at wot the car reaches around 4000 rpms then back fires so bad it won't rev any higher. I would like to know what is causing it. I was told today it the opti. what do you think? I can take it to the rev limiter aslong as its not wot, and it also runs fine any other time, like at 1/2 or 3/4 throttle.

 

check your coil and coil wire first. same thing happened to me and that was the problem