Every major carmaker owns and operates a wind tunnel, but those facilities are busy around the clock, as engineers work to stay ahead of the rapidly ­rising EPA-mileage tide. Inter-company rivalries are another issue foiling easy access.

The expert in charge at our clandestine test location explains: "All wind tunnels strive to accurately quantify the aerodynamics a car will experience in the real world. The vehicle and the tunnel constitute a system with complex interactions. As a result, drag and lift measurements on a particular vehicle can vary from one tunnel to another."

Here's a peek inside a typical modern automotive wind tunnel:

You're going to need a big-ass fan.

All wind tunnels use one or more electrically driven fans to push or pull air through a test section. GM's six-blade fan is 43 feet in diameter; other tunnels have 20 or more small fans. Some tunnels recirculate the air in a closed loop (called a "Göttingen type"); others draw it from the surrounding room at one end and exhaust the air after the test section ("Eiffel type").

Think small.

Under the tunnel's floor, sensitive scales accurately measure the horizontal (drag) and vertical (lift) forces and moments acting on the test vehicle. Other sensors record air temperature and pressure at key points in the test section. In addition to the electrical power needed to spin the fan(s), energy is expended in some tunnels maintaining air temperature and humidity.

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Park it.

Wind tunnels reverse atmospheric circumstances out of necessity; the test subject is static while wind flows over and around the car, posing an issue with the ground surface. Some wind tunnels replicate the real-world speed difference between the vehicle and the pavement with a moving belt. This is common in Formula 1 testing, where ground effects are crucial to performance. Other facilities achieve good results by evacuating what aero engineers call the "boundary layer" of air. In the boundary layer, flow velocity changes from zero at the floor surface to the undisturbed wind velocity at the top of the layer. The goal in any wind tunnel is to have the thinnest possible boundary layer to best replicate a car moving over the road.

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