Looks totaly different to the open ones which run 76 to 80cc's & are shaped totally diferentely.( See photos in book)- Big difference. IMHO the only hds.to use are these closed-chambered ones. The thing to remember in trying to suss-out your potential comp.ratio is to decide what pistons u will use in combination with these hds.My combo is; 4V, closed-chamber with flattop pistons. This gives -+ 11.00 to 11.50. depending on cc's in your chamber, which of course u will only determine on cc'ing the hd.chambers.( pop-ups will naturally give u more.)-which u should not go to, if u want run pump-gas. Before u say 11.0 is to high, let me say that I believe this to be incorect, in my experience. Only the Chevy-Boys will tell u this. I have run this combo for many years on 94 octane pump-gas here in South Africa and u guys, i recall have much better, higher octane gas.
OK, so now we have 4V, open & closed chamb.hds & 2V's with only open chambers. The 4v (fourbarrel) hds are the so-called 'trick' Ford production 351C hds.- with monster 2.19inch inlets, & 1.71 exh. valves & ports to match. (worth buying the one-piece, one-grove, up-graded valves since they are big & heavy, due to their size.) The 2V hds run 'small' 2.05 & 1.65's valves, still bigger than the biggest Chevy std. hds...( To make Chevy hds flow like 351C, 4V hds u have to spend alot of $ at machine shop or buy aftermarket fancy hds. The Ford Performance "bible" says that u should not touch the ports of the 4V's at all, just un-shroud the inlets to assit the incoming mixure flow coming thru the inlet port. (see photo in book.)-Thats ALL that needs doing for it to flow like gang busters. The 4V closed-chmb. 'quench' hds are good for almost 1.50 points compresion increase over the open ones. - These hds. are so-named cause they put out the combustion chamber flame-front coming from the iginited plug as the flame-front it moves across the chamber, & since the smaller chamber cools it all down more,- thus allowing it to tolerate higher compresion ratios. Plus,- and here's the thing: this design configeration creates turbulence in the chambers, which way improves the feul & air mixture. The Ford book says this occurs especially @ low revs giving more complete combustion & greater LOW & mid range torque. Magic.- Show me an un-modified prod. Chevy head that can make this claim.
So when the pundits say; dont use the 4V hds with big valves & ports on the street, as they are too big and slow down the air/feul mixture causing low-speed bogging-down or lack of torque @ low revs. B.S. -If the 4V hds are closed-chambered this no longer applies. Now as Jack has mentioned often, & one has the budget, u can further increase the incoming mixture velocity-flow (more torque) by using taller inlet-trackt manifolds & perhaps also running IR,(Induvidual Runner) inlets & F.I. or downdraft Weber IDA's with tall velocity stacks.
The 4V hds are basically the race-proven Boss 302 hds.
Anyhow, sorry to go on, but these hds are IMHO what makes it all this 'madness' worth while,- going the 351C 4V route. And thats why the Clevor-Combo
Clevland hds. & Windsor block,)- but thats another story. The Ford book to get is: "Ford Performance" by Pat Canahl - S-A Design Publishing Company, 515 W. Lambert. Bldg. E CA 92621-3991.-
Enjoy,- tai.