The SRT engineers stated in last week's Chat Session that the 392 in the Challenger will consistently run 12.6s. The fastest time they got was 12.4 sec.
It's interesting to note that if you plug 4,150 lbs. and 425 flywheel hp (6.1) and 470 hp (6.4) into a 1/4 mile calculator, you get the following numbers that are practically dead on:
425 hp= 13.1 sec.
470 hp= 12.6 sec.
1/4 Mile ET Calculator