why a 3" outlet and not an 3 1/2" or 4" outlet ?

ROD

I'm n/a and too large of a diameter will lower velocity.
I would like to test to see which diameter is better using the MAF but there are a few things that would make that may throw the readings off.
I may just go back to 3" and use somehting like this.

. . . . . but if you use 3 1/2" or 4" pipe and then neck it down to 3" before the MAF you get the velocity stack (X2) effect right before the MAF.

ROD

p.*. i have the 6" to 3 1/2" air ram, i would have gone with the 4" outlet but my 3 1/2" pipe is only 4.25" long.

Not exactly. The air will have to flow at 4...3.75...3.5....3 for a while before the maf. The velocity should increase but not as much as if it was transistioned sooner with more of a bell mouth. The main point of this is to smooth out the beggining boundary layer in the mouth of the pipe. Then the flow will have the maximum velocity in the correct diameter.