Once you have your after-market air intake and header installed, you'll want the engine to be able to expel these gases in the most efficient manner. This can be achieved by installing a performance exhaust system. Now you could always just purchase the muffler with a 4" tip but the exhaust will still have to pass through the OEM tubing to get to it. Because every exhaust system is limited in one way or another by its most constrictive component, the main advantage of buying a complete exhaust system from a single designer/manufacturer, is that the exhaust pressure wave can be properly managed; from the exhaust port to the exhaust tip. Most of these cat back systems are easily installed by attaching them to, or directly behind, the stock catalytic converter.