A friend who does port work flowed my J heads whe I got done with them. Just as a favor and I didn’t get a full written report. He said he got 262 at 500 and the same at 600. I had heard that these heads don’t improve past .500. Just my experience. I put 1.6 rockers with my .484 lift Mopar Performance cam to up the lift a bit and quicken the opening. Duration at fifty as I remember is 238. My performance goal is to be respectable. Not a race car.