I have a 1.8T 160hp A4.
I was wondering if the actual turbo from a 2.0 would directly fit onto the 1.8?

And if so, what is the size difference?
As it is now I have good power from 1,500-4,000rpm, and after 4,000 it has nothing. Feels like a diesel.

 

it should be fine. who know's? both turbos could actually be the same size. i think the power to displacement for both engines is 100:1.

 

I belive that the turbos are the same size. You should add some performance parts to help with your powerband. Ecu upgrades help alot, I would suggest that first before looking at turbos.