With the possible exception of the hunnish Dodge Viper, the C6 Corvette is unique among domestic vehicles in its ability to make just about any driver feel like a multiple ALMS champion. Amply girded with deep reservoirs of power, a denture-dislodging set of brakes, and a painstakingly refined suspension system-all operating under the benevolent aegis of a sophisticated computerized safety net-the latest iteration of Chevy's long-running sports car offers a peerless platform for pushing one's driving abilities to the limit with minimal risk to life, property, and reputation.
But for all its baked-in puissance in most operating environments, the C6-especially in base form-is not without room for improvement as a sometime track car. These inherent deficiencies came into sharp focus during one of our recent track sessions at Gainesville Raceway, when a colleague at another magazine decided to put his '05 Z51 coupe through its paces on the facility's 1.6-mile road course. Perhaps the most significant problem lay in the car's habit of spinning its inside wheel when accelerating out of turns, a tendency amplified by the aging all-season Michelins installed by the previous owner. And while the factory Z51 binders performed admirably up to a point, repeat lapping revealed a not-inconsiderable propensity for brake fade, resulting in a few hair-raising moments and at least one sod-scattering detour through the infield.
Much as we enjoyed witnessing some of these antics, professional responsibility ultimately prevailed, and we resolved to address the car's shortcomings with a few carefully selected aftermarket upgrades. We began by contacting OS Giken, a leading manufacturer of limited-slip differentials for foreign and domestic performance vehicles. Giken's Sean Fenstermacher recommended the company's OS Super Lock LSD unit, which is a direct replacement for the stock diff in C6, Z06, and ZR1 models. (The company also offers versions for the C4 and C5.)
According to Fenstermacher, the Super Lock's chief advantages lie in its unique construction. "The Super Lock is a clutch-type unit equipped with 28 steel clutch plates with very minimal preload. This provides a smooth lockup [all the way] to full lock. It also eliminates wheelspin, while maintaining smooth, almost open-diff-like corner entry. This results in faster exit speeds for our track and autocross customers."
Though not specifically designed for drag racing, the Super Lock is also said to pay dividends at the strip. "For drag customers, our diff provides a smooth and progressive lockup to both wheels equally, enabling increased stability during hard launches," says Fenstermacher.
Durability is also improved over stock, thanks to the inclusion of hardened stub axles, a forged case and pinion gears, and the use of heat-treated components throughout the unit. No break-in period is required, and fluid-change intervals are unchanged over the factory recommendations. (You'll need to use OS Giken's proprietary fluid, however.)
For serious racers, the Super Lock's preload, lock timing, and ramp angle are fully tunable, making it possible to tailor the unit's responses to best suit different suspension calibrations and track layouts. The configuration works so well, in fact, that track star Danny Popp is currently running a Super Lock diff in his NASA C6 race car.
A pro-quality differential demands a suitably stout housing, so we decided to yank the '05's famously frangible factory rear in favor of a heavier-duty piece from a C6 Z06. Fortunately, RPM Transmissions makes a conversion kit to facilitate this swap, complete with a media-blasted case, fresh seals, and micropolished gears in a choice of ratios. Although RPM typically loads these rears with factory Z06 internals, the company agreed to outfit ours with the OS Giken Super Lock hardware instead.
Next, to address the brake-fade issue, we pulled the 5-year-old factory pads and installed Hawk PC (Performance Ceramic) pads in their place. The PCs use one of Hawk's less aggressive compounds, making them suitable for street driving as well as occasional track use.
And finally, since all the braking and driveline upgrades in the world are meaningless without adequate rubber, we had a local tire shop strip off the car's old mud-and-snow Michelins and replace them with a stock-size set of Nitto's excellent Invo performance radials.
So, did our curative ministrations have the desired effect on the C6's track performance? We'll get to that soon enough, but first, let's take a closer look at the parts we used in the project and some of the work involved in installing them.