Shop Aftermarket Exhaust Systems and upgrade your 2004.5-2005 Chevy/GMC 6.6L LLY Duramax. A performance exhaust will typically add horsepower and torque, decrease the exhaust gas temperatures (EGT's), and can even improve engine efficiency. Many exhausts are offered in either 4" or 5" diameters. As soon as you begin looking for an exhaust, you will see a variety of construction choices and price points. Exhaust systems can be constructed in either stainless steel or aluminized steel and are available from many manufacturers. Exhaust installation is typically a DIY task that can completed right in your driveway. You can find exhaust systems from all of the leading brands like MBRP, Magnaflow, AFE, Banks Power, AP Emission Technologies, and more!