There's a ton of info there. I didn't think Id get this kind of a response.
I wouldn't know the first thing about figuring out harmonics for the best VE. Not that I wouldn't like to learn.
I have a limited grasp of what air does going from idle to red-line at this time.
I did find the stock diameter for a late E88 was roughly 1.25" and there's 6061 tubing 1.25 ID with .25" wall.
Should be plenty of good meat for tapering. Im going to ask a buddy in the tool and die room if he could taper these with 1.25" to get it close enough to the head for port matching and 1.50"/1.57" depending on the diameter of the throttle body I use.
There's alot of choices on Ebay for motorcycle ITB's. I wish I could make my own TB but I only have a Mini-lathe, no mill.
Nothing is set in stone. Still need to cast the flange that bolts to the head. Find with TB's I want to use and select the tubing.
This is a Inch/mm conversion chart I have bookmarked. Comes in handy for quick reference.
http://mdmetric.com/tech/cvtcht.htm