I have a 2005 POWERWAGON. It has punched cats, with the O2 simms that put the sensors out a little from theexhaust stream. I also have a superchips programmer on it.4"intake with a K&N filter.The other day I took a 5 hr drive and when ever I went down hill for an extended period of time it would backfire. The 1st time it did it I thought I had a blow out. It is a very LOUD backfire. Could this be from not enough back pressure due to the punched cats? It has had the cats out for about a year but this is the 1st time this has happened. The truck has 60,000 miles on it. I was hoping that it is just telling me that it is time to change the spark plugs (unburned fuel in exhaust). Let me mention that the times it did backfire I had been going down some long mountains with out my foot on the gas pedal.