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This design does a better job of harnessing that high-pressure area. The lip protrudes lower than stock and keeps air from going under the car. It also keeps air from slipping in from the sides, whereas the stock design actually encourages air to slip in from the sides. In most cases, a design like the one we are discussing results in both a better Cd (reduced friction from under the car) and more downforce (because the underbody area is at lower pressure). The front surface also looks to have a rearward slant to it, which will encourage more air to go over the top of the car. It isn't clear to me that the ducts in front will do anything to increase Cd, even if the front lip did not protrude lower. The main area you want to be slippery and smooth is at the edges. The edges in front of the tires and at the nose panel are still smooth with this design. This is all assuming that this design has more duct area than J-spec, which isn't clear to me. Let me point out the dimple in front of the rear tires. The rear tires create a lot of turbulence. Although this dimple creates its own turbulence, it will push the air around the rear tires a bit. It looks to me like there will be a net improvement in Cd. Finally, the sides of the rear valance end abruptly, which is better than rounded corners at the rear. I can't really tell if the rear diffuser is deep enough to do much, but in principle it is a very good thing to have. If you are a drag racer or a person worried about fuel economy, this is not an optimal design; you would seal up the ducts you mention to the degree possible. But for a road racer wannabe like me, it seems to do many things that I like. It even has a bit of a notch in the front lip contour, so that the downforce won't change radically under braking (air flow under the car won't be suddenly cut to nearly zero if the car pitches forward a bit). Cars without this notch can be very unpredictable; it is a very nice touch to include this. Without actual data, I can't tell how much aero improvement would actually be achieved with this kit, but it is clear this designer is using or at least aping designs that really work, instead of designs that merely "look fast".
- John
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12:27:36 04/16/07
