Mark Houlahan
Tech Editor, Mustang Monthly
September 5, 2003

Back in the early performance heyday of the 5.0 Mustang, the factory T5 five-speed transmission was usually able to cut the mustard. Even when a supercharger was installed, the T5, often rebuilt with better internals, would last behind these performance upgrades. But as times changed and more people found ways to make power, the T5 became easier to break--a stroker engine and gobs of torque going through a grippy clutch usually spelled doom for the T5.

The boys at TTC (Tremec Transmissions back then) took an existing truck-based transmission and modified it to work with the 5.0 Mustang and offer up even more torque capacity. But in a few years the aftermarket would manufacture enough power to make even the Tremec TKO or TKO II wince at the thought of being bolted up behind 600 or more lb-ft of torque.

Depending upon the application, the D&D Performance T56 conversion kit ($2,999) will contain a new crossmember and adapter, a 31-spline yoke, and, of course, the transmission. You have the option of purchasing a complete driveshaft instead of shortening yours. We opted for just the required clutch disc since we'd recently installed an 11-inch D&D Super Heavy Duty clutch ($299) in the car.

Tremec then offered the T56 six-speed in several GM platforms and eventually the Dodge Viper. Unfortunately, there was no easy way to get this stout transmission and its much-sought-after extra Overdrive gear into a Mustang.

Eventually, Tremec came out with Ford T56 aftermarket applications. Even the '03 Cobra bangs gears in a T56. But these models are rated at roughly 450 lb-ft of torque. This is much more than the T5 (300), the T45 (325), or even the Tremec 3550 (350-425 lb-ft depending upon model). The only other bolt-in transmission rated higher is the Tremec 3550 TKO II at 475, but it has one less gear and Fifth is a higher 0.82 ratio versus the regular 0.68.

For those who need even more torque capacity, D&D Performance--long a group of wizards in the ways of the manual transmission and clutch--began working on retrofitting a Viper-based T56 into the Mustang. The Viper's higher rating of 550 lb-ft means more strength behind the high-end street cars putting out 700 or more horsepower. And, with the Viper T56 you not only get the coveted extra gear, but also Sixth gear is a low 0.50 ratio versus the Ford T56's 0.62. After taking apart a T56, D&D "blueprints" it with the latest upgrades, including a steel 3-4 shift fork, carbon-fiber blocker rings on all gears (except Reverse, which still has brass), and verifying clearances and other minor details to ensure strength. The folks at D&D tell us in the five years they've been selling the Viper-spec trans to owners with "certifiably crazy power levels," they've never had a report of breakage.

The Viper T56 package comes with everything to install the transmission into your Mustang (there are Viper packages for the 4.6 and 5.0, and regular T56s for the 5.0) and only requires minor modifications to the crossmember mounting, shortening the driveshaft (or buying a new one), and adding fluid. Some applications will require a clutch disc change and a new, longer, speedometer cable.

Read on as we tackle the swap from a five-speed in Editor Turner's '98 Cobra to a new Viper T56.

Horse Sense: The T56 found in the Dodge Viper is not a direct bolt-in, of course. The staff at D&D takes the stock Viper-spec trans that has a torque capacity of 550 lb-ft (which they say is a conservative number) and blueprints the internals to bring the trans up to the latest updates and to ensure every- thing is perfect. Then they mate the T56 to a custom bellhousing, allowing a direct bolt-in behind a 4.6 mod motor (or a 5.0), having to do nothing more than shortening the driveshaft and relocating the transmission crossmember.