Thread necro because I found this thread from a Google search, so I know other people are finding it too.
In the first post, the OP says he threw an 80mm fan on a 6" PVC pipe. Being odd like I am, I always draft my solutions before I buy parts. And there's no way that combination fits those sizes.
This is what an 80mm fan on a 6" PVC pipe would look like.
Clearly, that's not what we see in the first picture. So I drew up alternatives.
The first row is 80mm fans, the second row is 92mm fans, and the third row is 120mm fans. The first column is 3" PVC, the second column is 4" PVC, and the third column is 5" PVC.
We can see that the 80mm fan is a good match for the 3" PVC pipe, the 92mm fan isn't a good match with any of the pipes, and the 120mm fan is a good match with the 4" PVC. Pretty sure we're looking at 80mm on 3" PVC above.
Note, this is using dimensions for Sch 40 pipe, by far the most common. Sch 80 pipe is much more expensive and harder to find in these sizes, so I didn't feel it was worth drawing those.