How much HP do you think Cast Iron Merlin Heads (Rect Port / Roller Rockers) on a stock 454 mag would produce over the stock GM heads? I am getting the motor rebuilt and decided to step up to these heads per the recommendation of many.
I went from nice 990 heads on a semi stock motor rebuilding it with ported polished Merlin cast heads with roller rockers and the log exhuast and jumped over 100hp per the impeller rpm hp chart. I was told by my builder that if I was to switch to some nice headers I would pick up more hp and possibly another 500rpms.
You should see a 50hp gain but it would depends on how restrictive your exhuast is.