I ported a set of 1971 915 j heads. Spent months on them. They went from 160cc to 186cc... you don't get there overnite , that's for sure. At first a few went 270..others went 257-260's cfm...so I pulled the trigger on a flow bench.. and began working them till they all flowed withen 5cfm of each other.
No tesla, turbo this or tht ,Benz beamer super cars that the rich 'a wads' drive around here can touch it. Street or freeway... I will leave you. Never got it to hook all that great.lol it would break tires going 65 or so if I dumped it.
It flowed better numbers than a rhs head at the time, talking 249cfm by .400 lift... 270scfm at .500...hell the damn .300 numbers are in the 200's ...like 204cfm.
Quench tops and .030 p/h clearance.
Paper thin push pinches.
It's not whether or not you can make 500+hp with factory iron..its whether or not you know how and have the time.lol