I believe I read it in Leo's book that he or some of the other racing head porters concluded that 64cc chambers seamed to be the smallest chamber size to be the best at getting flow without doing some intensive chamber work like Hank has done. When I first got Leo's book and going through it I really didn't think much of it because most of the book covered highend HP being more in line with all out drag racing performance and not street performance, at the moment I am unable to locate my book to see if that is where I saw this information.
On another note: I have a guy that does my Harley heads and during his learning experince with porting he drilled a hole down through one of the valves and used a micro camera so he could watch what was happening with flow and what was happening in the chamber and cylinder. Before working on Harleys he was an engineer working on jet engine design and performance, needless to say he builds some nasty street Harley motors, he also now makes pretty much most of his own stuff, Cams, Cases, Heads for Harleys.
