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Eaton Truetrac
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This is the Eaton Detroit Truetrac Limited Slip Differential (LSD). Unlike other LSD's that use clutches to lock the two axles together, the Eaton uses an all gear drive design to perform the same function. In addition to having no clutches to wear out, it operates totally smooth and silent. It will also last the life of the car without maintenance.
This is a 3 series carrier (for 3.42 and up gear ratios) taking 28 spline axles and a 7.5 in./7.625 inch ring gear.
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To go along with the Eaton differential, a new set of 3.73:1 gears will replace the factory 3.08:1 gears. This will make towing my boats easier on the car, and when not towing the gears will give quicker acceleration. Impact to fuel economy should be minimal. Make sure you have the correct carrier series for the gear ratio being used.
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3.73:1 Gears
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PCM Update
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A used PCM was sent for re-programming by PCMFORLESS.COM. They changed timing advance, shift points, corrected speedo for new gears, changed the radiator fan high/low on/off temps for 180* thermostat, and a few other performance changes.
The PCM I had reprogrammed came from another car, so a procedure called Case Learn was done by a GM dealer to reset the cranck shaft timing variation unique to each engine.
The new PCM and programming worked great. And most importantly, the speedometer is now correct with the 3.73 gears.
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The gears and differential were installed and after the 500 mile break in period, the gear lube should be changed. Took this photo when changing the lube.
The Reluctor Ring can be seen next to the Ring Gear. Its wider than the Reluctor Ring used on the carrier with 3.08 gears.
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Install Done
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Add Drain Plug
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To make gear lube changes easier in the future, I added this drain plug kit. Sealed it with 3M 4200 to guarantee against leaks.
The large round washer looking item is a factory installed magnet that collects metal flakes from wear. The new drain plug is below and to the right of the magnet.
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Reinstalled the cover and filled the differential housing with about 1.5 quarts of Mobile 1 70-90 Synthetic Gear Lube. Adding a short piece of plastic tube to the snout of the lube bottle makes filling the differential so much easier.
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Fill with Lube
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Buttoned up
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Everything works great, and the Felpro gasket w/silicone sealer has not leaked. A few coats of Rustolium spray paint keeps the cover looking fresh and rust at bay.
In about 30K miles, the gear lube will be changed, and it will be a snap with the drain plug compared to having to remove the entire cover and replace gasket and sealer.
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After the gear and differential change, the Firebird had developed very bad wheel hop. After a lot of research, I decided to replace these soft compliant bushings with Moog solid rubber bushings. That completely eliminated wheel hop on hard acceleration. The car also tracks much better on rough or washboard road surfaces, and expansion joints in the highway don't make the car shimmy.
Interesting to note that the original bushings that came out were not worn or damaged and only had about 35,000 miles on them. They were just too soft for the gear change. Solid rubber in the original control arms is the way to go with a street car.
I avoided poly bushings and boxed control arms due to suspension bind, stress cracks in the axle mounts, and cold flow problems with the poly material. The Moog bushings are hard rubber and eliminate problems introduced by poly bushings.
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Need bushing Upgrade
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