260 out of the 346/902/452 head is really a piece of cake.
270 requires a bit more time & effort.
280 requires a lot more time & effort.
260-ish out of a 906 ends up taking more time than the later heads because of the extra work needed at the short turn....... and unless you’re fairly skilled and/or have access to a flow bench........ you may have trouble getting there.
270 takes a fair amount of ST work for me get to that level, and 280 even more so.
Between the short turn and the much larger guide boss, I’d say it probably takes almost twice as much time to get a 906 head to 275-280cfm than the 346/902/452 head.
What I remember about my old '452 heads that tested 270/210...
- 2.14"/1.81" valves
- Originally "bowl hogged" for bigger valves
- Then "Brad Hogged" for a lot of hours in a below-freezing unheated garage*
IIRC, they were 240-250 / 180-190 before any "real" short turn work.
* I might still be blowing out cast iron dust from my sinuses after that experience
