What he is talking about with the magnaflows and the packs with a catted pipe is true. It is a matter of backpressure. Even though people think that they should open up there exhaust completely, it is wrong. On a stock or nearly stock car not making that much power, you will actually run worse if you open up completely. So in theory of backpressure, any straight thru muff will perform better with a hi flow cat combo, where as a chambered muff will perform better with a off road pipe. People will disagree with this that has the straight thru muffs and the off road pipe because it is loud. People think that when there car gets louder that it is faster and it is just not true. just my .02