I have a 2006 Impala LT 3.5 with 130k. I've done a full tune up but lately it seems like my mpg has dropped and i smell a rotten egg smell sometimes. I think the cat may be going bad but I have no cel on. Ideally I'd like to buy the magnaflow direct bolt in converter but money is an issue right now. I thought about gutting the cat but don't want the cel on. Does any company sell simulators and/or direct bolt in straight pipe? I'm looking for better performance and mpg.