When I got my truck last May, I had a SI/DO Flowmaster exhaust system put on it. I used the 2-1/2" stock pipe up until the muffler, an then ran two 2-1/4" pipes out the back. Did I use the correct size pipe? I ask this question because I need to know the right size pipe to use to keep the low end torque and HP. Also what is the best MUFFLER to use(not cat-back), to get the best possible power throughout the entire RPM range?

 

I have the 4.2 v6 by the way.