The Influence of Aero Beyond The Car
When you were racing in still air, each pound of air would have 1 pound of inertia keeping it there. It's therefore easy to deflect that air, and it compresses easily. A car drives through that still air at a high speed and disturbs the air in several directions. Then, the rear of the car sucks the air back toward where it was to begin with. The perfect shape for a race car would move the air out of the way with the least amount of force, and then allow the air to flow back gently, undisturbed. This can't be done realistically, but it's still your goal.

Remember when Richard Petty started the Daytona 500 a few years ago and his hat flew off? He was about 20 or so feet from the race cars, yet the cars were dramatically upsetting the airflow enough to blow his hat off. If the walls of a wind tunnel are too close, they will negatively affect your wind tunnel data. The closer they are to the car, the more inaccurate the data is. If you put a vehicle in a wind tunnel, you restrain that airflow disturbance and actually artificially reattach the airflow back onto the vehicle.

The data will show a higher downforce and higher drag, because you're compressing the air between the vehicle and the walls of the wind tunnel. This can also lower drag in some areas because you artificially reattached the flow onto the vehicle. One doesn't cancel out the other. You must manipulate all of the numbers from the wind tunnel data to have it make sense. Because you are putting a lot of inertia and energy into the air in a wind tunnel, the air doesn't behave the same way as it does in the real world. If it did, we wouldn't need test pilots.

When you see a smoke wand used in a wind tunnel, you see smoke roughly following the contour of the vehicle. The farther the smoke trail is off the surface of the vehicle when the wand is placed near the surface, the more airflow separation exists. This indicates more drag and less downforce. You want the smoke to lay on the vehicle from front to rear. The farther the smoke is off the car, the greater the flow separation and the greater the problem.

Changes to the rear flow of air will change the front flow characteristics and changes to the front flow will change the rear flow. Both drag and downforce are affected in this way. Any device you install across the airflow at the front of the car will separate the flow and significantly reduce good aerodynamic effects on the rear, as well as add drag. Never disturb the airflow at the front of the car.

Different Types of Airflow
Turbulent flow is where airflow boils and rolls in many different directions as it flows over the surface. This may be over the entire surface, or part of it. The less turbulent flow you have, the less drag you'll have. Most current race cars have turbulent airflow around them. The worst turbulence I ever saw was flight testing a certain low-wing aircraft. I saw some airflow actually going in the opposite direction of the aircraft, that means going forward!

Attached flow will significantly reduce drag and increase downforce or lift. If you get it attached over the entire surface, the drag reduction will really shock you. Usually, attached flow occurs over less than 10 to 25 percent of the front surface area. The more you get, the better you are. Attached flow is typically misinterpreted as laminar flow. The two are different.

If anyone talks to you about achieving laminar flow on a race car, he is misinformed. The only exception is with a very few Formula car wings. Those cars only achieve laminar flow over a very small part of the surface. Laminar flow is a very small, as in a few thousandths of an inch, layer of airflow that acts like ball bearings, further reducing drag.