I think I am correct on this...isn't the turbo inlet 4"? The pipe or plumbing is large but necks down at the turbo? I better have another look. I seem to recall having a piece of smooth bore flex hose on my test intake. Maybe it was 5" . I was sure it was 4" all the way from the filter/coupler to the turbo when I used the smooth bore hose.

 

I am going to do the *****'s whistler intake on my 6.7

 

turbo is 4" airbox is 4.5"