discouraged:( seeking help on serpentine probleme
 

I own a mustang 2006 with a 4.6l v8 roush supercharged, camshaft, forged internal, 12 lbs, machined a bigger crank pulley which weight barely nothing,
header etc..I broke 3 strap in 3 weeks.. 2 roush serpentine and 1 aftermarket cheap ass one, the two serpentine from roush broke the same way, it cutted in middle of the belt, and teared thet rest, they checked all my tensioner and pulley, everything seem ok, it is only happen when I get a hard launch pull...
Im starting to wonder if it might be the impact of the launch that make the serpentine to move a bit, and get out of the crank pulley range and get sliced while being a bit offset. Ive noticed that the machined pulley seems to be a little bit off in range then all the other, but barely 8mm, somehow, the slice in the serpentine once it broke/sliced/teared, seem to coordinate with the space leaved by the offest of the pulely. omfg im so ****ing unclear right now, im french and trying to explain term that i know barely nothing about is hard for me, anyone understand what I mean and have any opinion to share?
What could experience a belt to break most of the time when doing a hard launch in first gear, holding the clutch, reving it to 5-6k then bang dropping the clutch and pulling gaz, pulling second gear then bang not feeling the blower anymore, then the powerstering, **** this is a nightmare:(

8mm offset is more than enough to shred a belt. Get the pulleys to all line up somehow or it will keep happening.

If i put the oem pulley or just an reputed after market one, would i need to retune the car?

Early serpentine belt failures are often caused by misalignment. And, no, you won't need to retune.