Exhaust Manifold Headers for BMW E30 (1986-1991) 2.5L 2.7L L6High Flow 6 2 1 Exhaust Manifold Headers for 1986 1991 BMW 3 Series (M20 E30) What It Does: Replaces the restrictive factory cast iron exhaust manifolds with high flow tubular headers to reduce backpressure and improve scavenging for the M20 inline six engine. The Stock 3 Series Weakness The factory exhaust manifolds on the BMW M20 engine are constructed from heavy cast iron, a material prone to extreme heat retention and poor flow characteristics.