BD 3 Piece T3 Exhaust Manifold 1989-1998 12 Valve CumminsBD 3 Piece T3 Exhaust Manifold 1989 1998 12 Valve Cummins 12 valve 3 piece manifold. To make the most of your engine we need to take advantage of not only what goes in the engine but also what comes out of the engine. These manifolds are designed to maximize the exhaust air flow to reduce turbo lag time and cool exhaust temperatures. In the design of the manifold each port is analyzed and tested to provide a balanced flow to the turbocharger.