Opening up the diameter of the passage to the flapper is not enough. The turbine housing has to be reshaped to assist in allowing the flow to flow.

A complicating factor is the amount of metal in the turbine housing on the outside, as well as on the inside: I do not want to make a thin spot that will burn through.

First, I enlarged the diameter of the passage at the flapper; and started the blending of the sides and top.