The 920 273 heads have small ports and require porting and larger valves to get it to flow close to std J heads. Which is my goal to get them as close to J heads or better at .450" .5 lift. These smaller ports should help velocity and efficiency if I follow all of the good advice and pics this site has on the subject. But I'd be open to hiring it done. I know there are guys here that are far more skilled than I. I have a set of 920's on my 273 that I ported and they are working great. Just a mild bowl port and gasket match.