[wilf leek]

Tony - my primaries are 1 7/8" dia, running into 4" collectors.

I thought long and hard about this, and considered the fact that the primaries have to be reduced to match the port outlet and flange, and to get clearance for the bolts. Having said that, they are only reduced close in to the flange, and that is no worse than the actual port in the head??

I can't offer any practical advice on whether this dia primary is better, or by how much, over 1 1/2" ones. I just thought that since I was achieving big block cubes, I ought to follow big block exhaust practice. Or at least get a little closer to it.