HP = pounds per HP or how many pounds you take off to get 1 hp ((Weight/HP Data Taken from Edmunds.com)) Stock 2003 ZX3 Zetec: 2598 lbs 130 hp 2598lbs / 130hp = 19.9 lbs/hp For each 19.9 lbs you remove from your car, you will be getting the equivalent of 1 hp. HOWEVER, when you remove weight the gross weight of your car reduces (obviously) so to figure out how much you need to remove again, you must replug the numbers. Example: 2598lbs / 130hp = 19.9lbs/hp Remove 200 lbs? That gives you a gain of approx. 10hp. Now, let's say you want to remove more weight. 2598lbs  200lbs = 2398 lbs, your current weight. Since you haven't actually increased your engine HP, we are still using 130 as our number. 2398lbs / 130hp = 18.4 lbs/hp, so now lets say you remove 200lbs more. Now you've gained an approx. 11 HP. How do you gain HP from taking off weight? In reality you aren't, but since you're car is lighter, it's easier for your engine to move it. Instead of thinking of it as "gaining HP" think of it as "allowing more HP to be used to increase your cars speed/accel instead of trying to get it to move." A good example is pushing your friend in a shopping cart! (If that's what you do in your spare time. [thumb]) Your skinny friend is really easy to get going fast, whereas if you push just as hard, your fat friend will take longer to get going the same speed. By making the fat kid go on a diet, it would make him easier to get going faster even if you were pushing just as hard! So now you're thinking, "400 lbs = 21hp? Alright!" here's food for thought. Average AC in a car = 80 lbs Average Backseat = 40 lbs Average dampening tar = 15 lbs  Total? = 135 lbs, or about 7 hp from stock. Now the real question lies, is it worth taking away your backseat and your AC and making your car sound like a tin can for 6 hp? Not for me! [thumb] Taiden [ Added 11/8/05 9:23 EST ] Figuring Sprung Weight Equivalents for Rotational Mass Quote:
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The 1:3 Ratio of Unsprung Rotational Weight may just be referring to the rotational properties, and ignoring the fact that scale measureable weight is also being removed. To answer your question though, assume that you just use the ratio flat out without any conversions, and you'll be close. Want an example? Great! Here's one. 2600 lb Car 30 lb Flywheel stock (I have no idea what the actual weights of Flywheels are, I'm just guessing here.) 20 lb Flywheel upgrade Since we're losing weight on the flywheel, that's considered Driveline Rotational Weight, which is a 1:15 ratio. This means, take the weight, multiply it by 15, and that's how much "normal weight" you're removing. 30 lb stock 20 lb upgrade  10 lb loss in DRIVELINE ROTATIONAL WEIGHT 10 lbs x 15 = 150 lbs Sprung Weight Equivalent. This applies to accel/quarter mile based equations only, not handling/slalom! When you're thinking about handling, consider it to be 10 lbs Sprung Weight. Thank you Carrera for those Ratios! [thumb] 
LOL, nice point.

agreed, never understood the taking out spare tires/back seats, lol
Thats like leaning forward going downhill on a girls bike, you make look like you are going faster, but just look like a [limp] 
Weight reduction will improve performance, but not HP.

^^^ exactly.

And don't forget the ratio of weight loss.
Sprung weight = 1:1 ratio Unsprung Rotational weight = 1:3 ratio Driveline Rotational = 1:15 ratio Not to mention the much more impressive HANDLING benefits that result from strategic weight loss. Losing 80lbs of AC might not be a ton of help in the 1/4 mile, but it will make a very noticeable difference in handling. (Especially frontend response and grip) 
Those calculations are silly. If I take 1,000 lbs off of that 2,600 pound car, its going to be faster than a 50 horsepower difference. My bike weights something like 400 lbs and has 96 horsepower. If I took off 2198 lbs to make that focus 400 lbs then that would be a 110 hp increase? Sorry but a 240 hp 2,600 pound car isn't gonna keep up with my bike, but a 130 hp 400 lb. car would beat the hell out of it. Those calculations might make sense for maybe the first 100 pounds you lose.

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Example: if your car is 1000lbs and you remove 100lbs, and then another 100lbs you will get a different amount of HP "increase" with the equation than if you take 50 lbs off and then 150 lbs off. The more you rerun the equation and add up the gains, the more accurate you will be. If you run the equation for every 1 lb you remove, it will be accurate and make more sense. I hope that made sense. [8] To summarize, when your weight changes so does your lb/hp ratio, so everytime you change the weight you must run the equation a second time. To make your math more accurate, if you run the equation for each pound you remove it will be far more accurate. [thumb] 
what about rotational mass like wheels and brake rotors. how would you figure that out, cause its rotating and changes with the speed of your car.

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