G Shaw wrote:
>
> Hi
>
> I know that the engineers on this list will have an answer for this. Last
> night on a repeat of "Junk Yard Wars" an expert said that the power to
> weight ratio that we all know is so important in making cars go fast is not
> a simple relationship of dividing one number by the other. He said that as
> the weight doubles the power need quadruples. If this is so then a 200 hp
> 2000lb car would be twice as fast as a 400 hp 4000lb car, no? Is this true?
>
> Greg
Greg,
The statement is correct, in a limited range of comparisons. It really
describes the acceleration "potential" of two different vehicles. "Goes
faster" has a limited meaning.
For two cars that are OTHERWISE identical, in terms of gearing, and
power train losses, the statement is true for acceleration to speeds, IF
rolling friction and air friction are ignored. Rolling friction, at low
speeds, over a limited range of weight differences is probably close.
aerodynamics will NOT be the same, unless the frontal areas and shapes
are identical, and the weight is in lead bricks, not larger bodies. Even
so, at speeds up to 40-50 mph, the air resistance does not become a
large factor.
So, for two IDENTICAL cars, with one having a more powerfully tuned
engine, (not a much bigger, heavier, engine) the ratios are
approximately correct. Any tire spin will change everything. Any
dissimilarities in the shape of the power curve can lead to periods were
one engine is climbing in power, and the other is going flatter, making
the relationship momentarily non precise.
So, just like "there is no substitute for cubic inches", that
relationship depends on the often non stated "all other things being
equal, and neglecting frictional effects."
Such things as different gearing can change everything, even in
identical, but one heavier car, or one more powerful car, or any
combination of the above.
I hope that "clears" it up.
A good General Rule is "don't generalize". {9->
Steve
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