Spent a day at the track last week with my showroom stock 2011 grandsport: Auto/f55/duel mode exhaust. Had a great day, turned in a 12.39 1/4 after about 11 passes in the morning. It was about 75 degrees, overcast. tire pressure 30lbs all around, half tank of gas.
I was hanging with a couple of more experienced drivers and they suggested I take down the pressure in my rear tires to 24lbs. I did it, and by that time I had about a quarter tank of gas. Once I did this I could not get traction off the line. I tested with traction on/off, with/without active handling and every combination therein. My best times were mid to high 12's. The problem was I either had tremendous wheel spin, or I would try to roll it off the line, then launch once I had momentum (I know both scenarios cost time). But no matter what I did the traction control didn't seem to be working. I even raising the tire pressure back to 30. Could this have been because i overheated the system? Or because I had too little gas and therefore too little rear weight? I had about 7 minutes between runs and rested the car after every 5 -7 runs for 20-30 minutes. The other drivers said what I needed were track tires, which I know would be better, but I know the traction control system was working much better in the morning.
Also, I experimented with braking to 1200 for off the line starts, and just launching with no brake at all and timing the yellow lights.
Any advice would be welcome. Thanks.

Time Slip