If you were to place any modern car inside a wind tunnel, such as a Mercedes sedan, a type of car known as a three-box design-meaning it has a hood, a cabin and a trunk, all roughly box shaped-air would flow up the hood, over the windshield and across the roof. The majority of the airflow leaves the car straightly at the end of roof line. The dramatic drop of the rear window and decklid creates a low pressure area around the back of the car. This low pressure acts as a vacuum that sucks some air back toward the car, thus creating turbulence. In addition, the rear window's roughly 45-degree angle causes the airflow to be particularly unstable on a high-speed Mercedes. Turbulence always deteriorates drag coefficient, in effect adding weight to the car.

But, to know what exactly happens at the rear of your car at speed, you have to first start at the front. To help answer this, let's turn to an unlikely source, one of the industry's most beloved but least aerodynamically sound cars, an old Volkswagen Bus. "If a box on wheels is what they want," said VW head Heinz Nordhoff in 1946, "then a box on wheels is what they'll get." As the blocky shape of the Bus drives down the road, it literally punches a hole in the air, which is forced out of the way via the four sides of the box. This force is called Frontal Pressure, which creates high pressure as the air rams into the front of the Bus and packs in together. The usual measure of aerodynamic efficiency is the drag coefficient, Cd. It compares the drag force, at any speed, with the force it'd take to stop all the air in front of the car. Drag coefficients for the first boxy autos with large frontal surface areas were up over 0.70. Instead of letting the air slip past, they brought most of it to a halt right in front of the car (think of the flat grille and headlights of an older Volvo).

What is really happening is that the air slows down as it approaches the front of the Bus, and as a result, more molecules are packed into a smaller space. Once the air stagnates at the point in front of the Bus, it seeks a lower pressure area, such as the sides, top, and bottom of the vehicle. To give a few examples, the worst possible streamlining would be expected from a parachute, which is designed to maximize wind resistance. The Cd of a parachute is about 1.35. The lowest possible resistance is desirable in the airplane wing, which has a Cd of about 0.05. Automobile Cd figures lie between these two extremes. In the past 60 years, automakers have managed to cut Cd figures for production models nearly in half, from about 0.70 to about 0.40. In a practical sense, gas mileage is increased by 5 percent for every 10 percent improvement in aerodynamics.
At speed, the space directly behind the Bus is empty, devoid of air like a vacuum, a concept used for drafting in stock car racing. This empty space is there because the air molecules are not able to fill the hole as quickly as the Bus can make it. Push your hand through water and notice the divot that forms behind it (and notice how the water swells up in front of your hand). The air molecules attempt to fill in this area, but the Bus is always one step ahead. As a result, a continuous vacuum sucks in the opposite direction of the Bus, and this inability to fill the hole left by the vehicle is technically called Flow Detachment, where the air is yanked away from the car.

Now, back to the Mercedes. As the air flows over the hood of the car, it loses pressure, but when it reaches the windshield, it again comes up against a barrier and briefly reaches a higher pressure. The lower pressure area above the hood of the car creates a small lifting force that acts upon the area of the hood (sort of like trying to suck the hood off the car). The higher pressure area in front of the windscreen creates a small (or not so small) downforce and drag. This is like pressing down on the windshield and slowing the car, while the front end is lifted upward.

Where most road cars get into trouble is the fact that there is a large surface area on top of the car's roof and underneath the car, like with our example. As the higher pressure air in front of the windshield travels over the glass, it accelerates, causing the pressure to drop. This lower pressure literally lifts on the car's roof as the air passes over it, while the air passing underneath the car adds additional lift; all of this is a tight-wire act, balancing between too much lift and too much drag. The end result is aerodynamic efficiency.

Colin Chapman, an engineer who invented a new concept to provide downforce without altering drag, called ground effect (different from the aftermarket products of the same name). He incorporated an air channel into the bottom of his Lotus 72 racer, narrow in front and expanded toward the rear. Since the bottom is nearly touching the ground, the combination of channel and ground forms *******ly a closed tunnel. When the car is running, air enters the tunnel in the nose and then expands outward toward the tail. Air pressure is reduced toward the tail so that downforce will be generated.

The ultimate example of the downforce concept was the Brabham Alfa BT46B, designed by Gordon Murray for the 1978 Swedish Grand Prix, which actually used a cooling fan to extract air from the skirted area under the car, creating enormous downforce and hence amazing handling capabilities. After technical challenges from other teams, it was withdrawn after a single race, and then the use of skirts used to contain the low pressure were banned (replaced even by a stepped floor design). Another banned attempt at creating a low pressure underneath the car was the Chapparal 2J, which was raced in the 1970 Can-** series and featured a skirt around the side of the car to stop the outside air from rushing in to break the low pressure created by the top-mounted "extractor" fan. The list of innovations and innovators continued over the years, such as Robin Herds' March 701 and Peter Wright's Lotus 78.

I'm just curious. How is there high pressure?