Okay so i know my car is running a little rich because my exhaust tips always get black. When i rev my engine to 4500+ my car pops......i dont know whether its just the off road h pipe or not? cause it started doing that when i got it installed... anyways my car seems like it runs a lot better on 92 octane gas instead of the 89 i use. whats up with that? its mostly stock...but it idles a lot smoother and i get better throttle response from 92 octane? is this normal? could my timing be advanced or something? i bought the car used.. a couple years ago...