Not a fan of stacks, I got a few buddys with modded cummins rams ( both 400+ hp an 900+ tq) an they run 5" out stock location... To each there own. Wifey has 3 inch single exhaust no cat or muffler, my 626 is stock 2.25 with no muffler, deep. Revvy. Mazda v6 tone... The rain cap won't. Jack the back pressure up, its almost physicly impossible to have to much back pressure, more back pressure will give you more tq but kill hp!