I have been a Pontiac fan since my first car—an '80 Turbo Trans Am. I now own a '70 GTO, which I purchased when I graduated high school in 1997. My GTO has a 400 engine, a Turbo 400 auto trans, and a 3.23-geared, 10-bolt differential.
I wanted to add Safe-T-Track, so I located a 3.36 ring gear and a factory, four-pinion, limited-slip unit. I installed it and it seems to work just fine, with one exception. When I do a burnout, it will spin both tires for about 10 or 15 feet, then the power goes only to the passenger-side rear wheel.
I thought this was strange, so I referred to my '70 Pontiac Service Manual. In trouble diagnosis, testing the Safe-T-Track is done in steps: Hoist the car with the transmission in Park; attempt to turn either wheel. [It] should be extremely difficult to turn either wheel.
When I do this, it spins both wheels in opposite directions. What does this mean? Is the Safe-T-Track broken, and if so, where can I get it fixed?
Rocky Rotella responds: Brett, your GTO is equipped with an 8.2- inch-diameter ring gear retained by 103⁄8-inch-diameter bolts. Used by Pontiac, Oldsmobile, and sometimes Buick, it's commonly referred to as an 8.2-inch B.O.P. unit. (Chevy had its own version.)
The factory-installed, optional limited-slip differential was a cone-type unit, which, unfortunately, was not serviceable. The cones are clutches that act like the synchronizer rings in a manual transmission. Their conical face grips the differential case internally to provide power to both wheels. As miles accumulate and these cones wear during normal operation, they lose their ability to lock against the case, limiting the consistency with which both wheels spin. It sounds like this is what you're experiencing.
I haven't found any companies that can easily or economically rebuild the original cone-type differentials. However, there are excellent options on the aftermarket that will install easily into your GTO's original axle. I feel the Eaton Posi is among the best. Part number 19674-010 is designed for gear ratios from 3.36:1 and numerically higher. Mail-order retailers like Summit Racing stock it and sell it for about $560.
You may be able to reuse your existing 3.36:1 gear set depending on its condition, but if it must be replaced too, you can find quality replacement gear sets for the 8.2-inch B.O.P. axle from companies like Motive Gear or Richmond Gear. They do, however, range from 3.55 to 4.11:1, so be prepared for an axle-ratio increase if a new gear set is in order.
To the Point
I am working on my '69 Trans Am. Where does the vacuum advance go on the distributor when it's in the motor?
Rocky Rotella responds: John, the factory pointed the vacuum-advance unit in the general direction of the brake system's master cylinder when installing the distributor. I have found it to be an ideal location because it tends to provide the greatest range of motion when making timing adjustments, and usually requires the shortest length of vacuum hose, depending upon where it's connected.
The engine in a First-Generation Firebird is located rather close to the firewall, and that can limit your range of adjustment. If you find the vacuum-advance canister contacts the cylinder head or firewall and prevents you from reaching your desired base-timing setting, simply remove the distributor and rotate it one tooth in the opposite direction and then rotate the body a similar amount.
When moving a distributor, I prefer to place the No. 1 piston at top dead center (TDC) on the power stroke. I then align the rotor tip with the No. 1 terminal on the cap, which allows for easy startup and quick timing adjustments.
Mover and Shaker
I am putting an LS3 in my '79 Trans Am, but want to keep the Shaker scoop and hood. Do any companies make a new Shaker induction for an LS3 intake?
Valley Park, Missouri
Rocky Rotella responds: Marcus, to the best of my knowledge, no company presently produces a kit that retains Shaker functionality when retrofitting an LS engine to a Second-Gen Trans Am. Year One was using a setup it created on its heavily modified Bandit cars, but the kit has since been discontinued and there are no plans for subsequent runs.
I also spoke with Restore A Muscle Car in Waverly, Nebraska. Owner Dave Hall tells me that he's actively working with a company to produce such a kit, but he couldn't provide an estimated timeline for availability. I suggest periodically contacting RAMC (www.restoreamusclecar.com for future updates.
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