Dale Amy
March 1, 2001

Step By Step

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One powerful six-shooter. If you have visions of a six-speed dancing in your head, D&D Performance can set you up with its Viper T56, offering more than double the rated strength of some T5s and having 110 lb-ft more torque handling capacity than even the normal aftermarket T56.
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Here’s the D&D Viper T56 (top) beside a T5 (now also built by TTC). What is immediately apparent is the T56 is not a toploader as it lacks the T5’s removable top cover. Also obvious is that the mount bosses are cast in different locations, meaning the T56 can’t use the factory transmission crossmember.
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Up front, the Viper T56 has a 26-spline input shaft, whereas the T5’s is a 10-spline. You’ll have to ante up for a new clutch, unless you already have a Tremec TKO installed (which shares the T56’s 26-spline input). Of course, if your power level merits the Viper T56’s strength, you probably need a new clutch anyway. The T56 is a bit longer than the T5 and its output shaft has 30 splines in comparison to the T5’s 28, so a new yoke and shorter driveshaft are needed for the swap. The Ford aftermarket T56 also requires a short-ened driveshaft.
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To adapt the mighty Viper gearbox for pushrod Mustang use, D&D casts up a new midplate (T56 front cover) and bellhousing. The Viper tailhousing is machined to accept the speedo driven gear, and the plastic speedo drive gear modified to spline onto the T56 output shaft. The long Viper input shaft must also be machined for Mustang use, and the steel sleeve from an SN-95 input bearing retainer is used to guide the stock throwout bearing.
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The $2,995 D&D Viper T56 comes with a billet shifter. Add the $199 crossmember and bracket kit, along with the $49 speedo cable (for Fox applications) in the foreground, for a total of $3,243. If you can do without the ultimate strength of the Viper gearbox, D&D also sells the regular aftermarket T56 for $2,295. It too requires a $49 speedo cable and its own distinct $249 bracket kit (thanks to a different tailhousing), totaling $2,593.
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D&D can sell you a 26-spline clutch to go with your Viper T56. The company’s technicians like the Valeo heavy-duty pairing, or a Valeo pressure plate teamed with a McLeod Dual-Performance disc. A good driveline shop will likely shorten your existing driveshaft for around $150, but a new 30-spline yoke will be required to mate with the Viper T56 (or a 31-spline version for the regular aftermarket six-speed). D&D offers a Dynotech custom big–U-joint aluminum shaft (and yoke) for $399.
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On the right are various internals from the Viper T56—input and output shafts, countershaft, shift forks, Third gear, a synchro blocker ring, and a synchro slider—in stark contrast to their T5 counterparts on the left. Purely from the size of its components, the T56 appears to have been manufactured to bolt up behind a locomotive. Even Mr. Magoo could see why the D&D T56 has about twice the strength of most T5s.
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The D&D Viper T56 is on the bottom; the regular aftermarket T56 is on the top. Aside from the strength difference resulting from their respective gearsets, the bellhousings, midplates, and tail-housings are also distinct. Notice the relative fore/aft positions of the shifters on the tailhousings. D&D designed its bellhousing about an inch longer to correctly locate the shifter in the Mustang’s transmission tunnel, and this extra bellhousing depth has the added advantage of providing more room for clutch options. The regular aftermarket T56’s bellhousing is short enough to practically preclude use of a twin-disc clutch.
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Now that you’re convinced a D&D Viper T56 should go high on your Christmas list, you might be wondering what kind of wrestling match will be involved in installing it. To answer that question, we visited the new Walsh Motorsports shop (see sidebar, A New Kid in Town) where Don Walsh Jr. slid one of the D&D T56s in an ’87 coupe, in place of its existing Tremec TKO, as shown here. The TKO uses the factory transmission crossmember, so the process would really be no different with a T5, except for the clutch.
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After unbolting the driveshaft, dropping the crossmember, and unplugging the speedo cable and reverse wiring, the old tranny can be unbolted from the bellhousing (or in this case, scattershield). Obviously, the exhaust H-pipe was removed first. Speaking of exhaust, though our T56 fit just fine with the subject car’s long-tube headers (and driveshaft loop), D&D has not yet found an X-pipe that will clear either the regular aftermarket or Viper T56. Maybe you could try one at the rear of the car, à la the FR500.
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The old bellhousing, or scattershield in our case, can then be unbolted from the block. This brings up a point: Though fine for a dual-purpose car, the T56 is not really targeted at the dedicated dragstrip crowd due to the lack of an available steel scattershield. Who needs six speeds to cover a quarter-mile anyway? Most Viper T56 gearboxes sold by D&D so far have gone into killer, blown street cars, though the road racer is also an obvious market.
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Because of the car’s former TKO, the spline count on its clutch was OK. If removing a T5 or Tremec 3550 (both having a 10-spline input), remember that a new clutch will be required. As it happens, we had to temporarily remove the clutch and flywheel anyway, just to exchange the scattershield block plate for a factory version.
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The D&D T56 is shipped with the bellhousing attached. If you have short-tube headers you can likely bolt the whole assembly up into place at once. With the obstruction of long tubes, it may be easier to bolt the bellhousing up first, then maneuver the gearbox into position. If yours is a Fox chassis, its original clutch fork and pivot ball can be reused on the T56—those with an SN-95 must get a Fox clutch fork. One of the reasons D&D had to cast up a whole new midplate was to properly orient the pivot ball for correct clutch-linkage geometry.
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With the T56 (either regular aftermarket or Viper) bolted up, you’ll have to notch out a section of the factory block plate (arrow) for the clutch cable. This is best done with a steady hand and carbide cutter.
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You might be wondering why we bolted up the T56 before determining the new crossmember location. Don Walsh Jr. explains that due to such variations as the Mustang’s normal production tolerances, or changed engine mounts, the most accurate way to locate the new crossmember is to simply use the installed transmission as a locating template. With the supplied relocation plate and tranny mount in place as shown here, the crossmember and its brackets can be bolted up as an assembly.
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With the tranny jacked up, the crossmember’s brackets are then used as templates to mark the subframe for drilling. Though there is no set measurement, to keep things square make sure the brackets are located equidistant from the aft ends of each subframe. The subframes are then cross-drilled for through-bolts.
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Here, the crossmember and its brackets have been secured in place with the supplied hardware. You’ll need D&D’s longer speedo cable to reach around to the right side of the T56, where the Viper tailhousing has been modified to accept the Mustang speed sensor (unless yours is an SN-95, in which case the factory speedo wiring will reach). Likewise, the reverse-light wiring must be fed across the top of the T56 to the switch located on the passenger side.
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Finished, except for bolting up the H-pipe, the D&D Viper T56 fit just fine with the long-tube headers and driveshaft loop on our subject Fox. D&D offers its Viper six-speed for both Fox and SN-95 bodystyles, though the required driveshaft length differs slightly between them. Consult with D&D on the specific length for your application. 5.0
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We're big fans of Transmission Technologies' (Tremec) T56, a brawny six-speed gearbox that, we believe, deserves a good home in many a Mustang (and has finally found better-late-than-never Ford factory application in the 2000 Cobra R). In recent years, the T56 has become something of a generic household term when we T5 owners commence to salivating over the prospect of a hearty six-speed behind our 5.0s. What may not be common knowledge, however, is that there is more than one T56 variant, and though all are strong, there's always a lead dog in any pack. Two distinctly different versions of T56 are now available to the 5.0 crowd, the toughest of which is offered only through D&D Performance.

Rated at a healthy 440 lb-ft of torque capacity, a Ford small-block-specific aftermarket version of the T56 has been available for some time now. Internally this is the tranny as it appears in the new Cobra R. Having a gearset with a 2.97:1 First and a 0.62:1 Sixth, the T56 in this ratio configuration first appeared in the '93 V-8 F-bodies. This is an excellent transmission that is to the production T5 what forged steel is to pot metal.

Notably stronger yet, however, is the 550 lb-ft-rated version originally developed to buckle up behind the Dodge Viper's torque-spewing 8-liter V-10, and now extensively modified by the drivetrain specialists at D&D Performance to work in your pushrod Mustang. Don Walsh Sr. at D&D Performance undertook to adapt the Viper-version T56 because its specific gearset--with its 2.66:1 First and 0.50:1 Sixth gear--gives it a 110 lb-ft torque-handling advantage over the normal aftermarket version. Also, the Viper version's gear ratios really lend themselves to a nice, steep final drive ratio and high-torque applications. The increased torque capacity of the Viper T56 is directly attributable to this more favorable "headset" (input-to-countershaft gear ratio).

An idea is one thing, but the tricky part was adapting the Viper T56's physical peculiarities to the Mustang's--a job requiring, among other things, casting a new bellhousing and midplate (the midplate is sandwiched between the bellhousing and gearbox, and serves as the endloading T56's front cover). At considerable expense, those and the other engineering hurdles were ultimately overcome. The result is an absolute gorilla of a manual gearbox, along with an installation kit that makes harnessing one to a Fox or SN-95 small-block a straightforward proposition. If you need the strongest T56 money can buy, look no further.

In the accompanying photos, we'll examine the details of D&D Performance's ultimate T56 offering, compare some of its internals with those of the T5, and show you what's involved in bolting one up behind your 5.0 (or Windsor). Frankly, once you've sampled any T56, you'll be hard pressed to find satisfaction with a mere T5. With two now available, you can choose the torque capacity and ratio package that best suits your needs.

Horse Sense:
In case you're wondering, 550 lb-ft is more than twice the torque capacity of early factory-issue T5s. Though not inexpensive, that kind of brute strength ultimately makes the D&D Viper T56 far cheaper than an ongoing habit of disemboweling T5s--and you get a sixth gear.