By David Reher, Reher-Morrison Racing Engines
“A smaller, slicker car is the best of both worlds.”
Although I’m an “engine guy,” I’ve been around racing long enough to gain a working knowledge of aerodynamics. I’m also an avid amateur pilot, so I’ve developed an eye for airflow. While I don’t have formal training in the science of aerodynamics, I can look at a race car or a Cessna and come up with a fairly accurate notion of its aerodynamic characteristics.
When I started my drag racing career more than 30 years ago, aerodynamics seemed irrelevant. We were focused on elapsed time, and the speeds of our doorslammer cars were relatively slow. Back when we raced Mavericks and Stingray Corvettes, 130 mph was considered a big top-end speed. Today we’re racing 200 mph Pro Stocks, so aerodynamic performance has become more important. The arrival of Top Sportsman and other fast eliminators provides opportunities for weekend racers to experience speeds that once seemed unimaginable for production-bodied race cars.
Very few drag racers have access to wind tunnels and aerodynamic experts. Consequently there are some misconceptions about aerodynamic terms and principles. While I’m certainly not an expert in this field, I do have some practical experience with doorslammer aerodynamics.
Drag and downforce are two important concepts in aerodynamics. Drag is the aerodynamic force that resists the motion of an object through the air; it’s produced by friction, pressure differences and turbulence. Downforce (or “negative lift” as the aero engineers call it) is the downward force produced by the difference in pressure above and below a body moving through the air. High pressure above the body and low pressure below it increase the effective weight that presses the tires against the track surface, improving traction and stability.
Drag is important because it consumes most of the engine’s power at high speeds. Aerodynamic forces increase as the square of the speed, and the horsepower required to overcome this force increases as the cube of the velocity. In other words, doubling the speed of the car produces four times the aerodynamic drag and uses eight times more power. That’s why our race cars don’t accelerate at a constant rate over a quarter-mile; as the speed increases, more power is siphoned off to overcome aerodynamic drag, reducing the power available to accelerate the car.
Two other key aerodynamic concepts are frontal area and the coefficient of drag, commonly abbreviated as Cd. Frontal area and Cd are linked; the first measures the physical size of the body that’s moving through the air, and the second indicates the aerodynamic efficiency of its shape. In aero terms, a smaller Cd and less frontal area equals less total drag.
At the GM Aerodynamics Laboratory in Warren, Mich., technicians use a laser beam to trace the car’s frontal area onto a flat panel. The result resembles a cross-section of the car, like you had sliced through the body at its widest and highest points. The area of this cross-section is then calculated to provide the frontal area in square meters.
Now imagine that you traced this cross-section onto a sheet of plywood, cut out the profile, and put it in the wind tunnel. That flat plywood panel would represent the worst possible Cd for that car’s frontal area; let’s call that a Cd of 1.0. If we wanted to improve the Cd of this plywood race car, we could nail some tapered boards to the back side of the plywood. Since the turbulence behind an object is responsible for most of its aerodynamic drag, this modification might improve the Cd to around .70. Then we could attach molding with rounded outer edges to the front to help air move around the cutout; this addition might improve the Cd to something like .50.
If we kept adding boards and wood bits, eventually we might end up with a “car” with a rounded nose and a long, tapered tail like a streamliner for the Bonneville Salt Flats. When we finally arrived at a perfect teardrop, we would have achieved a shape with the lowest possible Cd.
The point of this exercise is to illustrate what happens during a wind tunnel test. Obviously we can’t nail a tapered wooden tail to the rear end of a Pro Stock to improve its Cd, but we can minimize drag as much as possible within the rules. That means smoothing the flow of air with rounded leading edges, eliminating protrusions that can disrupt the air molecules, and minimizing the turbulence in the car’s wake. What we hope to end up with is a Cd that’s the smallest fraction of the drag that would be produced by a flat plate that represents the car’s frontal area.
Cd is a calculated value that reflects an object’s aerodynamic efficiency regardless of its size. For example, a Boeing 747 airliner has a Cd around .11; an Indy car, with its open wheels and high-downforce wings, typically has a .75 Cd. But the crucial distinction is that an Indy car is much smaller than a jetliner; even though it is “dirty” in aerodynamic terms, it has a much smaller frontal area and consequently requires less power than a 747 to run 200 mph. While an airplane’s wings produce lift, an Indy car’s wings are designed to produce downforce. The penalty for this downforce is drag, but the increased grip produces faster cornering speeds.
The same principles apply to doorslammer drag racers. The goal is to get the most downforce with the least drag. It’s easy to add 40 pounds of downforce with 40 pounds of drag – but if you can get 40 pounds of downforce with only 20 pounds of drag, that’s a bargain.
The Cd of our race cars has also improved significantly over the years. The Pro Stock Camaro we tested in 1983 had a Cd of .28, which was remarkable at the time. Now the new generation of Pro Stocks has Cd figures below .20. But in addition to their aerodynamic shapes, the cars are now much smaller. Reducing frontal area is as effective as reducing Cd in lowering aerodynamic drag, so a smaller, slicker car is the best of both worlds.