Jan 7/69 427 chamber, open or closed?

[Greg L.]

I thought it was open but I'm now starting to think that it's closed. Could someone clear this up for me please and thank you.

Greg Linton
#45455

[Clem Z.]

Re: Jan 7/69 427 chamber, open or closed?

look like small port closed chamber smog heads. is there a casting ##

[Joe L.]

Re: Jan 7/69 427 chamber, open or closed?

Greg------

These are "closed chamber" oval port heads. Likely of GM casting #3917215, 3931063, or 3964290.

[Greg L.]

Re: Jan 7/69 427 chamber, open or closed?

They are casting #3931063. I guess they are closed then....always thought they were open though because of some combustion chamber pictures in my old "How to Hot Rod Big Block Chevys" book.

Would one of you guys happen to have a pic of what a open chamber head looks like of this vintage?

I'm also curious how you both managed to tell that it is an oval port head without seeing the ports. I always thought that the combustion chamber was the same on both heads but I see that I have lots to learn!

[Joe L.]

Re: Jan 7/69 427 chamber, open or closed?

Greg-----

The terms "closed chamber" and "open chamber" are somewhat "generic" terms. That's because there were variations of combustion chamber configuration that fall within each term.

What's confusing you is the "slant" which these heads have in the area below (as the head is oriented on the engine) the spark plug tapping. Most folks think that heads that have this "slant" are open chamber. That's not the case, though. Open chamber heads have a much more pronounced slant that tapers more-or-less evenly all the way from the deck surface to the top of the combustion chamber. This creates a generally larger combustion chamber than with "closed port" designs.

In the case of the '093' heads (and others like it), the combustion chamber is mostly "closed port" in configuration with the slight "slant" I described. The addition of the "slant" is why the '093' heads (and others like it) are a little larger in combustion chamber volume than a "classic closed port" design with similar valve sizes. Still, the combustion chamber configuration of these heads falls into the "closed port" design.

How did we know that these heads were oval port without seeing the ports?

1) The overall configuration of the combustion chamber is unique to oval port heads (although not all oval port heads have this exact design);

2) The intake valves look to be 2.06" rather than 2.19". Only oval ports use the 2.06" intakes.

[Duke W.]

Re: Jan 7/69 427 chamber, open or closed?

Maybe these should be called "semi-closed" or "semi-open" chambers.

The "slant" you refer to eliminates a substantial portion of the quench area on the spark plug side of the head. This is also the case of 462 compared to 461 SB heads though the plug side quench area on the 461s is much smaller as a fraction of total quench area compared to the early BB heads.

Large quench area was the key to the original "quench combustion chamber" OHV design pioneered by GM (Ed Cole is generally credited with the design, and it first showed up on the 1948 Olds "Rocket" V-8 and the next year on Cadillac's new OHV V-8 and ultimately all GM engines.) in the late forties that could tolerate higher compression on the same octane grade fuel compared to most other combustion chamber designs.

But then GM learned about a decade later when they started looking at emissions that large quench area yields higher engine out hydrocarbon counts than small quench area, so a compromise was necessary.

The fact that the 462 heads were first used in 1966, which correspondes to the first tailpipe emission limits (in California) is probably no coincidence!

The BB design migrated to "open chamber" primarily because it offered lower engine out HC counts due to the lesser quench area, and in the bargain, inlet flow improved due to a less shrouded valve. The "cost" was that the open chamber heads could not tolerate quite as much compression on the same octane fuel as the early full closed chamber variety.

Duke

[Greg L.]

Re: Jan 7/69 427 chamber, open or closed?

Well now that was very interesting reading...something else to add to my own personal archives! Thanks again guys.