I've heard both ways on it. All it does is lock up your differential, its not true posi traction. Posi Trac actual tranfers power to the wheel that is not spinning. The Phantom LSD just makes it so both spin in unison instead of one doing most of the work.
The Phantom LSD works best for straight line acceleration, especially with bigger engines in the Fiero. For AutoCross, its not recommended as it doesn't allow the wheels to spin at different rates around turns.
Be careful in the winter, or any other time the road is wet or icy. Coming out of any turn, if you give it even a tiny bit too much gas the tail and will take off. This can be very dangerous and nerve wracking, and if nothing else annoying. It makes whipping the back end around much easier, but also much more likely in some situations.
For your day to day driving, it gives you better grip when starting from a stop. If you have a big engine, it helps you to keep from chirping or spinning a wheel every time you start moving. Just be aware of the liabilities that come with it.
Theres also a rumor that they break or melt down under extreme situations, such as long hard driving as in racing. I don't think thats substantiated yet, but Cali Kid will be the one to find out if anyone will.
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