Jack Ryan
August 1, 2006

S197 Owners rejoice: D&D Performance has bundled together a bolt-in, six-speed package centered around its proven Viper T56 gearbox. The $4,995 kit has everything needed for the swap, including an upgraded clutch disc and a one-piece aluminum driveshaft. Well, OK, you still have to supply a couple buddies with strong backs.

Quick, if the new Mustang GT has one weakness, where would it be? Hmm, let's see, it has 300 horses right from the factory; rides, handles, and launches like no Mustang before; has world-class looks outside and in; and heck, it even has 31-spline axles and 3.55 gears. Ah, but its standard tranny is just a five-speed, and there's no six-speed on the option list. Well, friends, take heart, because D&D Performance now has you covered with a '05-'06-specific version of its legendary Viper T56 gearbox.

Now don't go taking this as undue criticism of the GT's factory Tremec 3650, which is a robust box that can handily shrug off OEM power levels. But let's have a show of hands from all you readers planning on leaving your S197 at the factory power level. Thought so. Strength aside, perhaps the S197's least satisfying tranny aspect, especially for the quarter-mile crowd, is a factory setup that has the shifter mechanism hanging back behind the transmission, connected to both the gearbox itself and the car's unibody. Naturally, the transmission, firmly attached to the engine, squirms around in relation to the body, so the shifter's link arm to the 3650 has to allow for this motion with a very soft rubber mounting bushing. Not surprisingly, this lends a correspondingly rubbery feel to the shifter in everyday driving; worse yet, it also makes a 2-3 speed-shift rather challening under the torque load of wide-open throttle.

Luckily, D&D's Viper T56 swap simultane-ously addresses both shortcomings. As its name implies, this bull-strong six-speed was originally designed for the monumental torque output of the Dodge Viper's 8.3-liter (English translation: 506ci) V-10. D&D's Don Walsh Sr. pegs its torque-handling capability at around the 650 lb-ft mark. It also offers nice, tight ratio spacing (see sidebar), starting with a 2.66:1 First gear, all the way up to a highway-friendly 0.50:1 Sixth. That deep Overdrive turns the GT's 3.55 axle ratio into a fuel-stingy 1.78:1 (as compared to the 2.41:1 overall ratio generated by the 3650's 0.68 Fifth gear.) While fuel economy won't be the primary reason you'll want a Viper T56, these days it certainly doesn't hurt.

As a real bonus, the D&D Viper T56 also does away with the half-tranny, half-body shifter mounting scheme, replacing it with a much more conventional-and precise-setup borrowed from the current Pontiac GTO parts bin, of all places. Another return to convention is the kit's included one-piece aluminum driveshaft, replacing the S197's stock, heavy, two-piece, articulating shaft, and necessary not just because of tranny-to-axle length differential, but also because the T56 reverts to a familiar slip-yoke output instead of the S197's factory companion-flange arrangement. If this all sounds a little muddy, our accompanying photos and captions should help clear things up.

Part of the Viper T56's Herculean strength comes from its 26-spline input shaft (the 3650, in Ford tradition, has a 10-spline input), so the D&D package also brings along a new D&D/McLeod Super Heavy Duty clutch disc for use with the stock pressure plate. Speaking of clutches, the S197's is hydraulically disengaged, instead of the cable release used on older Stangs. D&D's S197 version of Viper T56 is specifically engineered to accept the stock hydraulic throwout bearing.

In other words, this is a comprehensive, basically plug-and-play kit that D&D Performance offers, and installation is straightforward, as Don Walsh Jr. and crew at Walsh Motorsports demonstrate.