Write-up: How to determine approx. HP gains from removing weight! - Ford Focus Forum, Ford Focus ST Forum
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Old 08-11-2005, 01:04 PM   #1
Taiden
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Join Date: Aug 2005
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Location: Portland, Maine
What I Drive: 94 Grand Cherokee 55k mi, standard

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Post Write-up: 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 ZX-3 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. ) 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!

-Taiden






[--- Added 11/8/05 9:23 EST ---]
Figuring Sprung Weight Equivalents for Rotational Mass


Quote:
Originally posted by havoc7476
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.
Well, it depends on how Carrera is figuring these ratios. Carrera, in the rotational weights, are you including the subtraction of the raw weight itself?

Quote:
Originally posted by Carrera26
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
Example:

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!


__________________
I doubt anyone on a Focus board would be interested, but I'm selling a '94 Jeep Grand Cherokee with 55k miles, all power options and a manual tranny. PM for details. :)

Last edited by Taiden; 08-11-2005 at 08:37 PM.
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