Valve pocket/bowl hog tools....
I'm banking on the "real" standard of 28", and that sounds about right, more than that is difficult & usually ends up with some filling/welding unless You're ridiculously lucky w/the most gifted of heads. I managed 192cfm w/a 1.88" max-wedge exh valve on the intake, but the bowls aren't meaty enuff to make 'em flow at higher lifts,
and that was on a 3.500" bore.....................there was more there, but I shelved the test-head temporarily.....15yrs ago!!!!!:eek:
Which BTW, is approx. 2X the flow of an untouched port w/a stock valve and valve-job............