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Super Coupe Club of America > SC Literature > Super Ford's MN12 T-Bird Gear Swap

by Matt Stone
photography by the author

More rear gear sparks up the T-Bird; here's how to change them

It's not exactly news that a lower rear gear ratio (numerically higher) increases performance in just about any car, especially around town or at the 1320 ' A 3.55 or 3.73 ring and pinion might as well be standard equipment in 5.0 Mustangs, and it's typically the first performance enhancement recommended. But what about the T-Bird? The current MN12 'Bird is the performance choice of many, and it also shows substantial improvement with stiffer gears out back.

Though the Thunderbird sports an independent rear suspension, the center section is internally identical to the 8.8 live axle found in Mustangs, so any of the gears available in the SVO performance parts catalog will work. Once inside the differential; changing the gears is familiar ground to a 5.0 mechanic, but getting to the differential has a few twists of its own, which we'll examine here.

From a strategic standpoint, the plan is to remove the differential from the car via the passenger's side. Start by removing the brake caliper and hanging it out of the way. Then mark the bushings on all at the suspension members as shown. This ensures they can be reinstalled to match their previous alignment settings. If you do not remount the A-arms in the same spot, wheel alignment and ride height will be affected.
Most aftermarket exhaust systems, such as this Borla unit, will prevent dropping of the rear center section and will require removal. The stock system routes the pipes toward the fenders sooner and shouldn't get in the way.

There are two ways to approach this swap: do the entire job yourself, or just remove and reinstall the third member yourself, and take it to your mechanic for the actual ring-and-pinion change. Removing the center section, and saving most of the labor costs, is straightforward work for two people. Doing the entire ring and pinion change on your own bench requires a few special items such as a press and an inch-pounds torque wench, but it's do-able. To help you decide, our photos demonstrate the highlights of the suspension and differential removal Actually changing the ring and pinion is identical to setting up a Mustang 8.8 differential; familiar work to many enthusiasts and most shops, and the subject of previous articles.

Remove the through-bolt on the top portion of the upright, then swing the upper control arm out of the way, and wire it up. This allows the upright to swing down on the lower control arm.
Dropping the upright, arrows the half-shaft to be removed from the center section. Remove the through-bolt holding the lower control arm, half shaft, upright and brake rotor in place.
Now there's room for the rear center section to be moved toward the passenger's side of the car and be removed. Also, mark the driveshaft so it can be reinstalled in the same bolt holes, loosen the driveshaft and remove it. Reinstalling it a different way could affect driveshaft balance. If your car has ABS, remove the sensor units from the center section.
There are four long bolts holding the center section in place. Remove them, remembering to support the third member with a stand so it doesn't fall. There is no need to disconnect the suspension on the drivers side. It will probably take two people to work the center section loose, then drop it down and toward the passenger's side. The driver's side half-shaft will remain attached to it's upright, and stays up in the car. The center section should now be free, and ready for the bench.

Our subject, a 1993 Thunderbird LX equipped with the 5.0 V8 and four-speed automatic transmission, rolled out of Dearborn with 3.08 gears. It cruised comfortably on the freeway, but out-of-the-hole performance was sluggish. At 60 mph the digital tach registered only 1600 rpm, 70 mph just 1800. Blown SC V6s have 3.08s or 3.27s, and the 1994 and later mod-motor V8 cars also carry 3.27s. As these 'Birds obviously have long legs and tall overdrive gears, we judged 3.55s to be the right balance between increased punch and freeway comfort. They are also the favorites of the street 5.0 crowd.

We ordered our parts from Brothers Performance Warehouse in Corona, California, and local 5.0 guru Darin Hawkes performed the install. Was it worth it? Absolutely. Though it's difficult to gauge exact readings from the digital tach, it appears the new ratio increased revs about 250 rpm at cruise speeds: 60 mph now shows 1800 rpm, and 70 mph has the tach flickering between 2000 and 2100 rpm. The big difference, however, is off-the- line acceleration: the 'Bird feels like it lost 500 lbs, and is much zippier through first and second gear, while freeway cruising is not compromised a bit. This car is equipped with a K&N air filter and Borla exhaust, but is otherwise stock. Gas mileage, according to the car's computer, remains unchanged.

One of Brothers' owners has a 1996 TBird Sport V8 on order, and is contemplating whether to swap to the 3.55s, or try 3.73s. As the mod-motor is smoother than the 5.0, the 3.73s may be a good choice, but for a street driven '93-and-earlier car, 3.55 seems to be right number.

Two final notes on the installation. If your differential is a limited slip, remember to mix a bottle of Friction Modifier (Motorsport PN MI 9546-A) in with the new rearend oil. Also, you'll need to switch the speedo gear to compensate for the new ratio. For a 3.55 gear set, use the tan 19-tooth gear (Ford PN C7VY17271A).

Source:
BBK/Brothers Performance Warehouse
1605 Railroad Street
Corona, CA 91720
909.735.8880

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Document Last Modified: 07/29/04 03:34 AM
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