Overall, the Tremec trans case length is about the same as the Top Loader, but the mount is slightly further back. If installing the Tremec using a bellhousing spacer plate, slide the trans-support crossmember straight back from the forward C4/four-speed frame holes so it lines up with the rear C6 mounting holes, retaining the mount pad's normal forward orientation. If installing the Tremec using the optionally available short input shaft, use the rear C6 mounting holes, but flip the crossmember around backward (which actually puts the mount closer to the front).
The special "short" input shaft lets the existing driveshaft and 28-spline yoke bolt up to the standard Tremec 3550 with no mods required (with the spacer, you'll need to shorten the driveshaft 5/8-inch). Heavy-duty TKO Tremecs require a 31-spline C6-style yoke.
Dark Horse Performance
16617 SE 134th St.
Renton, WA 98059-6923
1600 Sierra Madre Cir.
Placentia, CA 92870-6626
44050 N. Groesbeck Hwy.
Clinton Twp., MI 48036-1108
A Flock Of 'Bird Questions
The '89-'97 Ford Thunderbirds came with an IRS; was there ever a limited-slip differential offered, perhaps with the Turbo Coupe? Are there aftermarket units available? What ring-gear size was used and are there performance gear sets available?
On another matter, was there ever a factory installation of a Borg-Warner T5 behind a 2.3L four-cylinder? I'm not sure which tranny was in either the SVO Mustang or the '84-'87 Turbo Coupe T-Bird. I need part numbers for the bellhousing if you could. Also, what type of clutch mechanism was used-cable/linkage or hydraulic?
T-bird IRS: Most '89-'97 T-bird (also '89-'96 Cougar, '93-'98 Lincoln Mark VIII, and '99 Cobra) IRS (independent rear suspension) rearends use essentially Ford 8.8-inch-type components. A few of the T-birds and Cougars came with the weak 7.5-inch diff; not suitable for performance use, it won't be covered in the following discussion.
Internally, the IRS units are similar to the corresponding Mustang/passenger-car solid rearends with much component interchange. Stock and aftermarket ring-and-pinions listed for the 8.8 differential fit the T-bird housing.
Ford Trac-Lok limited-slip differentials are a common factory option on both IRS and solid 8.8s, but differential interchange is a little tricky: Whether standard or limited-slip, Ford original-equipment production, Ford Motorsport, and aftermarket differentials designed for the 28-spline Mustang/passenger-car 8.8 rearend can't be swapped into the T-bird housing as complete assemblies due to differences in the axle retention method (31-spline "truck" 8.8 diffs don't fit because of dissimilar axle-spline counts). However, the current Ford original-equipment service-replacement 28-spline 8.8 Traction-Lok complete differential assembly (PN E9SZ-4026-B) is a universal unit that fits both the Mustang/passenger car solid rear and the IRS 8.8 housing.
A Ford 8.8 original-equipment Mustang/passenger car production Trac-Lok (as well as the Ford Motorsport diff, PN M-4204-F288) can be made to fit the T-bird by swapping in T-bird side gears (PN E9SZ-4236-C, two required). Auburn Gear recently introduced an 8.8 T-bird limited-slip diff (Auburn PN 542080) that's available through selected distributors like Tom's Differentials. Finally, Ford Motorsport also offers an 8.8-inch aluminum IRS carrier assembly (the entire housing) complete with Traction-Lok and 3.73:1 ring-and-pinion under PN M-4001-M373.