3904390 closed chamber heads

[427vette]

Hello guys,

I am building a 427 motor and am working out my static / dynamic compression ratio. I have read that most chevy heads cc more than the advertised volume, my heads are 3904390 '67 427 Corvette heads, the chevy spec. cc is 98.4cc,
I have measured mine twice to be sure, and both times came out the same at exactly 112cc.
Thats 13.6cc over the factory spec, is this normal for these heads, or have mine been modded ?Anyone familar with these 3904390's.
I have heard of the mod. to de-shroud the exhaust valve on these heads, is it possible my heads have had this mod done, thus the increase in chamber, anyone have any photos of what a stock versus de-shrouded chamber looks like ?

Any info really appreciated !

 

[grumpyvette]

heres two of the few side-by-side closed chamber vs open chamber pictures I have,
mortec
http://www.mortec.com/bbc.htm

and several other sites list your combustion chambers at 98cc, and most closed chamber heads I see average around 100-107cc so its likely your have had mods, keep in mind that if the larger valves available have been installed theres a good chance the valve seats were modified and sunk slightly and that could account for the difference, its fairly common for stock oval port heads to be modified for the larger 2.25/1.88" valves vs stock sizes like 2.09/1.72" valves

mark iv blocks

mark v blocks

(keep in mind that ALL '91 and later Gen.V and Gen.VI big blocks come with 4-bolt main caps. The two-bolt big blocks are no longer in production
MANY BUT NOT ALL aftermarket head designs have been modified to work on both the early MARK IV 1965-90 and later MARK V & VI blocks 1991-later.)
BTW, , on BIG BLOCKS the oil pumps and oil filter adapters are different due to the block oil filter recess and rear seals being different
GEN 4 or MARK IV

GEN V and VI

WELL WORTH READING THRU

http://victorylibrary.com/mopar/chamber-tech-c.htm

http://www.corvettefever.com/techarticl ... index.html


typical tool used to bevel the combustion chambers
http://www.goodson.com/store/templat...6e31d1882c7e8e
http://www.goodson.com/store/templat...6e31d1882c7e8e

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viewtopic.php?f=52&t=796

 

[427vette]

Here are some pictures of what I have,
thanks for the reply Grumpy !

BTW HERES SOME REWORKED BIG BLOCK HEAD COMBUSTION CHAMBERS

STOCK

SIGNIFICANTLY IMPROVED FLOW

 

[427vette]

I am going to change the pistons in this motor anyway, should I forget about these heads, and go to open chambers, I can get a set of 781's
all the info seems to be definitive that the open chamber heads perform much better, what is your take on this ?

 

[grumpyvette]

the correctly set up open chamber head engine combo will generally breath a bit better, at higher rpms but the closed chamber design tends to be a bit more tolerant of crappy octane fuel, and make slightly better low to mid rpm torque in my experience. those heads look ok, Id have them checked out and flowed, if they check out,and if its a street/strip car Id run them. guys get crazy chasing the last bit of hp that they can get, but its been my experience that a properly tuned closed chamber design BBC engine can put a great many cars with (THEORETICALLY SUPERIOR) open chamber heads on the trailer.
Ive built several closed chamber head 454-496 engines that ran 11 second time slips in the past and on the street thats a damn fast car