As the car travels through...
As the car travels through the air, some of the airflow is directed around the nose and to the sides of the front fenders and wheelwells. If the sides of the nose are angled from a top view, the air will flow out and away from the wheelwells, creating a low-pressure area just outside the wheels. The air under the hood is mostly stationary, and some of that air rushes to the wheelwells to fill the void created by this suction. This creates a low-pressure area under the hood, and the pressure differential between the top and bottom creates the downforce needed to provide more grip in the front of the car.
The average atmospheric pressure at sea level is 14.7 psi on all sides of an object, even our bodies. If we reduce the pressure under the hood to 14.5 psi over an area of just a square yard, we would generate about 260 pounds of downforce (0.20 [psi difference in pressure] x 362 [number of square inches in a square yard] = 259.2 pounds [total amount of downforce generated by the pressure differential]).
A counter effect to the air flowing out of the wheelwells relates to the air that flows under the nose of the car. This air will replace some of the air that is being suctioned out of the wheelwells. It also reduces the low pressure and the effect of downforce. That is exactly why we need the front spoiler, or valance, on the nose to be as low as possible. The popular big bar and soft spring (BBSS) setups tend to help this situation by allowing the overall chassis to compress into the track to help eliminate the flow of air under the nose.
Once we have modified the outside of our cars to provide the airflow that will create the low-pressure areas we need, the next step is to try to provide the greatest amount of surface area under the hood and rear deck that would be exposed to those low-pressure areas, thereby increasing the downforce effect. We also want to try to limit the volume of air that could flow under the nose and make what does flow less able to diminish our goal of low pressure by routing it away from our low-pressure areas.
One way to increase front downforce is to open the area under the hood that is available for pressure reduction. One area that we can utilize is the top of the radiator shroud or airbox. Most designs offer a flat top that is built very close to the hood.
We can redesign this structure so that the top of the airbox is curved from a sideview starting at the very top of the radiator. As it comes forward, it is at an angle of 70 degrees off horizontal and then curves forward to intersect the top of the openings in the nose. This shape creates a large cavity under the front part of the hood and nose that is now available for reduced pressure and added downforce.
 A low-pressure area in the...  A low-pressure area in the rear of the car, similar to the front, can be created with a little innovation. Remember the four crew chiefs that were suspended by NASCAR; work in this area was their undoing. They had either accidentally or deliberately created holes where low-pressure pockets could be formed to create rear downforce. This was needed due to the hard compound tires that they had to run. The cars were very unstable in early practice. |  We can modify the shape of...  We can modify the shape of the window posts to either promote the flow of air to the rear spoiler for added downforce at smaller racetracks... |  ...or to route the air away...  ...or to route the air away from the spoiler to decrease drag for larger and faster racetracks. |
If we build the bottom of the airbox wider at the front, less air will flow under the nose and upward to the low-pressure area. With less air being replaced, more low pressure is retained and more downforce is available.
This team built the airbox...
This team built the airbox in front of the radiator with the top angled down as it runs from the radiator to the front grille opening. The old box ran from the top of the radiator to the top of the nosepiece and did not allow enough air space for a low-pressure area to exist. The gain in downforce from this design is estimated at over 180 pounds, and the center of the force is located in front of the front axle.
At the rear of the car, we can manipulate the shape of the spoiler, the rear window posts, and the body just in front of the rear wheelwells. By routing the air that is flowing past the sides of the car out and to the sides of the rear wheelwells, a similar suction effect takes place to create a low-pressure area under the rear deck. There are obvious limits as to body shape in this area, but a little reshaping can help.
Racetracks that require less downforce and tracks where we can benefit from reduced drag will cause us to rethink how the air flows past the roof (greenhouse area) and onto the rear spoiler. If we reshape the post that connects the rear window with the side-window openings, we can direct air away from the spoiler and greatly reduce aero drag.
The rear spoiler produces two effects at the same time. It provides aero downforce generated from low pressure on the underside as well as aero drag, which creates a cantilever effect that redistributes some of the weight of the car from the front to the rear. Many racers mistake this effect for downforce because it tightens a loose car by adding more weight to the rear of the car.
If we put the car on a set of scales and have four guys pull straight back on the rear spoiler to simulate drag, we would see a change in the front-to-rear percentage of total weight distribution. This is what drag from the spoiler does. If we need the car to turn well (and who doesn't?), then the creation of a high drag effect cannot be in our best interest.
It has been found that the optimum angle for the rear spoiler is between 55 and 60 degrees from horizontal, depending on the type of racetrack. The longer and faster the track, the less spoiler angle we need to run.