I have noticed that many turbos either have no threaded 1/8" tapped hole on the compressor housing at all, some have them on the larger section of the "snail" compressor housing and some have the hole at the smallest area of the "snail". What are the advantages of having these vacuum ports at these locations?

No vacuum port:

Garrett GT40R



Vacuum port on the larger end of "Snail":

Turbonetics



Vacuum port on the smaller end of "Snail":

Precision



What are the advantages of having the vacuum port/nipple on certain locations of the compressor housing?