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Discussion Starter · #1 ·
Well, I was really bored at work today and started thinking about cars. My random thought of today was on how HP/PS/KW/(Power in general) is the most common measurement of a car's performance, but yet how unsatisfactory such a measurement is for really comparing the performance of two cars. Everyone knows that Power to Weight is a more accurate measure by which to compare two cars' performance, yet this measure seems to be mostly sidelined in favour of raw HP/PS numbers.

So I then started thinking about how I could convert/represent weight savings (in terms of kg) into equivalent HP figures. That is, how much HP a carbon fibre bonnet could "gain" for your car.

Here's the algorithm I came up with, I am wondering as to its logical correctness:

1) First we need to start up with some base stats, from which to base our gains. Everything is relative after all. Your current car's weight and power would be good numbers to use here:

Benchmark Weight (kg): 1260kg
Benchmark Power (rwhp): 245ps
Benchmark P/W ratio: 0.1945 ps/kg

2) Now, let's get some exemplery weight savings. I got these from jekyl&hyde's website.

Forged Wheels: 36.36kg
CF Doors: 18.18kg
CF Bonnet: 18.18kg

3) Now the algorithm. First, let's assume we got a new set of forged Wheels which saves us 36.36kg.

New weight: 1260 - 36.36 = 1223.64 kg
New power: 245 (unchanged)
New P/W ratio: 245 / 1223.64 = 0.200222

Comparison of New P/W ratio with Benchmark P/W ratio:
New P/W - Benchmark P/W = Increase in P/W
0.200222 - 0.194444 = 0.00577929 ps/kg

4) Representation of this increase in P/W ratio as PS/HP:
Power / Weight = 0.00577929
Power = 0.00577929 * Weight
Assuming weight is the same (i.e. you did a power mod instead of a weight saving mod), the corresponding increase in power is 1260 * 0.00577929 = 7.28 ps

5) Conclusion:
Installing a set of forged wheels, saving you 36.36kg in weight, is equivalent to getting a 7.28 ps increase in power.

Put differently, one could state the following in an advertisement and not get sued: "Volks Forged Wheels for sale. These wheels are guaranteed to increase your cars performance by 7hp!!"


Is this a valid conclusion?
 

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Weight savings performance is a very different from pure horsepower additive performance.
 

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Vaeth said:
Is this a valid conclusion?
I would say no. It is not adding any HP so the statement would be false.

Secondly, there are some errors in the power/weight argument. Especially when you are talking about wheels (rotating mass). Since the wheels are part of the drivetrain andthey are unsprung weight (under the springs) they show significantly more performance improvement when lightened than you would see from losing weight elsewhere on the car.
Simply put: losing 20 lbs on the wheels is MUCH better than losing 20 lbs in the interior.

Another thing to keep in mind is that losing weight improves all the physics, not just acceleration. You will also stop faster and handle better with a lighter car.
 

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Lowering weight also matter less than adding horsepower in higher speed applications as well, where aerodynamic forces are the primary resistive forces.
 

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Losing rotaional mass > Losing sprung weight.

:)
 

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Discussion Starter · #6 ·
Perhaps I should not have used Wheels as my example, to avoid the issues of rotational mass and unsprung mass. So let's consider instead 36kg of interior weight, is the conclusion still valid?

I would say no. It is not adding any HP so the statement would be false.
I did not intend to mean that losing weight added HP but rather that it added performance -equivalent- to adding HP (in addition of course to handling performance increase which I am not considering here).

Put differently, if you and a friend had (hypothetically and theoretically speaking) identical cars but he did something to increase his HP by 7hp whereas you decided instead to lose 36kg of weight, would both your cars still be equally fast in a straight line?
 

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No, because he would be a better driver :D

.... seriously, it is an interesting question, but I do not know if your math is right on or not.
 
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