we're going to need the rest of the timeslip to give you some better answers. I was running in the 15's with my car with the 3.55's in it. The converter allows you to leave at a much higher rpm and with some sticky tires, you get to use alot more of the motors power.
Tires and my converter knocked over a second off my et at almost the same MPH. I've run into the 13's at a little over 95-96 mph. The 60' is where you make up all the time.
I stand 100% behind all who sware by ungrading coverters. Even with a stock engine it will drasticly improve your launch and inturn improve your timeslip. It may not sound like alot but the stock stall to 2500 like i have in my car made a killer difference. But there is something fishy with your car running 15.9??? are you sure you were balls deep in the throttle????
throttle all the way to the floor, rebuilt trac-loc, i didnt feel any spin, time slip:
r/t: .105
60': 2.258
330: 6.384
1/8: 9.782
mph: 72.20
1000: 12.710
1/4: 15.183
mph: 91.08
Look at your 60'....what kind of junk tires were you running? Or you were spinning BIG time. You should be looking for 60' in the 1.XX's for anything close to a low 14 or 13. Get those times down and watch your et plummet.
I'm guessing that 12.40 car has some serious mods or a power adder. The secret to getting a near stock car cut some decent ets is to get those 60' down. If you check my sig, my car cuts 1.8's 60' and I'm in the 13's with stock h/c/i. Get yours down in that range and I wouldn't be suprised to see you cutting some 14.30's or better.