I'd like to reply to pvtkary about spinning both wheels with no LSD.
This has been a bit of a bugaboo to me in all these threads about LSD's. I think several people are out hunting for LSD's simply because of this question. They are spending time and money to solve a problem they don't have.
It is true that with an open diff AND a live axle (one-piece rearend from wheel to wheel like most rear wheel drive American cars) you will spin only one wheel, and the same one every time.
The effect is the same one we Z guys deal with when we rip out our front diff mount. When excessive torque is applied to the diff the pinion gear, which is connected to the power source, tries to turn the ring gear, which is connected, through the axles, to the rear rubber that is not moving and is stuck to the ground with the weight of the car. So, what you have is a pinion gear driven with a lot of force trying to turn a ring gear that doesn't want to. The result is that the pinion tries to "climb" the ring, thus causing the front of the diff to rise up. This same action also twists the whole diff case around the pivot point of the driveshaft. If you have a live axle this twisting action mashes one tire into the ground and lifts the other one up unloading the weight of the car on it and it begins to spin.
However, If you have an independent rear suspension (IRS), like on the Z car, that twisting motion is contained to the diff itself. The weight on the rear tires remains the same as it was before you hit the loud pedal. Two perfect black strips.
Now, this only applies when conditions are identical at both wheels. If one tire is wet, or the ground is smoother under it, or your 290-lb. mother-in-law is in the passenger seat, the strips may very in length. In this case the LSD will compensate. It will also compensate when powering through a corner when weight shifts to the outside tires. The LSD will also create more "push" or understeer which you may or may not want.
So, yes, the LSD is great, and will always help you get more traction. But if what you are concerned with is just leaving two black stripes save your money for more speed parts.
Hope this clears things up for some. I know I feel better.
BTW, my favorite movie is “My Cousin Vinnie”.
Dan