Performance Headers Manifolds Chevrolet Silverado GMC Sierra (2001-2016) Duramax 6.6L DieselHigh Flow Performance Exhaust Manifolds for 2001 2016 (6. 6L Duramax) Designed to reduce exhaust gas temperatures (EGTs) and lower drive pressure, these stainless steel exhaust manifolds replace restrictive factory cast units to accelerate turbo spool up and increase total airflow by up to 55 percent. The Stock Silverado & Sierra Restriction The factory cast iron exhaust manifolds on the 6. 6L Duramax suffer from severe internal bottlenecks,