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Tech Questions Answered in CC's What's Your Problem?

You Got Questions, We Got Answers

Track Time
Steven Patillo, California City, CA: I have an '85 Monte Carlo SS with a 350 and want better traction. I was on a road course this year and I kept spinning the right wheel while coming out of tight right turns. I checked out the Powertrax and other posi and limited-slip diffs, but the Powertrax causes an understeer and I can't tell if the other units do the same.

I want to be able to run the car on a road course, so would the Powertrax be a poor choice, or is the understeer just a small problem that can be handled with proper corner setup? By the way, after the road course, I installed disc brakes all around.

Terry Mcgean: You're right about the tendency of vehicles equipped with locker-type differentials to understeer or "push" in turns. The Powertrax is a sort of semi-locker, but the effects in turns are essentially the same. Many road racers dislike locking diffs because of their unpredictability in turns; a driver may enter a turn with the differential unlocked and then inadvertently lock the axles by applying power, upsetting the car in the process in mid-turn. Plenty of road-race dedicated vehicles used fully locked axles, either through the use of a spool or by using the old-school practice of welding the spider gears together. In these instances, understeer is obviously still there, but it becomes far more predictable, allowing the chassis tuner to set the car up accordingly, and the driver to use the locked axle to generate throttle-induced oversteer on command, rather than being a victim of understeer.

For many years, street/track cars have used factory-style limited-slip differentials for the track, though the problem, as you found with your stocker, is that these units usually aren't set up tight enough for track use, allowing the inside wheel to spin up in corners as it's unloaded. Common practice was to shim the clutch packs and tweak the preload springs to reduce the slippage, though the success of these methods is hit or miss.

A better alternative for you would probably be a torque-sensing-type differential, as offered by Torsen and Tractech. These use a series of gears to feed torque to the wheel that needs it the most, but without using slipping clutches or an in-or-out locker arrangement. This makes for smoother applications of power to the wheels and greatly helps in negotiating turns under power.

David Freiburger:
While I agree with Terry that average guys can freak out with a locker, I'd like to point out that NASCAR road racers all use Detroit Lockers. In my experience, a locker is ideal for open-track use, and if you're throttling back and forth in turning conditions where the locker might react negatively, then you need to learn to drive. A locker's push tendencies are, however, a problem for tighter parking-lot racing.

The Black Art Of Brakes Explained
Tom Kinney, Eagle Rock, MS: I have a '72 Chevelle and I recently performed a disc-brake swap using used parts from a '71 Monte Carlo. I reused the rotors but installed new wheel bearings and also used the Monte's calipers with a fresh set of parts-store pads. I transferred the Monte Carlo's power-brake booster and master cylinder along with the factory proportioning valve, and I plumbed everything accordingly. I know that I probably should have installed a new master, but the Monte was running and driving until a month prior to my taking the brakes, which worked just fine. I figured I'd get the system functioning and then replace stuff as needed. But now I have a couple problems.

First, I can't get the system to bleed to the rear. The Chevelle has new hard lines to the axle and new axle lines as well, so the fluid has a long way to go, but I still can't get fluid back to the wheel cylinders even if I depress the pedal with the bleeder screws open.

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