Being a curious person I am looking forward into installing back my free flow type muffler, but tha;ts later on after the CAI is in. Free flow gives better Hi-end power but sacrifice low end. I am not 100 percent sure about this, but that's what's on my mind right now. Since I have flanges on the muffler piping, there is no problem switching between sports and free flow muffler, although i need to make a match piping for the free flow muffler. I just want to see how will it affect my car :) Oh I almost forgot, if u get the genie header, u will have to make new cat back piping since the header is longer than std. The only thing left standard is the Catalytic convertor, the mech reccomended me not to change it unless i work out the engine. So hi flo cat is on the list too for future refrence :) I hope my R&D for this car will help all of you 1ZZ-FE owners :)