Hey Jack,
Good seeing you still around.
We're pushing 133cfm/sq-in through the MCSA of the head. Peak flow is 235cfm @ 28" @ .600" lift (230cfm @ .500) through the spigot.
With the carburetor and velocity stack bolted on, it goes down to 216.3cfm @ .600 and 208cfm @ .500
A TM-38 will flow 277cfm @ 28" H20 at full open, or 156cfm/sq-in.
This is by all means, not a 'maximum effort' deal. Looking to peak around 7000rpm on a 540cc deal on the street engine. A 40-42mm dia "MCSA" will allow the head to reach a bit over 240cfm.
The two biggest limitations with the stock Yamaha castings are:
1) material in the port sidewalls. This limits how far you can widen the short side radius. We're about at the limit of what we can do.
You can see here:
MCSA is at the port entrance to the head - it needs to be back that far as the port needs to gain area from there to slow down the air enough to get it around the short side radius without separation.
2) Valve lift. Being an OHC set-up, you're ultimately limited on how much valve lift you can run. On a pushrod set up, you can juts keep putting longer valve stems in and work with a rocker manufacturer and come up with a higher rocker ratio than stock. The Royal Enfield guys have done exactly this and are seeing over .600" valve lift without Myhaving to use more lobe lift. A win-win.
We're great on velocity - averaging about 300fps through the port. Believe it or not a bigger valve for us did not gain much in cfm, but did make the port more stable and quiet with work. A great thing since running depression can be over 3x what you can measure on the bench.