I have done extensive research on this. Mufflers will cause back pressure which will never let the valve open up. You have to run straight through design bullet type mufflers. I was just about to get the mufflers put on when I found a pair of Borla Header collector mufflers on RJ. I jumped on them but now I have to run a vacuum pump. The evac tubes only work at WOT so if your car sees even some minimal street time, it will suffer. The pump is the way to go. I have been searching ebay and RJ for a good used pump but they go quick all the time. New is almost $900.