Douglas Fir wrote:

Sorry Paul,

I wasn't trying to pull a fast one on ya.  I was trying to get a feel of the
car's  performance at fast cruise.  But now that you brought it up what is
your guestimate of the Hp you are using on a flat stretch of road.

I didn't get the video clip.  Probably because my box was full of pictures
of the garage.  Here is what a  13A looks like assembled and another one
compared to a 13B.  The 13A is on the left and quite a bit bigger.

Please send the video clip again

Doug in Japan

Aero Drag = Cd X Frontal area  X dynamic pressure.

Cd = drag coefficient for the car. Currently right around .3

Frontal area of cars are usually 80% of the height times width.

Dynamic pressure at sea level = .0026 X MPH^2 (26 pounds per square foot at 100 MPH)

Or .0012 X FPS^2

As a rule of thumb the aero drag and the rolling drag are equal at 50 MPH
for most cars. Marks Mech Eng Hand Book has some actual rolling drag coefficients 
for rubber tires on concrete.

The rolling drag increases directly with speed while aero drag increases
with the square of speed. Add both together at the speed you are interested in.

HP is a force (drag) times a velocity. 550 pounds going one foot per second
is one HP. 

You now have all the info you need :)

As a guess, without running the numbers, the HP is in the range of 30 to 
40 HP at 60 to 80 MPH for the RX8.

Let us say it gets 25 MPG at 60 MPH. 2.4 gallons burned in one hour. 

14.4 pounds of fuel burned in one hour. 

BSFC = .5 pound of fuel burned per HP generated. 

Therefore 28.8 HP.

Paul Lamar
 
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