Aerodynamic Tweaking - Racing Drag
Aero-tweaking for an efficient coefficient.
HOT ROD StaffWriter
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.
1 Though it may look reasonably stock, this Late Model Racecraft-prepped ’08 coupe can throw as much as 1,700 hp at its Hoosier road race tires. Wearing Z06 bodywork, a ZR1 front splitter and side skirts, along with an aftermarket hood and rear spoiler, the C6 started off with a drag coefficient (CD) of 0.373, an overall lift coefficient (CL) of 0.023, front lift (CLF) of 0.155, and rear lift (CLR) of -0.132. Note that the minus sign on this latter number denotes negative lift, that is to say, downforce.
<STRONG>4</STRONG> The next few rounds of mods focused on the C6’s underbody air dams. First, they were removed altogether, which increased drag. Then, trimmed versions were installed which effectively accounted for the car’s slightly lowered ride height, and basically returned underbody air dam ground clearance to the factory-normal 90mm “sweet spot.” We ended up using untrimmed versions of the corner dams and the overall result was a desirable change from front-end lift to downforce, at the cost of slightly increased drag. These mods were worth keeping.
<STRONG>7</STRONG> After taping off the rear brake ducts just forward of the wheelwells with virtually no effect, the team then turned its attention underhood, taping off or plugging airflow gaps around the radiator. This accomplished little, once again probably because the external grille openings had already been sealed off so there was relatively little airflow present. But you have to try…
<STRONG>10</STRONG> So on went a little Z06 spoiler, which proved surprisingly effective in reducing both lift and drag. Okay, so the GM aero guys weren’t quite so surprised since they’d helped shape it in the first place for the production Z06. But overall lift was still present.
LMR has fortified the coupe’s engine bay with a twin-turbo 440-inch LS7 producing 1,700-rwhp in Texas Mile trim. With boost dialed back for street use, the combo drops to a mere 1,000 hp. No efforts have been made to reduce the weight of the car; it still has functioning air conditioning and audio systems.
Every wind tunnel needs a fan, and GM’s is 43 feet in diameter—a statement given scale by the puny people huddled beneath. Inside its center cowling is a 4,500-hp electric motor. The facility is shaped like an oval racetrack, with the fan located at one end of the “back straight” and the vehicle test platform situated in the middle of the “front straight.” Roughly 20,000 cubic yards of concrete went into tunnel construction.
<STRONG>13</STRONG> In these days when online videos are all the rage, LMR wanted to test the effect of mounting a couple different configurations of camera to the car. Let’s just say that, given their large suction mounts, they all acted like aerodynamic boat anchors. We therefore expect that any onboard video from the Texas Mile will be shot from inside the car.
<STRONG>16</STRONG> Tom Froling then suggested altering the car’s rake by lowering the nose somewhat. Rather than mess with suspension adjustments (nearly impossible with the car on the platform) the crew simply added sufficient ballast weight under the hood and in the cabin footwells to drop the nose by about 12mm. The effect was rather dramatic, lowering drag by 8 counts, and benefiting downforce to the point where the overall lift coefficient was down to 0.001.