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Aerodynamic Tweaking - Racing Drag

Aero-tweaking for an efficient coefficient.

Dale Amy Dec 28, 2011
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Drag is the enemy when seeking maximum velocity, and while a production C6 Corvette is an impressively slippery object, its overall aero efficiency is necessarily compromised by styling and safety considerations. It is, after all, primarily a streetcar. But forget the street for a moment. In a perfect world, if you wanted your C6 to simply go as fast as it could in a straight line, wouldn't a day of aero experimentation in GM's famous wind tunnel be the coolest way to eliminate those compromises? Welcome to the perfect world.

Late Model Racecraft, out of Houston, Texas, recently had just such an opportunity to harness the air- and brain-power of GM's world-class wind tunnel facility-properly known as the Aero Lab-for a full day of intensive aero tweaking. And GM invited us to watch-offering a rare glimpse into this otherwise secretive structure within GM's North American Technical Center. The subject was an LMR customer's twin-turbo 2008 C6 coupe that is headed back to Texas Mile competition-a standing-start exercise in nailing one's right foot to the floorboards and going for max trap speed at the mile marker on this former naval air station near Beeville in southeast Texas. The velocity goal is lofty: to surpass the current (though controversial) 250.1-mph record held by a Lamborghini Gallardo Superleggera, and bring the speed crown back to America where it belongs.

Surpassing 250 mph in a standing mile is clearly no mean feat, even with the roughly 1,700 rwhp this coupe has on tap thanks to mods done by LMR (see sidebar.) Obviously such speed demands minimal drag, but the aerodynamic balance of the car must also be configured such that both ends remain firmly planted at all times. So, while it may be tempting to simply strip off anything and everything that might induce drag, some downforce must remain, front and rear, otherwise the car might well morph into the world's prettiest Scud missile.

The GM Aero Lab's Lead Development Engineer, Tom Froling, is intimately familiar with Corvette aerodynamics, having been responsible for aerodynamic "tuning" on both the production C6 as well as the world-conquering C6.R racers. So if LMR had been going road racing, Tom might well have had all the aero answers right at his fingertips, but with the peculiarities of pure straight-line speed in the curiosity crosshairs, we settled in for some good old experimentation.

First task was to establish the coupe's baseline aero performance-the overall drag coefficient (CD), as well as lift/downforce measurements at the front (CLF), rear (CLR) and overall (CL)-as it arrived at the facility. From there, the LMR team members, with advice from GM's aero staff, began making one change at a time, followed immediately by firing up the wind tunnel to accurately quantify the results. More experimental tweaks were then tried until the clock-and/or the ideas-ran out. With that in mind, settle back and see what we learned. In a nutshell, by day's end, drag had dropped from 0.373 to 0.307, and overall lift had become downforce (see sidebar) by dropping from 0.023 to -0.030. A good day's work that should translate well on the Texas Mile.

The Thrust to Overcome Drag

Though it started life as a base '08 coupe, about the only body components left stock on this still-street-driven, air-conditioned C6 were "the doors," according to LMR co-owner, Steven Fereday. Wide-body Z06 bodywork had been fitted front and rear, and a ZR1 chin splitter and side skirts were in place, along with an aftermarket rear spoiler. Oh, and a very necessary drag 'chute (which must raise eyebrows around the streets of Houston). In this form, it had previously run 231 mph-in a stiff headwind-at the Texas Mile, making it the fastest Corvette there to date. To do so required about 1,700-rwhp thanks to a Late Model Racecraft-developed drivetrain that included the following:

  • World Products Warhawk aluminum LS7 block
  • All Pro LS7 heads
  • 440 cubic inches
  • LMR-fabricated twin rear-mount 76mm turbochargers, air-to-water intercooled
  • FAST LSXR 102mm intake manifold
  • Capable of 25-30 psi of boost with ice in the 'cooler tank
  • RPS prototype quad-disc, carbon-on-carbon clutch
  • RPM-built TR6060 tranny with G-Force guts
  • RPM-built rear end with 2.50:1 gears

Understanding Drag and Lift Coefficients

The coefficients discussed in this article are dimensionless numbers used to quantify the amount of drag or lift created by an object in a fluid environment such as air or water. The only units applied by the GM aero guys in discussing such coefficients are "counts," so that a drag coefficient of, say, 0.385 might be referred to as 385 counts of drag. In conversation, for example: That change we just made lowered drag by 13 counts.

Logically, a smaller number in a coefficient indicates less lift or drag and, since we're dealing with a car here and not something intended to fly, we are in fact seeking the opposite of lift: downforce. Downforce, then, is simply negative lift, and is indicated by a lift coefficient preceded by a minus sign (i.e. -0.012.) For LMR's straight-line top speed mission, we didn't want very much downforce at all --just enough to keep the car earthbound-- since any downforce (or lift) contributes to overall drag.

Tunnel of Love

Gotta love the GM wind tunnel-oops, we mean Aero Lab. Completed in 1980, this hurricane-in-a-concrete-loop can generate breezes of up to 138 mph at the vehicle test location, thanks to a 5:1 reduction in tunnel area that creates a big-time venturi effect. The tunnel's 75 tons of air is put in motion by a 6-blade fan that is some 43 feet in diameter. Its enormous blades are made of hand-formed, laminated Sitka spruce (tipped in balsa) and are spun at speeds of 25 to 250 rpm by a 4,500-hp electric motor. We left out the exclamation marks, but you can insert them where needed.

Data measured in the tunnel includes airflow velocities, pressures, temperatures, wind noise, as well as forces and moments acting on the subject-the latter components measured via a weighbeam balance system sensitive enough to detect a 10 gram differential with an 8,000-pound vehicle on the test platform (there are approximately 454 grams to the pound-do the math...)

For production vehicles, the tunnel is used for aerodynamic shape development, cooling airflow optimization, and wind noise reduction. Obviously, many of GM's various factory-supported race programs also benefit greatly from its aero analysis. But, in a recent development, GM has now decided to make the facility and its staff available to privateers-make that well-funded privateers-during periods when not in GM corporate use. Start saving those pennies.


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