You think of air as flowing over your car at speed, right? Actually, you're ramming into it and shoving it out of the way-punching a hole through a wall of fluid that weighs about 0.08 pounds per cubic foot. It fights you coming and going, pushing against the nose of the car as you compress it then leaving a low-pressure wake that sucks backward on the tail. That's all bad. As any veteran bench racer knows, if a car is running at its top speed, then to double that speed would require eight times the horsepower. More practically, to go just 25 percent faster-say, from 125 mph to a bit over 155 mph-requires double the horsepower.
Or you could go faster without adding a single horsepower by making the car sleeker, being more friendly about inviting the air to step out of your way. That means getting familiar with aerodynamics, the study of the interaction of gases and bodies passing through them. It's a complex discipline, and there are so many variables that can affect how a car slices the wind that you're never going to learn how to optimize it from a magazine article, though you can get a good start with the story "Aero Tricks You Can Use" from the Sept. '05 HOT ROD (see it now at www.HOTROD .com). Even professionals can only make educated guesses, but as Gary Eaker says, one good test is worth a thousand opinions.
Eaker has made it possible for you to get that test. He's a former senior project engineer from GM's Advanced Aero Group and was later the aerodynamicist for Hendrick Motorsports. He helped develop NASCAR roof flaps, Top Fuel body side deflectors, and the EV-1, with the lowest-ever production-car coefficient of drag (Cd) of 0.19. His latest project is a wind tunnel known as A2, located alongside his advanced AeroDyn tunnel in Mooresville, North Carolina. AeroDyn operates nearly 24 hours a day with pro race teams, but A2 is a lower-content tunnel without a simulated moving ground plane or the capability of testing a car in yaw, where street-car guys and sportsman race teams can learn lots about their cars. We first covered A2 in the Oct. '05 issue ("Kingdom of Air") when it was under construction, and in mid-2006, we had the opportunity to bring the HOT ROD Magazine Spl. Bonneville race car to A2 so we could tell you how it all works, impart some tips, and, of course, make our car faster. The learning curve and new world of opportunity are better than even your first day on an engine dyno.
The A2 tunnel can accommodate anything from motorcycles to small dragsters, but nothing with too tall a rear wing (such as a winged Sprint Car). Anyone can schedule an appointment by calling about a month in advance. The price for first-time visitors is $345 an hour for the first two hours and $490 an hour after that. About half an hour of that time will be eaten by load-in and -out. Eaker suggests a minimum two-hour appointment to learn anything significant; four hours is better for good testing, and you can complete a fairly thorough regimen in six hours.
Once the car is placed squarely in the tunnel with each wheel on a scale, a measurement of the car's frontal area is taken. This does not have to be precise as long as the same number is used throughout testing and in any future testing at A2 (what's important is the comparative analysis of A-B testing throughout the day, as specific aero data will vary even from tunnel to tunnel). A few initial trials may be required for the A2 staff to adjust the roof line of the tunnel, which varies to match the shape of the streamlines as they expand off the shape of the particular vehicle being tested. With that complete, the fans will fire up, drawing an 85-mph wind over the car as measurements are taken. A complete test takes about four minutes.
Data is output into an Excel spreadsheet that may be manipulated on a computer provided solely for customer use. You may bring your own pressure taps for up to 16 channels of data-logging, and the A2 information will baffle you with a number of raw data outputs that reveal aero force and moment (torque) coefficients. As a first-timer, the information of concern will be coefficient of drag and coefficient of lift (Cl).
Coefficient of drag, as simplified by Eaker, "can be thought of as the proportion of airflow energy not recovered by the car." In other words, the ideal situation would be to have the pressure at the front of the car be equally balanced by pressure at the rear of the car. That's impossible, but the closer you can get, the lower the Cd will be and the faster the car will go. The Cd measurement is independent of size; in other words, a 51/48-scale model of your car has the same Cd as the fullsize car.
Coefficient of lift is translated into actual pounds of positive and negative lift (the latter is often called downforce) as measured at each wheel of the car. You can input your target mph to see what the lift numbers are based on how fast you intend to go. Lift coefficient does not change with speed, but the number of actual pounds of lift is affected by velocity.
So Cd and Cl are the first two steps in using aerodynamics to go fast in a straight line, but in the tunnel, you'll also learn about vortices (the plural of vortex), which are energy-expending swirls of air created by the movement of the car through the air. Energy is expended in generating vortices, and that use of power generally creates drag, though in some cases vortices can be useful in filling in low-pressure areas to trick the airflow in beneficial ways. More commonly, vortices interrupt optimal flow, and often the best you can do is control the direction of the vortices, such as in getting them off the back of a rear spoiler rather than letting them roll off the sides.
While the learning process will certainly give direction to your testing, it's critical to come in with a specific testing procedure based on your goals, which could vary with the type of racing in which you're involved. That's where the fun begins. Show up with lots of duct tape, foam core, cardboard, aluminum panels, tin snips, rivets, and modeling clay with which to make shapes on the car, plus plenty of people to help. Even if an aero mod is not legal in your class of racing, it can be important to try it so you can identify aerodynamically sensitive areas to find ways to trick the wind. Eaker notes that, "Aero is notorious for interactions between seemingly independent mods," meaning the test results you get on your rear spoiler may change when you add a front air dam. "If object B is behind object A, then there is probably an aerodynamic interaction-but it might be good or bad."
He told a story about testing station wagons with the deflector at the back of the car to keep road spray off the rear window. The deflector alone added drag, and a roof rack alone added drag, but when the roof rack was added to the deflector, drag was actually reduced. Of course, the combination was not as good overall as a smooth roof, but the process is a good example of nonobvious interactions.
All the more reason for a logical test regimen supported by willingness to be flexible as you learn. It gets really fascinating as you go, and the most rewarding part is that you can try virtually anything with the tape and cardboard and in four minutes you'll know if you moved in the right direction or not. You can test hoodscoop drag and ram effect; shape your wings, spoilers, and air dams for optimum Cd; seal bumper edges; find and solve unforeseen trouble spots; test pressures at various places on the car with your own taps; and determine airflow across radiators and intercoolers. Aftermarket companies can develop body components for drag and flow.
The testing of our top-speed Bonneville car was focused on finding the lowest possible drag while balancing lift to keep the car planted on the ground. A top-speed racer is the easiest possible case for wind-tunnel testing because, as Eaker puts it, "The closer the car operates to terminal velocity, the more difference aero improvements can make." In that case, a 10 percent reduction in drag equates to about a 2-3 percent increase in speed. But is it relevant to your car? When pressed, Eaker admitted that benefits are greatest for top-speed cars, second best for circle-track or road-course cars, and third for drag cars, which expend more power overcoming inertia than parasitic drag. Even so, the closer your drag-race competition (think Comp Eliminator), the more important aero mods can be. Even at Drag Week(tm), quickie blockage of grilles has seen improvements around a tenth in e.t., and that can add up.
Overall, a day of Wind Camp at A2 is a fascinating experience for the serious performance enthusiast. It's very close to the HOT ROD Top Speed Challenge at the ECTA event in North Carolina March 31 and April 1, so if you're coming to race, consider making an appointment.
Top 5 Easy Aero Mods That Almost Always Work
We pressed Eaker to give us clues about simple aero tricks for typical musclecars-stuff that will work nearly every time even without tunnel testing. The delta between Cd numbers is often referred to as "counts," or thousandths of a point, so the difference between a Cd of 0.250 and 0.200 is 50 counts. Eaker gave us some rough estimates on how many counts of Cd you might improve when these mods are applied to a typical older musclecar. Try these next time you're at the track.
Lower the ride height: "Dropping the car-front or rear-will always reduce drag." On our Camaro, raising the tail 1.5 inches changed the Cd from 0.201 to 0.227.Improvement: At least 20 counts per inch
Block the grille: Eaker's tip is "Always get air around the car rather than through it." Stopping air from entering the grille has shown dragstrip e.t. reductions of a tenth or two even on 120-mph cars. This will also reduce front-end lift, and when the front end raises due to lift, drag increases even more. (Bonus tip: Stiff front springs can also help prevent front-end lift). Grille blockage is more effective on older cars than on newer ones with sealed radiator areas, and obviously, it can only be used for short-duration events due to reduced engine cooling.
Improvement: 15 to 30 counts of reduction in Cd and 50 to 100 counts in reduction of Cl
Add a front air dam: "You can usually get 90 percent there just by adding a dam straight down from the front bumper, just like the one on your Camaro." Even Bonneville guys ask us why we have that big barn door on the front of the car, but keeping air out from under the car both reduces drag and neutralizes lift for solid aero gains.
Improvement: 20 counts less drag, 50 less lift. The taller the car sits, the more important the air dam is.
Seal the back of the hood to the cowl: This is another area that's less critical on newer cars that are already well sealed. Cowl-induction hoods have a high-pressure area at the base of the hood. Air does not travel out the back of the hood; it is forced down into the engine compartment. That air must escape through the car, causing flow disruptions and drag.
Improvement: 10-20 counts less drag, 50 to 75 in lift
Remove the outside rearview mirrors: Factory mirrors hanging off the doors are almost always obstacles to airflow. The '94-up Chevy Impalas and Caprices are notable exceptions; on those cars, the Cd actually gets worse when you remove the mirrors.
Improvement: 10 to 20 counts of drag
Aero Stuff That Doesn't Really Matter
In addition to our list of five tricks that almost always work, here are some things A2 customers might want to try that are really a waste of time.
Wax: Contrary to what you may read on your favorite message board, well-waxed, smooth paint is no more aerodynamic than the worst spray-can, flat-black primer job you can imagine.
Golf-ball dimples: They do not work on cars, regardless of the scale of the dimples, unless your car is a 1.68-inch-diameter sphere spinning through the air with no ground plane.
Taping seams: Rarely if ever are body-panel seams so large and misaligned that smoothing them with duct tape will make a measurable difference in Cd. We tried it on our Camaro, and it did nothing.
Here, Eaker is examining the stock Z28-type side spats, which did not alter the Cd but added 40 pounds of rear downforce. He thought they may be more helpful in yaw but says testing 0 degrees to the airflow gives you 75 percent of the data you'd need at slight yaw. We also tested the Moon discs, which reduced drag by two to four counts. The farther the wheels are from the outer edge of the fender, the less the discs matter.
Smoothing rivets and hood pins: The removal of minor surface burbles, such as rivets (remember the Howard Hughes movie?) and hairpin-type hood pins shows no measurable improvement in Cd.
Dropping the tailgate: On a pickup, lowering the tailgate does not usually reduce drag. If you are racing a truck, know that extended cabs and crew cabs are more aero than regular cabs.
The biggie: windshield rake: According to Eaker, "Here's a myth I can bust. Once the windshield is past 45 degrees of rake-and many stock cars average like 60 degrees-you will not see an improvement from laying it down at an even steeper angle." We proved this on the Camaro, building a hugely sloped "windshield" out of foam core. It did nothing.
