#3 is the shortest of the four runners. That fact has its own problems to be dealt with.

Besides opening up the manifold outlet to improve overall flow, I want to try to improve the flow of each individual runner, if possible. I looked at the stock casting to visualize and ascertain if anything could be done.

I determined that if I could assist in redirecting the flow from each runner from where it flows, as cast, across the plenum aimed at the other side; and shift that aim point towards the middle of the turbine housing inlet and scroll; then that would improve overall flow AND assist each runner's flow.

Making the outlet larger made it possible to shape the sides and merges of the runners to redirect the aim. I did not want to diminish the goilet in the back of the plenum; but wanted to reshape things to assist the goilet's function.

This view of #3 shows the aim of the runner across the plenum. Its target is the flange edge and the side of the turbine housing when installed.