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."
Our '05 Z51 coupe had a habit...
Our '05 Z51 coupe had a habit of spinning its inside wheel when accelerating out of corners. This OS Giken Super Lock limited-slip differential should help tremendously without scuttling street driveability.
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.
This exploded view of the...
This exploded view of the Super Lock shows the 28 steel clutch plates incorporated into the diff. More plates mean a smoother transition to full lockup, and a firmer lock once that point has been reached. The stock C6 diff uses 16 plates, only half of which are steel.
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.
Having experienced the '05-issue...
Having experienced the '05-issue Corvette rear's self-destructive tendencies firsthand, we decided to step up to a brawnier C6 Z06 unit. RPM Transmissions supplied one of its Z06 swap kits, which comes with everything required to do the job. (Note: The kit typically comes with a stock Z06 differential, rather than the OS Giken diff shown here.)
Micropolished gears in a choice...
Micropolished gears in a choice of ratios are another standard feature of the RPM kit. We chose a stock-ratio 3.42 gearset to preserve the Vette's outstanding fuel economy, which nears 30 mpg on the highway.
The comparison shot shows...
The comparison shot shows the difference between the '05 C6 (left) and the '06-up Z06 rears. Note the additional strengthening ribs employed on the top of the Z06 unit.
As illustrated here, the stock...
As illustrated here, the stock (left) and Z06 tailhousings differ greatly in construction. Most notably, the Z06 piece uses two additional bolts and one extra stud to better tie the transaxle together. Because of these differences, the Z06 piece will need to be swapped onto the existing trans.
Because of the considerable...
Because of the considerable amount of specialized work involved, we strongly recommend having this installation performed by a shop whose techs are experienced in Corvette driveline modifications and repairs. We had ours installed at AntiVenom, a highly regarded Vette-tuning firm in Seffner, Florida.
Since the all-season Michelins...
Since the all-season Michelins on the car had long since passed their sell-by date, we decided to replace them with a fresh set of Nitto Invos, in the stock sizes of 245/40ZR18 (front) and 285/30ZR19 (rear). The Invos are an ultra-high-performance street design, making them ideal for daily driven cars that see occasional light track use.
To reduce brake fade, the...
To reduce brake fade, the factory-issue pads got the hook in favor of these PC (Performance Ceramic) pads from Hawk. The PCs use a low-dust, low-noise compound that's easy on rotors and suitable for everyday driving. The more-aggressive HP-Plus and HPS compounds are likely a better choice for serious road-course and autocross flogging.
The last step was to replace...
The last step was to replace the existing fluids with these high-performance variants from Amsoil. These specially formulated lubricants will ensure that the engine, brakes, steering, and differential operate in peak form on the track.
With the upgrades in place,...
With the upgrades in place, the car's lap times at Gainesville tumbled by more than a second, from 1:09.08 down to 1:07.95. While that might not sound like much, in point of fact it represents a tremendous improvement on a short course like G-ville. The driver attributed the difference to increased corner-entry speeds-thanks to the rubber and brake upgrades-and the new OS Giken LSD's ability to spit the car out of turns without lighting up the tires.
Improvements on the dragstrip...
Improvements on the dragstrip were no less dramatic: 60-foot times dropped from 2.15 to 2.06 seconds, indicating that the new diff and tires were really putting down the power off the line. (Quarter-mile times were essentially unchanged, likely because of the higher ambient temperatures recorded during our follow-up test.) Factor in greatly enhanced driveline durability and the fact that none of these modifications exacted a penalty on the street, and this project rates as a success on all counts.