I've been hanging out a bit lately on the Do The Ton forum. I quite like the aesthetic side of the custom scene and find some jealousy for the ability of those who do it well, not being overly creative myself. Although the technical side of it is often rather frustrating, and while many of those involved seem completely oblivious to the consequences of the supreme low budgetness often employed, I just can't get excited about a set of $100 shocks.
Anyway, in the Engines section I found a post with some links to work by David Vizard. Who is David Vizard you ask? davidvizardseminars.com calls him "a Race Engine Building and Performance Tuning Guru. Author of many technical books, he now lectures to engineering audiences worldwide ..." He has done lots of stuff. The images are found on the Eurospares website.
The two links I found of interest were for inlet trumpet design and inlet tract length, as below. Best to download the inlet trumpet design page from the link so you can magnify it to see all the detail.
The thing I find interesting about the inlet length diagram is that 10,000 rpm is given as 10 inches, or 250 mm. There's quite a lot of bikes that are much less than that, and a big part of that is available space. 8,000 rpm is given as 12 inches or 300 mm, which is a lot of length to find on a motorcycle. I'm sure the "long" manifold system on the Mikuni carburetor 2V Ducati motors is not a lot longer than that. Maybe I'd better measure it on Monday.