i have a 4 banger, haha, i already have one muffler.
taking off that one muffler and going to a single muffler set up, will give you more tq. on my old accord, also a 4 cylinder, i had a single muffler with 2.25" piping so i could still have some tq, haha. going bigger with piping is good, but too big is bad. thats why i said 2.75" piping, cuz i think 3" would be overkill for a single muffler set up on a n/a j30.