Tremec six-speed manual transmissions have served the Corvette since the introduction of the C5 generation in 1997. The T56 appeared in all models until the C6 Z06 introduced the stronger Tremec TR6060 in 2006. Two years later, the 430-horse LS3 brought the TR6060 to the Corvette lineup across the board. The '09-up ZR1's LS9 is also matched with the TR6060, which happens to be rated for 700 lb-ft of torque.

The transmission, of course, transfers torque to the rear axle, and here again, the C5 and C6 have featured a number of different rearend assemblies of varying capacities. Not surprisingly, the C6 Z06 and ZR1 models offer the largest and strongest. Beyond what the factory offered, there are aftermarket upgrades for the trans, the rear axle, and everything in between. These are capable of handling performance upgrades ranging from basic bolt-ons and road-course setups to high-boost power adders designed to send a Corvette down the dragstrip in single-digit e.t.'s.

But it isn't quite as simple as ordering a "Viper" upgrade for your T56 or swapping in a Z06 rearend to ensure your modified Vette is best equipped to handle big power. Take the tried-and-true T56 six-speed, for example: While it may not be the most refined manual transmission in the universe, it has nonetheless performed admirably in the realms of performance and efficiency. You just can't beat the satisfaction of clicking into Sixth gear on the freeway and approaching 30 mpg with more than 400 hp on tap.

It's when the engine's output is increased significantly that the T56's limits are revealed. In the C5 and early C6s, this transmission's maximum torque rating was 450 lb-ft. That's fine for most mildly modified street cars, but in this age of 700-, 800-, and even 1,000-horse street engines, the T56 just doesn't have the strength to cope.

"Once you get to about 700 hp, the T56 is done," says Rodney Massengale, at RPM Transmissions, in Anderson, Indiana. "It's a very good transmission, but it just wasn't designed for the kind of power that LS engines have been pumping out for the last few years."

And what about that vaunted Viper version of the T56? The biggest difference is the output shaft (also known as the main shaft). The Viper unit uses a beefier 30-spline shaft (1.290 inches in diameter), versus the Corvette's 27-spline shaft (1.175 inches in diameter). That's it, really. Some enthusiasts will tout the Dodge's steel 3-4 shift fork, too, but having torn down countless transmissions over the years, Massengale says his technicians find steel forks only occasionally in the Viper versions.

"You can upgrade the T56 with the Viper main shaft and some other internal upgrades, but the gears and synchros will be the same," he notes. "If you really want to step up to the next level, you've got to look at the TR6060."

The TR6060 has its roots in the venerable T56, but it was designed for much more powerful engines such as the LS7 and LS9. It's also more refined, with smoother, more direct shifts.

The TR6060's greater strength comes from larger and stronger components, as compared with the T56. Take First gear, for example. It measures 4.9 inches in diameter and is 0.98-inch thick in the older trans. In the TR6060, First is still 4.9 inches in diameter, but it's 1.19 inches thick--a significant 22 percent increase. It's a similar comparison for all the gears, and the TR6060 also gets a 31-spline output shaft measuring 1.36 inches in diameter.

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Automatic options

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.

Awesome axles

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."

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.

SOURCE
RPM Transmissions
800-406-1109
http://www.rpmtransmissions.com
Driveshaft Shop
4530 Southmark Dr.
Salisbury
NC  28147
704-633-2380
www.driveshaftshop.com
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