This image shows the driver...
This image shows the driver side of our '96 C4 just prior to installing the lowering kit. Note that while the rear of the Vette rode visibly higher than the front, the difference probably wasn't so dramatic as to warrant corrective measures on its own.
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.
This lowering-bolt kit from...
This lowering-bolt kit from Vette Brakes & Products (PN 32332) includes two 10-inch Grade 8 bolts, four new rubber spring cushions (polyurethane is also available), a pair of Nylock nuts, and a quartet of washers. At only $31.99, it's a true rarity among suspension upgrades: one that just about anyone can afford.
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.
E.T. Update
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.)
The passenger side was quite...
The passenger side was quite another matter. Here, the gap between the fender lip and the tire tread measured almost 5 inches at the top of the wheel arch, lending the car the kind of high-riding stance more commonly seen on four-wheel-drive trucks.
A view from the rear provides...
A view from the rear provides a good look at the car's pronounced sinistral slouch. Our hope was that the lowering bolts from Vette Brakes & Products would allow us to even out the rear suspension to achieve a more balanced stance.
The stock spring bolts offer...
The stock spring bolts offer some degree of adjustability, but not enough to compensate for worn-out spring cushions. Surprisingly, the two passenger-side cushions (shown here) checked out OK, meaning they weren't part of the problem. That being the case, we'd have to hope that the additional adjustability provided by the VB&P bolt kit would be enough to bring that corner of the car back down to earth.
Moving to the driver side,...
Moving to the driver side, we found this oddball suspension bushing wedged into the top spring-cushion perch. Not only was it the wrong part for the job, it practically fell apart in our hands when we removed it. Ah, the joys of working on older cars.
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.
Since we covered the rear-suspension-disassembly...
Since we covered the rear-suspension-disassembly process in detail in our last installment, we won't go over it again here. Fortunately, accessing the spring bolts only requires that you decompress the spring itself and remove the castellated nuts that hold the bolts in place. Once the old bolts are out, simply install the VB&P units in their place along with the new cushions and hardware.
After you've installed the...
After you've installed the bolts and reattached the spring, use an air gun to adjust the car's ride height to your desired setting. It's unlikely you'll be able to nail the right stance the first time, but using the stock bolts as a reference point, as shown here, should at least get you in the ballpark.
After you've made your initial...
After you've made your initial adjustments, lower the car and drive it a short distance to allow the suspension to settle. If additional ride-height tuning is required, raise the car again and tighten or loosen the new spring bolts as necessary. Repeat the process until you've dialed in just the right stance.
This photo shows the rear...
This photo shows the rear aspect of the car after the lowering-bolt installation. Once again, the tremendous overall improvement in ride height is at least partly offset by the remaining right-left imbalance. On a brighter note, it's extremely unlikely you'll run into the same problem on your car.
While the passenger-side rear...
While the passenger-side rear corner looks much better than before, it's still a hair higher than we'd like-this, despite our having run the VB&P bolt to the limit of its adjustable range. AntiVenom's Greg Lovell told us he'd never seen such a pronounced corner-height imbalance on a C4 before, and attributed the mismatch to a worn-out rear spring. Looks like we have more suspension work to do.
After having the Vette aligned-a...
After having the Vette aligned-a necessary step after any suspension work-we snapped a few more pictures to show the car's ride height in post-install form. If you compare this driver-side shot to the previous one, you can see that the rear now sits just a tad lower than it did before the installation. From an aerodynamic standpoint, it's always best to maintain at least a slight front-to-rear rake.
Nitto's NT555R is a DOT-approved...
Nitto's NT555R is a DOT-approved radial tire with a tread compound and sidewall designed to maximize acceleration on the dragstrip. For optimum results, you'll need to perform a moderate burnout (water optional) to remove any surface dirt and get the rubber nice and sticky. We ran ours at 20 psi at the strip, then aired them back up to 35 psi for the drive home.
The Nitto NT555 features a...
The Nitto NT555 features a tread design similar to that of the NT555R, but its deeper blocks and harder rubber compound make it more suitable for fulltime street use.
The Vette netted an even 20...
The Vette netted an even 20 mpg over the course of our trip-a respectable figure given the 75-80-mph speeds involved and all the abuse heaped on the car at the track. On the highway, the C4's tach showed 2,400 rpm at 75 in Overdrive.
Although not the quickest...
Although not the quickest the car has run, our day's best pass of 13.307 seconds at 104.57 mph did set a new high for trap speed. Pairing a looser converter with the new 3.54 gears should yield a major improvement in e.t.