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      10-29-2012, 01:47 PM   #25
highrevs
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Drives: 2007 335xi
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Quote:
Originally Posted by THE-FN-MAN View Post
Is the new GT500 a lot more aero dynamic than the previous model? I've seen (actually rode in) my buddy's stock Vette vs a pulley/tuned '11 cobra and the Vette had the advantage as the speeds climbed.
I guess you're in luck because I'm an aerospace engineer stuck at home in the middle of a hurricane!

Estimating the aerodynamic characteristics of any car is a huge challenge. Typically we're talking absurdly expensive computer models, wind tunnels, and a lot of man power and time.

But there are simpler models that can usually give us the "90% answer". In this case we want to look at drag force. Like others have said, the drag coefficient is just one part of this equation. The other critical piece is cross-sectional area; imagine putting a 5 ft by 5 ft piece of cardboard against the wall and swinging it outward as fast as you can; going to be a lot more difficult than the same with a 1 ft by 1 ft piece of cardboard (and this is basically independent of drag coefficient since it's the same for both cases, same material and shape). To estimate a car's cross-sectional area we can multiply width and height.

Attachment 1 has the formula for drag force (think that's actually from one of my old papers); ignore the wind velocity part - that application was crosswind induced lateral movement of aircraft; in this case velocity of the car plugs right in assuming no head wind. From there we need the drag coefficients and dimensions for both cars. Couldn't find a reference for the GT500 drag coefficient so I just went with the value (0.35) that Peteypab listed. Everything else was easy to locate. I plugged everything into Excel (attachment 2).

Looking at attachment 3 (plot of drag force vs. speed up to 160 MPH for both cars), there doesn't appear to be a huge difference. At 160 MPH the difference between the two cars is less than 100 lbs of air resistance. You can definitely see the effect of velocity squared in the equation though.

Here's where it gets interesting. Looking at the difference in drag force (attachment 4), it also seems negligible, e.g., 30 lbs of difference at 100 MPH. That's deceptive though. What really matters is the area under this curve - that's the cumulative amount of air resistance that a GT500 accelerating from 0 to 160 MPH will experience beyond that of the Z06 doing the exact same thing. To estimate that I just did a polynomial regression curve fit in Excel (top equation in plot, to define the curve) and then calculated the integral (area under the curve, bottom equation). So how much more cumulative drag force will the GT500 experience from 0 to 160 MPH? Roughly 4000 lbs. Or to imagine it another way, lifting a CTS-V.

You can look at that two ways though. The Z06 is more aerodynamic, experiencing 4000 lbs less drag force over the course of a 0 to 160 MPH run, enabling it to be almost equally as fast with less power. Or alternatively, that the GT500 is such a brutal monster it plows through an extra 4000 lbs of force against it and ends up being as fast at the Z06.
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