I am running an hx35, which the exhaust housing has an exhaust port that's 2.5". I am going to run a 3" exhaust out the bumper (forward facing manifold), so I have 2 options for the exhaust.

A. Weld the 3" vband flange straight to othe exhaust housing even though the housing is 2.5" technically

B. Weld a 2.5" to 3" transition diretly to the housing and weld the 3" vband flange to the transition.

Would the sharp change in exhaust size have enough of a negative effect to need the transition?

 

... What I did. H1c.
The flange is .25in bigger on each side.. So the weld almost takes up the space, formed a little contour

 

I got a flange for the holset and my buddy weld a 3" Vband onto it. You should be able to find them on eBay if you wanted to go that route.

 

Or you can message this guy and ask him if he can make one to your specs.
http://bit.ly/1bouox5

 

I have no need to pay anyone, I'm making it all from stainless and tig welding it all. Just trying to figure out if it's worth the tube cost and effort or if it's negligible difference.

 

I would never weld anything directly to the turbine housing. get a v band flange and do it right.

 

Very common practice to weld a vband flange straight to the housing. Absolutely nothing wrong with that.