It feels really good the first time you cross it too! I actually ran 101 at 14.03 before I finally got into the 13's. Good luck!
I always just use the performance calculator on hardcore50.com its as close as anything I have seen, but just like most guidlines, its perfect "set-up" senerio. and it will give a disclaimer of +- 5%.
Something is flexing if you have everything lined up. No way a bigger pulley with a deeper groove should throw a belt if it's all lined up. Make sure all your accessory bolts are in and tight.
Enter your car's weight and horsepower Weight:2820 HP:192 1/4 mile ET:14.25 1/4 mile MPH:95.64 1/8 mile ET:9.0 1/8 mile MPH:77.25 that is as close as it can get actual numbers are 14.220 96.92 mph
the only other thing i can think of, is the belt is shaking/vibrating/so much that it is actually coming off itself. have you tried overly tightening it?
There is a formula that will tell you how quickly you can get through a quarter mile with a given HP. You could work it around to figure How much HP you have with a given time. The problem with math is that there are a lot of variables that have "normal" values plugged in so they are never exact. Also if you have less traction your times will be slower. That will tell you that you have less HP than you really do. The best way to find out how much HP you have is an engine Dyno and the best way to get your times is to run down the strip.
