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.
1 Broken parts are hard on...
1 Broken parts are hard on the ego...and the wallet. Whether it's a broken transmission, snapped axle shafts, or, as seen here, a shattered differential housing, an insufficiently upgraded drivetrain won't stand up to big horsepower forever.
2 Let's start with the manual...
2 Let's start with the manual transmission. The TR6060 used in LS3-, LS7- and LS9-powered C6s has a 700-lb-ft torque capacity--nearly 56 percent more than the 450 lb-ft capacity of the T56 used in C5s and early C6s. The TR6060 also features triple-cone synchronizers on gears 1-4, a stronger input shaft, stronger shift forks, and other enhancements. Photo courtesy GM
3 A close-up profile of TR6060...
3 A close-up profile of TR6060 (left) and T56 Second gears shows the dramatic difference between them. The TR6060 gear is 1.200 inches thick, compared to the T56 gear's 1.075-inch thickness.
"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.
4 The 3-4 synchros (left)...
4 The 3-4 synchros (left) are larger and stronger for the TR6060, too. This one has a 4.760-inch diameter, compared with the T56 part's 4.115-inch diameter. It also has fine-tooth gear angles, with more teeth (64 vs. the T56's 36, in the case of these examples).
5 The main shaft in the TR6060...
5 The main shaft in the TR6060 (left) is also much beefier, with a 1.360-inch diameter and 31-spline output section vs. the 1.175-inch/27-spline shaft of the T56 unit. The TR6060's thicker shaft contributes greatly to the transmission's higher torque capacity.
6 The 1-2 synchronizer-assembly...
6 The 1-2 synchronizer-assembly retaining ring in the T56 (right) is known to break on occasion. The TR6060 retainer has been upgraded to prevent this potentially catastrophic failure.
7 Both the TR6060 and the...
7 Both the TR6060 and the T56 come with a plastic shifter insert (right), which is prone to wear and cracking. Replacing it with a bronze insert is a must, regardless of the transmission you're using.
8 Bronze fork pads should...
8 Bronze fork pads should be used in place of the factory plastic pads. The pads press onto the shift forks, which move the sliders back and forth to engage the gears. When the plastic pads break or wear, the sliders can't fully engage, causing the transmission to pop out of gear.
9 When it comes to automatic-shifted...
9 When it comes to automatic-shifted powertrains, swapping out the stock four- or six-speed trans for a prepped 4L60E like this one can be an easy way to improve durability and performance. With a factory shifter in the console, the conversion looks OEM, too--even in Z06s and ZR1s.