Here's what CHI say about their 228cc 4V head exhaust port:
"4. All new 220+cfm "UNI" Exhaust Port will fit either 2V or 4V headers with NO port matching, and features true Standard Height Exhaust Port & bolt locations."
And about the CNC option:
"Owners of raised dome, high compression pistons to suit 4V closed chamber heads, can now order CHI heads with a 4V CNC chamber. Our 4V CNC chambers are 74cc in volume and retain our relocated spark plug location. These can also be ordered as an option by people that require increased chamber volumes for blown or stroker applications. The need to replace those high compression 4V pistons is now a thing of the past."
74cc shouldn't be too far off from what you have I'm guessing, depending on what all has been done to your current OCs.
I agree with what Chris Minchew said about the CHIs; those heads are right, they are as advertised, and flow what CHI says they do OOB. I have a set of AFD 4Vs, and they are way less than stellar OOB, and far from what is shown on the web-site, as far as flow. I just freshened my co-worker's 380cid Clevor (4.02 X 3.75), and the only real change was a set of 225cc 3V CHIs, OOB. He's mad at me now, 'cause the chassis is being overpowered, BIG-time ('71 Comet GT).
Some people on the forum have posted about their swapping to CHIs from their deadly serious, max effort iron heads and not seeing any appreciable gains, but if you changed "up" from milder irons on your Fastback, I'd be surprised if you didn't get a bunch of extra power. I know after my one experience hands on with 'em, my next (if ever) set of aftermarket canted valve heads will be CHIs.
"If you're under control, you're not going fast enough." ---Parnelli Jones on Trans Am racing.
