Without the cat in place the ECU will throw a check engine light. Evidently the ECU also compensates by dumping extra fuel into the motor. We think the extra unburned gas ate away at the fiberglass packing on my muffler. It blew the packing on a trip up to Atlanta I replaced the muffler and it began doing it again shortly before I decided to sell the header. Right now the muffler on my car has quite a bit of the packing left but you can see fiberglass coating the inside of the tailpipe.

Even though we suspect the car war running rich it still got an average of 35-38 mpg on the highway...stock the car gets around 30 mpg on the highway. However in city driving it did lose gas mileage even with very conservative driving (mostly to keep noise level down).