Dude, but many of us here
try to quantify our mods, though outside of a windtunnel it's only an approximation, as Neil noted. Look and ye shall find. There are a few how-to threads, calculators and spreadsheets on coastdown and rolldown testing. Also, many members have included results in their modding threads (MetroMPG's cardboard boattail is good for a 15% reduction). In some cases it's easier to test the reduction in fuel consumption, which is usually about twice the Cd reduction (very ballpark here).
Quote:
Originally Posted by talldudenumber5
to see what each mod or collection of mods actually decreases the CD by. i was curious if we could attach a reduction number to say" adding a front air dam reduces CD by .2 on average"
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It's impossible to say that "such-and-such mod reduces Cd by 0.02". At best, you can put it in %, but beware that this may not always be true. Every car is different, so the same mod (airdam, bellypan, wheelskirts, etc.) on different cars may have a totally different effect. It also depends on other mods, for example MetroMPG noticed while testing the cardboard boattail that it behaves differently depending on whether he had a bellypan or not, similarly whether the side mirror was folded or unfolded, etc.
Another thing to keep in mind is that a good coastdown session takes
lots of time, so noone wants to test each individual mod and configuration separately.
EDIT: I see Wyatt posted first
I'll add that my rear wheel skirts + 45cm (18") Kammback reduced my Cd from 0.334 to 0.324 (only about 3%), with fuel consuption reduction around 7% @ 70km/h and 8% @ 100km/h. The testing was in less than ideal conditions, I'm still looking for a better place (straight and flat for 4-5km with no traffic).