My 412ci small block is right at 12.55:1 compression, with a dynamic of 9.44:1. Obviously this will not run on pump gas. Here's the deal: My #492 camel hump heads have 64cc chambers. If I can get this up to 74-76cc, I can get the dynamic from 8.1 to 8.3:1, perfect for 92 octane if I'm careful.

Can the chambers be enlarged by 10-12cc? Here's why I think it's possible:
-Polishing chambers (small enlargement)
-Unshroud intake valve
-Unshroud exhaust valve (probably not necessary, but makes chamber larger)
-Possibly match chamber edge to my 4.200" head gasket

The only one I'm not sure of is the last option, because I don't know where the water jackets are. These heads are heavily ported and work well for this engine (as does the camshaft), so I'd hate to have to spend $900 on some new heads. Well, what do you guys think? Can it be done?

I doubt you will be able to get to 74-76 with what you are planing.
Consider that a four eyebrow flattop pistons is only about 6cc's for all four eyebrows & you will get an idea of how much you need to remove.
12 cc's is quite a bit.
I have angle milled a few 64cc heads to 52-54cc's & it takes about .080-.100" to get there.
Flat milling takes about .0065 per cc so 12 cc's would be about .078" & that's the whole chamber

You can get there. Use a .051 compressed head gasket and take a little at the valve reliefs in the piston as well. You can also sink the valves a little if needed.

As mentioned above i would go for a thicker head gasket pkg.

I have seen in other posts that either Felpro or mr Gasket offers stackable (Copper?)head gaskets for reducing compression some. But I dont know how much compression can be reduced in the manner but the gasket mfg and or good machinest could answer this question.

I know you like your exisiting cam but maybe you should consider stepping up to a little more cam to bleed off some compression if your setup would allow for this.

I would definately call and Talk to UDHarold now at Lunati Cams because he definately has the experience & know how to design/grind you a cam to help out in reducing your comp some while retaining the current running aspects of the motor with the current cam setup or at least something acceptable/close to where your at now which you stated you want to stick with.

So to sum it up i would think that a thicker head gasket pkg along with some more cam timing suggested by UDHarold to reduce some cylender pressure should hopefully reduce your compression to a lvl you can live with on the street.

Other than that it would look like you would need to dish the pistons or worse yet buy another set of heads. You could buy a new set of heads and sell your exisitng heads
to offset the cost a little. This avoids the work of tearing down the motor to remove & dish the pistons if you need to reduce the compression more than the thicker head gaskets and cam change could take care of.

Scott

Matt,
The .051 gasket will gain you about 2.5cc's so you still need at least 9 or 10. I agree it can probably done it is a lot more meterial than it sounds like.
I have unshrouded a lot of 64cc heads to the gasket etc & the gains in cc are usually on the order of 4-6cc's at most.
We have done quite a few of these for classes where you cannot do any porting but I can do some pretty "creative" valve jobs with cutters :D
Are you running a dome piston? If not your compression is not 12.5. With a flat top at zero deck it is about 11.6 with a 64cc head & a .040 gasket assuming 5cc's for valve reliefs.
And I would advise against sinking the valves unless it is a last resort. It will hurt flow a bunch. The further you can keep the valve from the short turn the better.

Thanks for all the suggestions, guys. Keep 'em coming!
The compression is a true 12.55:1 (9.45 Dynamic), I've CC'd everthing including ring lands. If I use the FelPro .051 gasket, that drops me down to 12.1:1 (9.1 dynamic). Gettin' closer...
Now, we all know that Comp Cams grinds their cams with 4* advance. If I install it straight up, that brings the dynamic down to 8.8:1. If I deshroud the valves and polish the chambers to gain Wolf's figure of 6cc, that brings it down to 11:3 static and 8.2 dynamic, which should be about perfect. I could play with timing to avoid detonation that that point. The problem would be that my quench would be way far from ideal.

What do you all think? Is 8.2-8.5 dynamic doable with pump gas and iron heads? I'd really hate to tear this engine apart, and I'd also hate to replace these heads.

I have those heads and they were ported to the gills. I got them from a friend who did the port work and when I had them cc'd they came out at 71cc (so I had to shave them too high for me). So if you port the chambers you will be pretty close.

My heads were also heavily ported, too. The engine builder who worked on them spent many many hours on them, and it flows more air than most 210cc heads. He's got a flowbench and a air density probe, and ports heads for many 6+ second mountain motors. This is why I'd hate to get rid of them.
71cc would work for me. Do you know possibly what was done to the chambers?

The material adjacent to the spark plug hole is fairly thick if I'm not mistaken, and couldn't you retard your cam timing to reduce the dynamic compression. Use valves with the bowl shape head maybe?

reduce your rocker ratio, use break in rockers maybe

The rocker ratio only effects the duration at .050, not the advertised. Valves still open at the same time, only quicker with a higher ratio. Timing doesn't effect dynamic compression either, with this compression I'd have to retard it severely, losing alot of power.

I've thought about grinding the area you speak of. We'll see how things go.