Did you run the 15.6 in good weather? I have seen times drop by as much as a full second between the 105* Texas heat and a nice 50* night with low humidity.

The vibration thing is a huge issue. I know it killed me and my 60' times. I'm still chasing down a vibration in my driveline at about 5-10 mph. It has cost me some pretty good times. I'm trapping as high as 81.x but only cutting 2.15-2.20 60' times even with 26" ET Streets and a ss3600 torque converter.