So whatta we got here, some kinda Frankenstein? Nope, but it's a mischievous compilation all the same. A conjoined six-speed transmission created from Viper and LS1 parts may sound strange, but follow along and you'll see why it's just what the good doctor ordered for our monster Biscayne.
The Tremec T56 six-speed, double overdrive transmission was initially designed by Borg-Warner to handle the brute torque of the original Dodge Viper in 1992. A year later, General Motors bought into its virtues, and Ford came along quite a bit later and planted one in the Mustang Cobra. Though there are many external differences among the transmissions, internally they all look unmistakably the same.
Other than making one fit in a strange chassis along with the hydraulic clutch setup, the one drawback to the T56 is its Bonneville-gear overdrive ratios, but there are two schools of thought on this. Dropping 600-800 rpm with a normal overdrive transmission is usually a great improvement, but making a hi-po powerplant lope along at 1,500 rpm, frequently well below where it starts making power, is another matter entirely.
As a result, Sixth gear was all but useless unless you live in Montana or drive the Autobahn on a regular basis. It also takes some serious horsepower to be able to push that steep of a ratio. For the more pragmatic amongst us, >> these same deep ratios yield amazing mileage numbers, even behind highly modified motors. In its strongest incarnation, a stock T56 will routinely absorb 450 lb-ft of torque, a value assigned after a dizzying WOT test that lasts for 24 hours. The two overdriven gears are 0.74 and 0.50:1, respectively. Some of us like 'em, but others figured they were simply stuck with the crazy gearsets.
George Kreppein, vice president of Manufacturing for Rockland Standard Gear (RSG), has used his relationship with the folks at Tremec to manufacture the proper combination of parts to improve upon the situation. Now, 0.80 Fifth and 0.74:1 Sixth gearsets are available, along with a new 25-spline input/30-spline output mainshaft to make it all work.
The new mainshaft features longer splines where the Fifth and Sixth gearset normally resides, allowing for individual Fifth and Sixth gears--the stock setup employs a one-piece Fifth and Sixth gear assembly. There are four main pieces to the Tremec T56: The case, adapter plate, clutch housing, and extension housing--all made from die-cast aluminum. This helps keep the weight down while maintaining superior strength. The fully assembled transmission weighs about 130 pounds. Like all modern manual transmissions, the T56 employs an internal rail-shifter system that protects the linkage and shift rods within the housing. The shifter itself bolts into the extension housing.
The T56 is a robust and reliable transmission for both the street and track right from the factory. It features tapered roller bearings front to back, constant-mesh synchronized gears, needle bearings under the speed gears, and the double-overdrive design allows for closer-ratio gears throughout the range of the transmission (at least until you hit Sixth).
By upgrading the shifter-gear fork inside the transmission during the buildup, RSG addresses the jumping out of gear problem, a nit common to the T56. And paying close attention to internal tolerances as the unit is assembled ensures long transmission life.
Rockland Standard Gear has been building race versions of the T56 for years. RSG builds various drag and road racing transmissions, including one for the company president's weekend ride: a Grand-Am Cup Z06 Corvette. Mike Weinberg's team, Powell Motorsports, has won the Grand-Am Cup in this road racing series four years in a row (1997-2000) and runner-upped in 2001.
RSG has a bunch of its T56 combinations in the real world living behind engines putting out well over 650 hp at the rear wheels. It has built these bulletproof iterations for road- and drag-racing applications, along with normal replacement applications for people who want the improved gear ratios and internal upgrades for their street ride. The privilege can be yours too, for a mere $3,500 (retail).
With the proper tweaks and know-how, it is now possible to custom build a T56 with alternate overdrive ratios for many applications, and fabricators and transmission rebuilders (e.g., Keisler Automotive Engineering, www.keislerauto.com) have been creating all-inclusive kits and custom adapters to fit the T56 into various vehicles for quite some time. This gear change is good for an additional 800 rpm at the end of a straight, so it'll certainly help our torque monster Biscayne to be more driveable.