[DeTomaso] Speed at 7,000 R's with

[Ken Green]

Dan,
   
        But how much HP does it take to just spin a 351 at 7000 RPM?  that part of the HP does not drop with altitude.  I think it cound be a large fixed term in the equation.
   
  Ken

Daniel C Jones <daniel.c.jones2 at gmail.com> wrote:
  > On TV this weekend they said that the average motor looses 3% of it HP for
> every 1,000 feet of elevation.

That's due to the air density decrease. At the same time, the aerodynamic
drag is also decreasing for the same reason, so it takes less power to
achieve the same top speed. A first order approximation estimate of a car's
top speed can be computed using the following formula:

/------------
15 / 1100 P
Vmax = ---- \ 3 / -------------
22 \ / Cd S rho
\/

where

P..........Power in rear wheel horsepower
Cd.........Drag coefficient
S..........Frontal Area in square feet
rho........Density of air in slug/cu. ft.
Vmax.......Speed in miles/hour.

If power decreases linearly with density, then the power decrease is
offset by the density decrease and top speed should be unaffected.
However, acceleration (which is dependent upon mass) will be reduced
so it will take longer to get there. When Mad Dog says he can feel
the power loss, it's the reduction in acceleration that he's sensing.
Given a long enough road, he should be able to achieve a similar top
speed but it will take much longer to do so.

Dan Jones
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