Natebrody and Polarco are talking about the right things. The turbine 64.5, 67, 71 (mm size) etc are the blades that move the air. Think of a ceiling fan - the smaller the fan blades (64.5) the sooner they are able to turn with less effort/resistance. Although they turn easier to start once they are spinning to full potential they can only move a certain amount of air. The larger fan blades (71) are a bit more difficult to get moving and momentum but once spinning is able to move more air because it is larger.
The larger numbers (71+mm) is when you will hear about lagging or taking more time to build boost and get going. The 67mm is the best overall performance for the 6.7s from all the research I have done. It spools like stock but has larger blades to move more volume than stock giving you more top end power. 1900+rpm is when the big boost hit will come and it lights instantly. Anything less than that will be the 0-10psi range depending on throttle % and load.
Overall there are ways you can mix and match the turbine (64.5), compressor housing (68), and exhaust housing (T4 divided .91)
Turbine - blades in turbo
Compressor housing - space around the turbine (its home)
exhaust housing - shape and cut that connects to manifold. There are different shapes and sizes for these
The 64.5 will spool real quick since it is smaller than the 67 that does fine. You may want to watch EGTs depending on how hard you tow and what terrain. Still an improvement over stock.
Someone please let me know if I am off on this!! I am not an expert just what I have gathered after ALOT of reading over the years
To Capt Point- there are improvements to the housing to help increase flow. The Borg SXE turbos flow extremely well- you can get away with a smaller 64.5 and have it flow like a 67 because of how they machined inside the housing to increase the air moving.
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