In our last issue, we detailed the job of installing a Zip Products HD Dana 36 differential in our '96 C4 coupe. Although the installation itself went off without a hitch, all that loosening and retightening of suspension components exacerbated what previously had been only a minor problem with the Vette's rear ride height. Indeed, no matter how hard we cranked down on the stock spring bolts, the freshly modified car persisted in evincing the kind of arse-in-the-sky stance you might expect from a '79 Camaro running a pair of Gabriel Hijackers. Not cool.
To preserve our C4's handling balance and restore a measure of aesthetic respectability, we contacted Vette Brakes & Products in St. Petersburg, Florida. VB&P has been fulfilling the chassis and suspension needs of the Corvette market for more than 30 years, and today boasts what is arguably the most wide-ranging product lineup in the industry. The company sent us a kit containing a pair of custom-length lowering bolts and all the hardware required to install them on any '84-'96 model.
As we have so often in the past, we decided to farm out the work to AntiVenom, a highly regarded Corvette specialty shop in nearby Seffner, Florida. Although this is a job a skilled mechanic could conceivably perform in the driveway, access to a lift, an air gun, and a specialized spring-compression tool will reduce installation time considerably. In our case, AV's Greg Lovell and Casey Willard had the bolt kit in and the car's ride height set in just under an hour. Follow along now to see how they did it, and to check out the finished product.
With our C4's new rearend properly broken in, we day-tripped it up to Gainesville Raceway to see what effect the differential's shorter 3.54 gearset had on quarter-mile acceleration. The jaunt also afforded us an opportunity to see how the fresh ring-and-pinion affected the car's fuel mileage and driveability, the two areas that traditionally suffer most as a result of such installations.
Since the lower (numerically higher) gears promised to boost the car's off-the-line snap, we augmented them with a pair of Nitto's NT555R radial drag tires measuring a brawny 315/35R17. Unlike some of the other "drag radials" on the market, the Nittos feature a wear rating of 100 and a tread pattern that makes them suitable for occasional street use. (Just be sure to keep your Vette's traction-control system engaged in day-to-day driving and keep a close eye on tread depth, especially if you plan to use these tires in the rain.)
Paired with the NT555Rs were a brace of Nitto's NT555 street-performance tires, sized 275/40ZR17. The standard-issue 555 has an overall look similar to that of its competition-bred "R" sibling, along with a treadwear figure of 300 and a more well-rounded overall performance envelope. If the idea of using the same set of tires for street and strip duty appeals to you, this combination of front 555s and rear 555Rs could be just what you're looking for.
As for driveability, the four-hour round trip revealed no new bad habits stemming from the differential swap. The Zip rear remained as smooth and noise-free as when we installed it, while the 3.54 gears put the car's 75-mph cruising rpm at a perfectly tolerable 2,400 rpm. A check of the trip computer at day's end, meanwhile, revealed an average-mpg figure of 20.0, which could easily have been improved upon by a less excitable driver.
Unfortunately, our track session proved something of an anticlimax. Given our previous best pass of 13.268, we were hopeful that the new gearset would, at a minimum, drop the car into the 13.1-second range. Instead, we could do no better than a 13.307 in the midsummer Florida heat, followed by a 13.321 before the sky opened up and cut short the day's activities.
Interestingly, the car did pick up more than 1 mph in trap speed (104.57, as opposed to our old record of 103.35), perhaps an indication that the lower axle ratio was putting the big LT1 more deeply into its power band at the top of the track.
Given that the C4 displayed no inclination whatsoever to spin the gummy Nittos off the starting line-and believe us, we tried-it's clear that either more power or a higher-stall torque converter will be needed at this point to make a significant impact on e.t.'s. We'll be addressing both in the near future, starting with a performance-oriented rebuild for the car's tired 4L60E trans. Stay tuned.