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To put it bluntly, you C6 guys with the six-speed 6L80E slushbox have few options at the moment. There are plenty of aftermarket upgrade systems for four-speed-equipped '05 models and C5s, but the computer "tuning" of the six-speed autos hasn't been fully cracked yet. Accordingly, if the planned rear-wheel output of your Corvette is north of 700 hp, retrofitting a beefed-up four-speed might not be a bad option.
In fact, when it comes to high torque capability, a properly built automatic transmission is sometimes preferable to a manual. Just look at the quickest Corvettes on the dragstrip these days--even the Z06s and ZR1s--which don't run stick shifts. Specially built 4L60E, 4L80E, and even Turbo-400 transmissions are the ways to go.
Switching to a 4L80E or Turbo 400, however, will require surgery. Each is a few inches longer and taller than a 4L60E, so tunnel modifications are needed, along with a shorter torque tube and driveshaft. It's a serious commitment to step up to one of these transmissions, but they're strong and dependable.
"The 4L60E is great for cars running from the 11s into the 9s, as I can personally attest," says Massengale. "If you're aiming for bottom 9s or the 8s, you'll probably want to go with the 4L80E or Turbo 400."
And when it comes to deciding between the two, the three-speed Turbo 400 is ultimately more adaptable for performance use, with a greater range of gear ratios available. It also offers a faster trans brake, and, unlike the 4L80E, there's no lock-up converter. In short, it's a true racing transmission--complete with all the positive and negative traits that distinction entails.
10 Even if the stock transmission is retained, its main shaft should be swapped out for a
11 When the capabilities of the 4L60E have been exceeded, a 4L80-series or Turbo-400 (sho
12 RPM Transmissions modifies the Turbo-400 (bottom) to fit the C5/C6 chassis, but the ca
As for the rear-axle system--that is, the differential, axle shafts, and so on--it's once again the C6 Z06 and ZR1 components that deliver the greatest strength. C5 and C6 Corvettes have used a variety of differential housings, with the C5's being the smallest and weakest. While they're all mostly upgradeable, a C6 Z06 or ZR1 housing will bolt right into a C5 chassis using a C6 chassis cradle. The Z06/ZR1 diffs are easily upgradeable, too, with even-heavier-duty components to ensure bulletproof performance on the street or track. The Quaife torque-biasing differential is one notable example.
"The Quaife is the best option for high-powered cars that are going to see some time on the dragstrip or road course," says Massengale. "We can upgrade stock differentials for cars mostly used on the street, using compressed-carbon clutch packs in place of the stock bronze clutch packs, among other modifications. There are also a few aftermarket differentials that provide greater capacity than the stock diff, but for serious power and performance, the Quaife is definitely the way to go."
Unlike other differentials, the Quaife does not incorporate clutch packs. It's strictly gears inside, for maximum strength. Sets of floating helical gear pinions mesh to provide "normal" performance, but in the presence of wheelspin, torque bias is generated by the axial and radial thrusts of the pinions in their pockets. The resulting friction forces enable the differential to transmit a greater proportion of the torque to the appropriate wheel. According to Quaife, it's a progressive process and doesn't entirely lock the diff. At around $995 for the unit alone, it's not a cheap solution, but it's a bulletproof one.
While upgrading a C5/early C6 to the TR6060 trans and a C6 Z06/ZR1 axle will deliver the torque capacity to support a high-horsepower engine combination, Massengale advises that the rest of the driveline needs to be addressed, too.
"The biggest killer of transmissions and rearends isn't necessarily too much horsepower or torque channeled through weak parts, it's abuse and wheelhop," he says. "We've seen cars with only about 400 rear-wheel hp break the differential housing because of wheelhop, and we have customers who send twice that power through a basically stock manual transmission because they know how to launch correctly and shift precisely. The stronger parts are fundamental, but they're no replacement for driving skill."
13 Eliminating the transmission/differential housing's natural tendency to rotate upward
14 C5 and C6 Corvettes have come with a number of different differential housings over th
15 The difference in size between a C5 (right) and a ZR1 housing is significant. Note, to
Massengale walked us through the setup his shop put together for Ohio customer Mark Carlyle, whose Atomic Orange, single-turbo '07 Z06 has run as quick as 7.75 at 183 mph using an automatic transmission and the production-based independent rear.
"Starting at the engine, Mark's car runs a new flexplate and bellhousing, along with a new driveshaft and driveshaft couplers," says Massengale. "The transmission is a Turbo-400, and the differential is a Quaife set in a ZR1 housing. And, of course, the output shafts are stronger, and even the CV shafts and wheel studs are stronger. It's a complete system from end to end, backed up with the right suspension."
Along with the trans and rearend, Carlyle's car runs a racing-spec flexplate, bellhousing, torque tube/driveshaft assembly, and more. That's more than you'll need when bolting a blower onto your C5, but it does illustrate the importance of assembling the right combination.
"It's easy to spend too much on parts you don't need, or to pick the wrong part and wonder what went wrong later. [For this reason] upgrading your driveline shouldn't be done by blindly picking parts out of a catalog or off a website," says Massengale. "Because the production parts are basically interchangeable, it's easy to put together a transmission/axle package that's tailored to the power level of a vehicle. For example, we can use a T56 with a C6 Z06 differential--or match a TR6060 with the ZR1 diff, mount it to a C6 cradle, and slide it up into a C5. You really just need to consult with an expert to pick the right parts for the expected performance level of your car. You'll save money and frustration in the long run."
Be smart and honest about your performance upgrades. If you plan to drive your car primarily on the street and keep its output level below the 700hp threshold, there's no reason to spend the extra money on a TR6060. RPM Transmissions and a number of other drivetrain specialists can deliver a strengthened T56 with a 30-spline output shaft, solid synchronizer keys, bronze fork pads, a steel 3-4 shift fork, and other upgrades for roughly half the cost of a full TR6060 swap. The same goes for the rear axle.
A little forethought will help put your Vette's power to the pavement efficiently--rather than putting expensive parts on the pavement in tiny, expensive pieces.
16 A C6 differential housing will fit in a C5, but it's necessary to swap out the entire
17 Quaife's torque-biasing differential contains no clutches, which can slip and burn out
18 When using the Quaife with a C6 Z06 or ZR1 housing, there's only one ratio choice: 3.4
19 RPM Transmissions worked with the Driveshaft Shop to develop output shafts that mirror
20 One of RPM Transmissions' unique replacement parts is the torque-converter drive plate
21 Next in line for an upgrade is the driveshaft. Shown here is one of the Driveshaft Sho
22 Matched with the stronger driveshaft are stronger couplers. These urethane-based parts
23 Get the combination right, and consistent performance is virtually guaranteed. This is
24 This photo amply illustrates the importance of upgrading the couplers. This is a stock