I have the 18.5 exhaust housing and it looks like I don't need a larger turbo. I do need to go to four inch exhaust.

The four inch down pipes that clamp directly to the turbo do not look like a smooth transition from the small outlet to the four inches. The elbow looks to take care of some of the transition.

What would be the best combination? It seems to me that keeping the elbow and going to a four inch downpipe made for the stock HX35 would be the better choice.

Am I thinking right here? I don't really like the abrupt change from the turbo outlet to four inches and don't think it would be beneficial.

For my uses I would not think a larger turbo, even a hybred would benefit, I can build 30 pounds with the current setup. The pressures are just about what I was trying to achieve.