Given that uncertainty, how can you choose the best aero kit for your car? Damn fine question. Let's start by examining some aerodynamic basics, and then ask some experts for their advice on how to cut cleanly through the air.
 One of the best ways to evaluate...  One of the best ways to evaluate the aerodynamic properties of a body kit is in the wind tunnel. Here, a Mugen kit is put to the smoke wand test at the JARI wind tunnel in Tsukuba, Japan. Wind tunnel time is very expensive, though, so not all body kit makers have the resources to test this way. |  A tale of two RSX body kits....  A tale of two RSX body kits. The Mugen Aero kit, wind tunnel-tested and shipped from Japan through King Motorsports, increases the Acura's downforce by 168 percent and reduces its drag by 11 percent. The cost for all that engineering? Retail is $4,780. |  This RSX kit from Wings West...  This RSX kit from Wings West does not spring from the wind tunnel, so its exact effect on downforce and drag is unknown. However, the domestically produced kit retails for $1,466, a fraction of the Mugen kit's cost. |
Aerodynamic Basics
Anyone who has stuck their hand out of a moving car has experienced the effects of aerodynamics At freeway speeds you can feel how hard the air is buffeting your hand; imagine how strong the forces are on something as big as your car.
Those forces of wind resistance against your hand-or your car-are called drag. Engineers measure a car's ability to slip through the wind by assigning it a drag coefficient (CD), which is calculated through a mathematical equation. For you math whizzes out there, the CD equation is:CD = D/(1/2xpV2xA), where D is drag, p is air density, V is velocity, and A is the car's frontal area.
The sleekest production cars right now (the '02 NSX, Porsche 911, or Corvette, for example) have CD values in the 0.30-0.29 range. A mid-'80s Honda Civic, on the other hand, was in the 0.37 to 0.39 range. An old-school VW Beetle? Halfway to a barn door at 0.49.
Reducing drag and lowering that CD is one of the main goals of aerodynamic improvements, as getting a car to slice cleanly through the wind has several benefits. Probably the most important one is efficiency: Reduce the drag that's acting on a vehicle and you can reduce the power needed to push it through the air. An engine that doesn't have to work as hard gets better fuel economy, which is why aerodynamics are so important to the new car industry.
There's a flip side to the efficiency deal too. If you can clean up a car's aerodynamics so that it needs less horsepower to push it through the air, there will be more power available to push that car faster. That's why all those land-speed-record cars look more like airplane fuselages than cars.
How do you reduce drag? By smoothing the flow of air over, around, and behind the car. OE designers have come up with all sorts of tricks to lower drag, from reducing a car's frontal area to molding in small winglets under the trunk area to diffuse the air that comes out from underneath it.
But drag is just one aerodynamic factor that's at work on your car. Two other forces you've probably heard about before are lift and downforce. Look at the side view of a modern car and the shape looks something like the cross-section of an airplane wing, doesn't it? Well, if it looks like a wing, you'd better believe that at certain speeds, the air flowing over and under the car is going to make it work like an airplane wing and lift it, somewhat. It may not leave the ground (at least, we hope it doesn't), but there could be enough lift to unload the tires and suspension, which will affect the car's traction and handling.