Not an engineer, but imagine the optimum transport pipe is probably ~5".
Force/unit area calcs show 6" less than half the four.

"Joe Emenaker" wrote in message
om...
igor wrote:

.... to a number of people here who say
that bigger is better with ducts. I can see that as a general
proposition,
but at some point, relative to the size of the fan, won't velocity
suffer
in a significant way?


Where the magic cross-over point is depends upon the CFM of your DC,
how long and curvy your ducting is, and how resistant to airflow the
inside surface of your ducting is.

- Joe