What am I missing? According to the factory service manual, the valves are the same between hipo and non-hipo 289 heads. Seeing how the heads supposedly both flow the same, I imagine they aren't that different.

 

In order to get 10.5 compression ratio from that engine the combustion chambers have to be smaller than the regular 289 heads.

 

i always thought it was a difference in piston and not the head since ford bought badger pistons (they got a ford part number) for any thing that needed more than a cast piston the reason i know this is a gentleman cam into my store one day and asked about my mustang and he proceeded to tell me about he was the sales rep that dealt with ford and how he sold them their pistons in the sixties and seventies he seemed to have a good knowledge so i believed him (i am pretty good at telling when a person is lying). the only difference in the hip head is the machined spring seat and the screw in studs but guess what the reason that the hipo heads worked better was that machined seat allowed for greater lift but what do i know i am just a kid that has lots of books and access to the internet

 

http://holley.com/8124.asp That's the one I run, redisgned version of the Action Plus to work with newer carbs/cams/heads etc. If you're going towards a performance street build, this'd be a better manifold than either the Eddy Performer or the older Action Plus. And I can tell you for a fact that it'll pull like a beast to 6,000rpm+ with the right setup