Closed Chamber??, Open Chamber??
Closed Chamber??, Open Chamber??
09-15-2004, 09:51 PM
09-16-2004, 12:41 PM
It all depends on what engine family you're talking about. The two most commonly refered to as "open" and "closed" are the big block Chevy and the Pontiac.
Early BBCs ('65-'67) are "closed" chamber. That is, the chamber is oval-shaped and the spark plug is exposed to the flame front on one side, only. The same is true of the Pontiac. In '68, they "openned" the chamber to allow for better flame propegation across the top of the piston. The main engineering rationale was to reduce emmisions, making the engine more fuel efficient at lower RPM. It worked. Pontiacs needed an AIR (Air Injection Reactor) pump to pass '67 federal standards. In '68, with the new chamber shape, the pump was no longer needed. It didn't do as much for cleaning up the Chevy, but it did significantly improve performance.
I've heard people refer to small block heads as open or closed, but that's not really accurate. Small blocks have "large" and "small" chambers. The 327 and early 350s had a small (64 CC or less) chamber in the performance engines, for more compression with a flat top piston. The lower level engines had "large" chambers (70 CC or more), to keep the compression down. There are a couple of variations with the early stuff. There are almost as many chamber designs as there are people making heads with the modern stuff.
Piston design for BBC varies between chamber types. Most others use flat tops, therefore, no difference until you get into racing heads and pistons. While you can use "closed chamber" pistons in an "open chamber" build with BBC, it isn't recommended as flame travel is degraded. You cannot use "open chamber" pistons with the "closed chamber" heads, as the bump on the piston will hit the head.
A picture is worth a thousand words, but I don't have any handy. I hope this helps some.
09-16-2004, 11:16 PM
It is also my understanding with BBC's that a closed chamber head will increase your compression compared to an open chamber head.
09-16-2004, 11:58 PM
One of the BEST examples was/is the early Ford 351 Cleveland head. Closed chamber, 4 barrel heads were what they used for the base Boss engines.
09-17-2004, 07:54 AM
50bulletnose, At the risk of a disagreement, the Cleveland heads are all "open" chambers. The early heads are what we call "quench" heads, as they have a much smaller chamber, with "quench pads" on both sides of the bore. The spark plug "sees" all the way around. The later ones had a full-round chamber, dropping compression ratios significantly.
The BBC "closed chamber" heads had a smaller capacity, thus raising the compression ratio over the "open" chamber heads. There are some open chamber heads as small as 106 CCs. Most of the "closed chamber" heads were 106 CCS, as well. The 366 and 396 heads had smaller (99, I think) chambers. Some of the later 396s and the 402s used a small "bump" on the piston to maintain the higher compression ratio. Virtually all of the BBC Hi-Perf engines used domed pistons.
09-17-2004, 11:58 AM
Thanks for the reply MrPbody. Your concern for the risk of disagreement is well founded. :) I believe what we have here is a case of semantics. The heads do have an open appearance, however are well described as closed. I agree that there is a larger quench area than the BBC, but the effect is virtually the same. Also, Ford has catagorized them as closed. From a purely engineering standpoint, I have to agree with you. The shape of the Chebby chamber more closely fits the classic standard.
It sounds as though you may have some experience with comparative head engineering. May I inquire as to that experience, as you've mentioned "we"? Please don't tell me that you stayed at a Holiday Inn Express last night! :smile:
09-17-2004, 10:33 PM
On a BBC, by how much do you think a closed chamber will raise the compression as opposed to an open chamber head? Im about to put a set of heads on my Nova with a casting of 3964290 which is said to be a closed chamber square port head with a 101 cc chamber. What would you expect my compression to be with a stock piston?
09-18-2004, 12:43 PM
50bulletnose, Nope, not a Holliday Inn, but good shot! Actually, I build engines for a living, and have significant experience and education in that arena.
I must say, however, after looking over a set of Australian "Clevelands" (just happen to have a set in the shop right now!), I would have to back up on my previous statement. The spark plug is fairly shielded from the exhaust side. While not the classic "bathtub" shape, I suppose they could be considered "closed".
The term "we" refers to myself, my co-workers and employees.
Robsnova, the only way to determine the answer to your question is to actually measure the chambers (CC them) on both heads, and do the math...
C = compression ratio
R = 1/2 the bore diameter (radius)
H = Stroke (or height)
Pi = 3.14159
CC = chamber capacity
CF = 16.378 (conversion factor from Cubic Inch to Cubic Centimeters)
C = Pi x R(squared) x H x CF / CC
The above method is the approved formula by SAE. It will indicate a static compression ratio of 1 full point lower than most compression calculators on the net, as it does not add "dead volume" back into the equation, but allows only for "swept volume".
Lastly, the term "square port" is strictly shade-tree. The intake ports are refered to as "rectangular" in the business. Rectangular port heads make MUCH more power, regardless of CID, over 3,500 RPM. They're pretty much dead below that, except in engines over 500 CID. For a driver or street/strip application with a smaller engine, late '60s 396/427 oval port/open chamber heads are the hot lick. Port efficiency is much better at the lower volume required by engines operated in these conditions.
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