what does the SJ stand for ?????

The basic trim level for the Grand Prix was called "Model J"; the "SJ" trim designation was for "Sports J", and the "LJ" trim was the "Luxury J". Some people say that the "Model J" name was meant to remind people of the supercharged Duesenberg Motors "Model J" cars of the 1930s.

The J model was named after its designer, John Deloran.

John DeLorean was a young engineer that left Ford and designed the Grand Prix that’s why they named it a model J for John DeLorean

Many people believe that the Model J was named for John Delorean's initial. However, it has also been said that the models were named after Duesenberg. If one looks at some of Pontiac's history, we see model name designations borrowed from other manufacturers of exotic vehicles. For example, the GTO and the 2+2 nameplates were "borrowed" from Ferrari. In looking at the Duesenberg models in the late 20s and 30s, one will find that there was a Model J, a Model SJ (supercharged) and even a few Model SSJs. In addition, these models sported long front ends with rear of the car being short. Take a look at photos and you'll also see the inspiration for the boat tail rear end of the 71/72 and later models. So, from my perspective, since these are also the exact same model designations for the 69 to 72 Grand Prix, with the Hurst models bearing the SSJ moniker, I am more inclined to believe the theory that they were "borrowed" from the Duesenberg line and have nothing to do with John Delorean's name. I have also seen reference that the grill design for the 69/70 models was inspired by a vintage Miller race vehicles of the 1920s,