With its racing pedigree, the Corvette's natural home is on the racetrack. Many owners are not content to be trackside spectators, but are compelled to let their Vettes run free on the open tarmac. Eric Birkness is one of those owners, and his '06 Z06 has been designed with a racing flair.
Following more than a decade of competitive shifter-kart racing and several years of motocross and enduro experience, Birkness set his sights on a means of excitement with less risk than motorcycles and less punishment than karts. "The biggest problem with karts is that you get bruises on your sides and on your legs every race weekend, just from the cornering force," he explains. To achieve this goal he decided to move on to taking fullsize cars on the track.
Like many Vette owners, Birkness grew up with a passion for America's sports car. He recalls a particular fondness for a pair of scale-model Stingrays from his youth. When he finally was able to acquire a 1:1 scale Corvette, Birkness pounced on an '01 Z06 and fitted it with a supercharger for 550 horsepower. The C5 Z was short lived, as a batch of bad fuel led to detonation during his first lap at a track day and destroyed a pair of pistons. Rather than attempting to sort out the blown (and blown-up) C5, Birkness felt it best to move on to a car better suited to the track.
The pursuit of a proficient track car naturally led Birkness to the C6 Z06, and he purchased one of his own in December 2005. Initially, the plan was to keep modifications light to preserve street use, but that changed during a track day at Watkins Glen. While lapping the track, the car got "a little sideways in the esses" at 120 mph, and the instructor suggested that the car be equipped with a rollcage. Birkness was at first reluctant to carve up the interior of his brand-new Vette, but he ultimately decided it was in his best interest to follow this sage advice. In this act, the die was cast and the Z06 began its transformation to its current radical form.
While at another open-track event at Watkins Glen, Birkness was awestruck by the Whelen Motorsports C6 race car's comparative advantage at carrying speed through corners. Seeking such performance of his own, he inquired with Kerry Hitt of Advanced Composite Products (ACP) about outfitting his Z06 with the racer's bodywork.
The ACP ZR/T body kit goes beyond the mere cosmetic enhancement of most exterior modifications by providing enhanced performance. Originally built for use in the SCCA World Challenge professional racing series, the kit boasts performance developed from aerodynamic testing in the GM wind tunnel. ACP claims its kit will generate in excess of 700 pounds of additional downforce at speed. It features a heavily vented hood to reduce lift, a large wing to keep the rear tires planted, and rear fenders widened by 1/2-inch on each side to fit wider rubber. Birkness offers a glowing review. "The aero stuff really works," he explains. "The car turns better and stops better. It makes the car way more stable at high speed and really plants the rearend out of corners."
Ultimately, the visual impact of the body kit is equally as impressive as its functionality. The combination of aggressive lines, racing appointments, exposed carbon fiber, and the black finish lend the car an exceptionally menacing appearance. This runs somewhat contrary to Birkness's original intent, as he explains: "I wanted a black car so it would look understated and not get so much attention, but with the bodywork and big wing it gets lots of stares." The car even has a tendency to slow traffic when being transported to racetracks on an open trailer.
Sharpening the Z06's corner-carving abilities was a must, and the suspension has been significantly enhanced with T-1 springs and sway bars installed front and rear. A set of 1 1/4-inch-drop billet uprights and Moton shocks at each corner give the car a hunkered-down stance befitting its capabilities. In order to shave off speed in a hurry, a Stop Tech big-brake kit was installed and cooled by a set of custom carbon-fiber brake ducts. On the street, the Z wears a set of CCW SP500 wheels shod in Michelin Pilot Sport Cup tires; CCW C14 wheels with Hoosier R6 slicks are used at the track.
Such a serious aerodynamics package required more chassis tuning than a common application. As Birkness explains, "It took a while to get things set up right since there was so much downforce, the front of the car would scrape with gentle braking if you were going over 150 mph." After the suspension was adapted to the Z06, it began to work in concert with the aerodynamics to provide a thrilling driving experience.
Generating 505 horsepower from the factory, the LS7 engine in the C6 Z06 is nothing to sneeze at in stock trim. However, Birkness decided to squeeze more power out of the small-block with the help of Hutter Racing Engines. In an attempt to avoid repeating the experience of his '01 Z, the plan began with building up the already formidable stock aluminum block to handle the added strain. The original hypereutectic pistons were replaced with custom 4.130-bore JE units connected to a lightened 4-inch Bryant crankshaft with Crower billet rods. Internal enhancements also include sleeved lifter bores and a priority oiling system.
To ensure adequate breathing for the athletic Vette, a Lingenfelter air intake and MAF were added; they feed factory LS7 heads that were treated to both CNC porting and a valve grind. The valves themselves were upgraded with larger Xceldyne titanium and Manley Severe Duty units on the intake and exhaust sides, respectively. The stock camshaft, meanwhile, was yanked in favor of a Comp Cams 251/256 stick with a 112-degree LSA and a towering 0.660-inch lift. Valvetrain upgrades also include Manley springs, Jesel lifters, and Manton 3/8-inch pushrods.
The factory manifolds were replaced with a set of LG Motorsports long-tube headers for rapid exhaust expulsion. The original Z06 after-cat exhaust was retained, but an LG exhaust bypass switch was installed to allow Birkness to manually open and close the muffler butterfly valves to control the volume. This setup affords the Z06 with a proper race-car rumble at idle.
A supporting cast of modifications further speaks to this car's special nature. The oiling system has been updated with a four-stage dry-sump setup from Dailey Engineering, including a billet pump and pan. The engine also features an ATI balancer and a Ron Davis radiator with a custom shroud.
With dyno tuning by Hutter Performance Center maximizing the combination, the engine produces peak output of 734 horsepower at 7,200 rpm and 608 lb-ft torque at 5,400 rpm. This translates into top speeds of 165-170 mph on the main straights at Pocono Raceway, Watkins Glen, and Virginia International Raceway. As Birkness explains, "Down the straights you can pass almost anything short of a Le Mans prototype car."
Combining increased power production with track duty necessitated a sufficiently beefed-up drivetrain. The stock TR6060 six-speed has been replaced with a Rockland Tranzilla close-ratio transmission that allows the car to go approximately 70 mph in First gear, 100 mph in Second, and 130 mph in Third. Further upgrades include an RPS twin-disc carbon clutch, a DTE Stage 5 Super Duty differential, and transmission- and differential-oil coolers to ensure efficient operation. All told, these enhancements help meet Birkness's goal of reliable on-track performance.
As with the rest of the Z06, the interior has been optimized for track performance. Hard cornering capabilities call for fixed Sparco Evo seats with Schroth Racing five-point harnesses to keep the occupants in place. A rollcage built by Phoenix Performance was added for protection; it necessitated door-panel surgery by Xtreme Auto Interiors just to fit inside the cockpit. The stock gauge cluster is augmented by a SPA Design 3 stage shift light in a custom shroud, and custom removable speaker cabinets were built for the rear hatch by Kerry Hitt of ACP. The GPS navi-gation system is still functional, and a Race Cam in-car video system captures all of the excitement at the track. In keeping with the car's race-oriented nature, an in-car fire-suppression system was also installed. While the factory heater was retained, the dry-sump upgrade required removing the air conditioning. Birkness uses a helmet cooler with fan and ice chest to keep cool and prevent his glasses from fogging up.
The end result of his labors is a unique Z06 that is also a potent track car. As Birkness attests, "With the combination of downforce, power, brakes, etc., it is a very competent track car on the straights, braking, and in the turns." The car has been used at numerous HPDEs held at tracks such as VIR, Pocono, Beaver Run, Nelson Ledges, and Watkins Glen.
Birkness feels that his past racing experience gave him a good lead-up to the Z06, with motocross eliminating fear and karts teaching essential skills. He further attests that racing karts at 100 mph while one inch above the ground is a uniquely thrilling experience. "Things happen three times as fast on a shifter kart as in the Corvette; there is not time to think." However, he appreciates the more civilized nature of the Z06, as well as its ability to rapidly cover ground and push him back in the seat with spirited throttle application, even at high speeds. He also has adjusted from a racing mentality to one of pure enjoyment, stating that, "I just drive it as I'm comfortable that day, and my trophy is bringing the car home in one piece."
In spite of the car's track-oriented nature, it's still a capable street machine. The largest concern is some surging at lower speeds when driving in traffic. However, such an extreme setup demands a focused driver. As Birkness explains, "You have to be careful feeding the throttle, or it will snap sideways if there is any crown in the road." As would be expected, the primary issue is the Z06's attention-grabbing power. "Wherever I take it people want rides, but the problem is you can't show what the car can do on the street," he says.
With such an extensive array of modifications already accomplished, future plans for the Z06 are focused on tweaking the current package for optimal results. A cam with slightly less overlap is being installed, and the tune adjusted to reduce the low-speed surging. Ron Hutter is also working to strengthen the LS7's heads to handle higher spring pressures and rpm levels during extended track use. Birkness is also set on finding a way to replace the air conditioning, as the vented hood transmits heat from the engine compartment into the cabin.
As with any such endeavor, Birkness has many people to thank for their help in the project. In particular he credits Kerry Hitt of ACP; Ron, Matt, and Trevor Hutter of Hutter Racing Engines; Kurt Omensetter of Phoenix Performance; Xtreme Auto Interiors; and Turner GMC/Buick. Above all others, Birkness is grateful to his wife for putting up with his hobby.
With a busy schedule as a partner in a large construction company and a standing gig as a bass guitar player in a few groups, Birkness does not get to use the Z06 as much as he would like to. But when he does, the car is both a stunning visual achievement and a celebration of the Corvette's racing lineage. As he sums it up, "It out-handles anything, other than the occasional non-streetable race car, and it hits 170 mph or more on some tracks. For a street car I drive to church, I think this is OK."