[bfrank1972]

Originally Posted by gtengr

What I did was set up a spreadsheet that compares speed vs. drag vs. horsepower required in 5 mph increments (and put rolling resistance and associated power required on the same sheet), then looked at the Cd of other production cars to make inferences about what aggressive aero might do on the BRZ. For example, the Cd difference between a stock Viper and the Viper ACR is 0.37 vs. 0.54, an increase of 0.17. Knowing how aggressive the aero is on an ACR, I estimated that a BRZ with a big wing + splitter might be around 0.27+0.17 = 0.44. Plug that into the spreadsheet to observe the difference in horsepower required vs. stock, and usually that ballpark answer is enough to satisfy my curiosity. There are more caveats than this, since Cd isn't the same at all speeds, ride height, etc, but those are the basics.

If you prefer a road test, you could install the aero mods and then re-measure the top speed with a precise instrument (not the speedo). If you know the new drag limited top speed, have a dyno graph of your rwhp, and have checked your spreadsheet with the stock top speed to ensure your rolling resistance power estimate is close, then you can back-calculate how much power was lost to aero at this new top speed, and from that back-calculate the associated Cd with the aero on.

Or you could download this

http://www.cartestsoftware.com/