Depends on how much power you are making, I believe anything under 500hp 2.5 inch is fine. I would still choose the best flowing mufflers because they can actually make a significant difference in power. Again depends on how much you are making, anything above 350 hp I would prob want the best free flowing mufflers. I'm sure other guys will chime in. 2.5 will leave you with room in case you make upgrades later on
