IQ, I like reading your replys and your #' are impressive. could it be that the open chamber nature of the 906 would naturally out flow the closed 915. thanks.
Well, actually that does have a little to do with it, but the 915 chamber can be modified to compete with the 906 on the lower lift flows.
The struggle I was having was in the upper lifts. My son had to straighten me out on the reason for the difference in the higher lifts.
Even though the ports in the 906 and 915 look the same, they are not. There is a difference in the thickness of the casting in places. You can do more extensive grinding in the bowl of the 906 before you hit water than you can do in the 915, and there are other subtle differences.