This shows the blending of the merges from #2 and #3. This allows the flow to expand upward into the turbine housing. The opening up and blending helps the goilet direct the flow, without compressing the exhaust gases any more than necessary. I want the exhaust to have as direct a shot at the turbine housing inlet as possible. Inside the housing scroll is where the squeezing down of the exhaust should occur; not in the manifold. Or, at least, as little as possible.