Choosing the right exhaust system for your BMW E82/E88 (2004-2012) will vary on your choice. There are three main types of aftermarket exhaust systems on the market including axle-back, cat-back, and header-back. The axle-back is the type of system that replaces the system from the rear axle to the muffler and has the least effect on performance and fuel economy. A cat-back system replaces the stock exhaust system from the catalytic converter back to the muffler, and has a moderate effect on fuel economy and engine performance. A header-back exhaust replacement completely removes your stock components and replaces them with aftermarket options. This has the greatest impact on performance and fuel economy.