HOTRODDERS: We often see the "10lbs is the equivelant to 1HP" claim on websites or in magazines, but that is very misleading. The truth is there is no one weight that = 1 hp, it is a matter of power to weight ratio, and the "weight" part of the equation is a floating value.
As a public service message to the board i have typed up the following explanation to help people understand what's at play, and to more accuately estimate just how much weight really does = 1 HP for their application.
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There is no one 'right' answer as to how much HP you
gain from weight savings, as it is a floating number
based on power-to-weight ratio. For instance, here is
a couple examples to show what i'm talking about:
Vehicle A, 1000lbs gvw, 300hp
Power to weight ratio: 3.33lb/hp
Vehicle B, 2000lbs gvw, 300hp
Power to weight ratio: 6.66lb/hp
Vehicle C, 3000lbs gvw, 300hp
Power to weight ratio: 10lb/hp
Vehicle D, 4000lbs gvw, 300hp
Power to weight ratio: 13.33lb/hp
So by looking at the above chart we can see that for
vehicle A, 3.33lb = 1hp, whereas for vehicle B 6.66lb
= 1hp, for vehicle C 10lb = 1hp, and finally for
vehicle D 13.33lb = 1hp.
The US 83 928S A/T(my car) has a real world curb
weight of 3350lbs, which with a stock 234hp motor
yields a 14.32lb/HP power to weight ratio.
Let's take a 300lb reduction in weight, and figure
out the new power to weight ratio based on a stock
engine:
3350 - 300= 3050lbs / 234HP= 13.03lb per HP.
So in order to 'gain' 1hp we'd need to reduce weight by
13.03lbs. However, let's say we reduce weight by
another 200lbs, well, that changes the
power to weight ratio, and therefore changes the 1HP=
X number of pounds ratio. As i said earlier, is a constantly changing
figure.
So if we deduct another 200lbs we now have a vehicle
weight of 2850lbs, which
with the stock 234HP= a P/W ratio of 12.17lbs per HP.
So at that point 1HP will = 12.17lbs.
As can be clearly seen, as vehicle weight drops, the weight required to equal 1 hp also drops.
See it's not that we're 'adding' power by decreasing
weight, what we are doing is increasing the power to
weight ratio. So as weight is decreased it actually
requires progressively LESS weight to = 1hp.
But let's say we add a free flowing exhaust that adds us 20hp to the above mix. Now our P/W ratio in that same 2850lb 928S US car is 11.22:1.
So at that point 11.22lbs = 1hp.
We'll use my car for a final example based on my
estimates of horsepower(based on actual 1/4 mile ET of
13.67seconds) and weight(estimated at 3050lbs with no driver at the time of the run) using this simple program:
http://www.race-cars.net/calculators/ho ... lator.html
312 crank HP divided by 3050lbs(no driver)= 9.94lbs
per HP.
So for me(at that time), every 9.94lbs in weight reduction is the
equivelant to adding 1 more HP. As i decrease vehicle
weight(or increase actual HP) that value will drop
proportionately.
Hope this helps to clear that issue up for anyone interested.
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Snipers...and targets.</b>
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