How advanced is your timing? I dont think an exhause system would do that though...When I was running my 89 5.0, I advanced the timing by 4 degrees and the engine would ping when I ran regular fuel...when I put high octane fuel in my car the pinging went away...Ill talk to my mechanic back home and Ill let you know what he says, but I really dont think that the exhause would cause your car to ping cause normally that would be an internal issue.