I would never go over 2.5 for charge piping. In most cases 2.5 is too big for most applications

 

Mine is gonna be 2" ID

 

werd. it's not going to benefit you to have larger pipes just to be closer to the actual t.b. size. you will still benefit from the larger bore t.b. simply for the fact that it will allow more air in and faster, so no need to try to match the size.

 

http://www.burnsstainless.com/Alumin...minumtube.html
http://www.roadraceengineering.com/mandrelbends.htm

 

To convert metric to english, divide it by 25.4 67mm is like 2-5/8"...

I didn't put much thought into my setups. My turbo was 2" and my side mount was 2", so I went with 2". My TB is 2-1/2" so I added a reducer after my IC and converted to 2-1/2"

But now my FMIC is 2-1/2" and my TB is 2-1/2" but my turbo is still 2", so I added a reducer right after the turbo and went 2-1/2" throughout. I have yet to really drive it, so I don't know how much better or worse it will be.

 

The only reason to go bigger NA or boosted is to lessen the presure drop in the pipes. Pipe friction which causes pressure drop is related to the velocity in the pipe. The bigger the pipe the slower the charge has to move through it for the same volume flow.

This might get you thinking that bigger is always better...not true...The charge pipes have a bigger volume to fill when the pressure rises (LAG) and if you have to reduce for the TB then thats a big pressure drop in reducing neglecting what you got with the bigger pipes.

One sentence answer...Match the size of the charge pipes to the TB.