"i believe that you do still need back presure on a turbo car most people say that turbo cars dont need back pressure to get full power but they are wrong. on a 4 cilinder car it is ok to open the exhaust to 3" you have 4 cilinders cming out of one pipe so its a good sze for maxamum power for z's we have 3 cilinders puching out the exhaust so i would stick with 2-1/2" exhaust. but that dosnt matter anyway because hypers have 2-1/2" exhaust. and i believe after you hit about 600hp its ok to open the exhaust up to 3"-deesz