Advantages and disadvantages between open chamber and closed chamber BB heads? - Team Camaro Tech
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post #1 of 9 (permalink) Old Nov 4th, 00, 06:31 PM Thread Starter
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What are the advantages and disadvantages between open chamber and closed chamber big block heads? I want to build a reliable 500hp 454cid motor. I'm getting ready to send out my heads to be rebuilt. What modifications should I do to the heads (ie valve size, porting, spring tension, etc.) The head casting number is 353049.I believe it is a semi-open chamber heads with 113cc, 2.06 in, and 1.72 ex. I'm open to ideas!!
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post #2 of 9 (permalink) Old Nov 4th, 00, 08:01 PM
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Far as I know, there are no advantages to the old closed chamber heads. They provide higher compression, but less power than the 049's you have.

Popular Hot Rodding had a series on the 049 heads a couple of years ago. Dave Vizard built a pair, changing the seat angle to 30 degrees from 45, to pick up low-lift flow. Interesting articles.

Add big valves (even at 45 degrees) and whatever springs your cam supplier recommends, clean up the bowls and ports a bit, bolt 'em on and have fun. (super bargain: Take-out 2.19 and 1.88 valves, complete set from Competition Products is around $60. Heavy, but under 5,000 Rpm, who cares?)

Lingenfelter built a 496 using (modified) GM oval port heads, got 600+ horsepower. Not bad!
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post #3 of 9 (permalink) Old Nov 5th, 00, 05:27 AM
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The mag's say that -049's and -781's are the best of the open chambered early large oval port heads.
Make sure you cc them, though so you will know real CR.
I went with Merlin oval port for my 496 because after reading an article that was written up about John L's 502 buildup using "ported big valve peanut" and ported big valve oval port heads, that the Merlins intake port CC stock was what he ended up with after his port job on the large oval heads.

Apparently the closed chamber heads shroud the valves tooo much at rpm to really make alota power. Thats why GM went with the open chambered heads design. Even with the about 1 point drop in CR., they still produce more power then the closed chambered heads.

BUT, the closed chambered heads WILL make more power then open chambered heads when used in a low rpm combo due to the detrimental effect of shrouding being less down low rpm-wise or so I've read.

One interesting combo is taking the 98/103 cc closed chambered heads and planing down about .100" to get 90 or so cc's and running a true flat top piston with a below 230 duration at .050" cam. Plus minimum deck height dimension.
Pop Hotrod ran an article on this combo and said that it will run on 87/90 octane gas and not knock with an 11/11.5 to 1 CR. due to combustion chamber efficiencies being better then open chambered designs.

Also, the General changed from closed to open due to the closed being dirty pollution wise since it has "more" cold metal in the combustion area that inhibits a complete burn of the fuel/air mixture. But shrouding was lessened so power was better. Go figure.

This stuff gets into the "Fueling" heads that I've tried to stir controversy up with.

A fascinating subject this is; combustion chamber, valve size and seat angles and valve /head deck angles. Hope this helps and isn't tooo confusing.

I wish that the Chevy High Performance head tests would have included the "SB2", the "Dart/Buick 10 degree" and the small block "splayed valve" (big block design) heads to finish the test completely.

As always, if I get tooo windy, just tell me to shut up!!!! pdq67

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post #4 of 9 (permalink) Old Nov 5th, 00, 05:30 AM
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Only real advantage is the ability to use flat tops and still get good compression.
post #5 of 9 (permalink) Old Nov 5th, 00, 07:48 AM
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pdq67, Amen brother, not long winded at all, just givin' the facts!

Allen M, if you want a reliable 500hp, stick with your "049" castings! I used them on my big incher (496ci), had them ported, cut new seats with larger valves. The "049" and "781" castings have the largest flowing runners of any oval port heads(310cc or something like that, don't have my spec book with me). In fact, a good ported set of "049" heads have been known to outflow rectangular ports up to 6500rpm, with attention paid specifically to the short turn radius and smoothing the exhaust ports. I don't know too many running on the street as a "reliable" big block that are spinning faster than 6500rpm, so I don't know how you could go wrong with them. And to answer the question about closed chamber, don't waste your time. The amount of metal taking up the area in the head SEVERELY shroud the valves. You are better off with the open chamber heads and put the metal on the piston, with a domed piston to bump your compression. That way piston is out of the way on downstroke allowing the open chamber head to flow freely! By the way, I've cc'd two pair (4 heads) with the "049" castings and they measured between 114cc and 116cc in the chambers. This will get you in the ballpark for compression when selecting pistons!

If you want better flow, go bigger on the valves. I think my stock ones were 2.07"Int/1.76"Exh for the "049"'s. I went with Speed pro swirl polished 2.19"Int/1.88"Exh. Choose springs based on the cam your using. I went with the Comp Cams Xtreme Roller 294 K-kit. It comes with Springs, locks, retainers, stem seals, pushrods, roller lifters, cam, timing set. They have all the specs for setting spring pressure for the components provided.

1956 Chev 150 2dr sedan 302ci
1965 Chev Malibu 2dr hdtp 496ci
1967 Chev Camaro 327ci

[This message has been edited by [email protected]$TeR (edited 11-05-2000).]

[This message has been edited by [email protected]$TeR (edited 11-05-2000).]
post #6 of 9 (permalink) Old Nov 17th, 00, 05:26 PM Thread Starter
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Thanks for the info. Anyone else have any suggestions on what else can be done to the motor to 500hp on pumped gas?
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post #7 of 9 (permalink) Old Nov 18th, 00, 05:39 AM
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An engine like you are talking about as follows;

9.5CR., P&P oval's w/ big valves, a Holley Strip Dominator single plane and 750 carb., small 1.75" four tube headers per Dyno2000 produce:

Cam = 280/290, 110/106, Max lift.

Hydraulic = [email protected], [email protected];
Solid = [email protected], [email protected]/4500rpm and
Solid street roller = [email protected] and 570/[email protected]/4500rpm.

Take your pick. pdq67

As always, these are simulations but will give you some idea whats going on. I like the 282S solid cam from CC, myself.

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post #8 of 9 (permalink) Old Nov 18th, 00, 06:39 AM
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By the time you put the money into those heads, you'll wish that you bought Merlin heads. I have a real nice pair of #049's with everything done to them and 1 weekend of time on them in a boat. They have alot invested into them and I might be swayed into parting with them.

Everything I own is for sale. What were you looking for?
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post #9 of 9 (permalink) Old Nov 18th, 00, 02:01 PM
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Thats why I went with the new Merlins.

It took me sooo long to gather the nickels and dimes to buy my stuff a piece at a time that I wish I woulda waited and bought the stuff all together now. I woulda went for a set of Canfield 300+ rectangular ports, me thinks. They didn't exist when I bought my Merlins. pdq67

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