Re: 427 Closed & Open Chamber Pistons
Here is an '842 head that shows almost an identical combustion chamber.
Here is an '842 head that shows almost an identical combustion chamber.
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Re: 427 Closed & Open Chamber Pistons
Here is a 3919842 "closed" chamber head. This is what I would expect them all
to look like. Yet, a previous image shows a more '074-like combustion chamber
found on another '842 head. As a matter of fact, if you search eBay, you will
find two or more '842 heads with this same open chamber design. Any thoughts?
Could these have been owner modified, or cast that way? See how confusing this
gets.
Steve
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Re: 427 Closed & Open Chamber Pistons
Steve,
In mid/late 1969, it seemed everyone that was racing anything with a 1st design L88 wanted the new open chamber heads, especially the Pro Stock Camaro guys. Some bought new OC heads but many just had their 838's modified to be open chamber. It was "the thing to do" in that era and it seemed just about everyone did it. They expected tons of new found horsepower. (it didn't happen)
I've never seen an original 838 head that was factory open chamber. Any that we see today have been converted. There's no logical reason why GM would have been making these heads with an OC design, especially because of the fact that they were still being used for service.Comment
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Re: 427 Closed & Open Chamber Pistons
Michael,
Thanks for the historical input. I thought this might be the case.
So, I now have a new question in response to your post. If one is trying to
lower the compression ratio to a more useable number ( 9.5:1 ), will this
combustion chamber modification to the '842 heads lower the compression ratio
by any significant amount? Do the '842 heads have enough wall thickness in
this area ( adjacent to the spark plug hole ) to facilitate this grind back?
Do you run the risk of getting into the water jackets? Can they be welded
up if you do hit the water jacket? Is it worth it today? I'm leary about
laying back this area in my stock '842 heads. What do you think?
SteveComment
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Re: 427 Closed & Open Chamber Pistons
i have seen this done and no problem BUT i have also seen them leak. aluminum castings back them were not that good and i have had stock ones we had to "water glass",sodium metasilicate, the water jackets to stop the leaks. we used "water glass" to preserve eggs back during WW II. NEVER try to "open chamber" closed chambered cast iron heads because they all will fail.Comment
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Re: 427 Closed & Open Chamber Pistons
Steve,
I think your last question, "is it worth it today" is probably the most important one. I suppose it really depends on what the end use for the engine is going to be but, all in all, no. The overall design of the heads is actually just slightly beyond caveman by todays standards so if your looking for serious HP, look elsewhere. There are a number of later design heads that look similar, externally, but produce much better torque/HP numbers without spending thousands on old obsolete parts. Truth is, there never was a great HP increase as a result of the open chamber design. Certainly not enough to justify spending a lot of money to convert.
The problem you mention about "accidently discovering water" is quite possible. I've never actually had that happen to me but I've sure heard a lot of stories about it over the years. Aluminum is actually easily repairable but, again, the cost of all the machining is not near worth the results.
If I were going to build something that LOOKED correct for a period restoration with 838 heads, I would consider flat top pistons if I was going to drive the car from time to time. That would probably drop the CR numbers down to around 10 or so. (I would have to run the numbers to get actual CR).
On pistons, I think Joe Lucia has good information about what's actually available today. I don't get involved in engine building now so I can't make any recommendations.Comment
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Re: 427 Closed & Open Chamber Pistons
Steve-----
I would say that these are definitely converted heads. Note that the "open" portion of the combustion chamber appears to have been "ground"; it is not the as-cast appearance seen in factory, open chamber heads.
Quite frankly, the heads HAVE to be converted irrespective of the above. There's no way that GM would have made open and closed chamber heads with the same casting number. As I've mentioned previously, the combustion chamber configuration is part of the casting. If anyone got the idea that the factory would have converted closed chamber heads to open chamber, the idea is absurd. First of all, such a conversion would have been WAY too labor intensive for a factory operation (assuming that the heads were a normal PRODUCTION or SERVICE piece; a GM "experimental-type" piece would really not be a lot different than a field converted piece). Second of all, if the factory had made such a conversion the chances are very great that the casting number would have been ground off, in whole or part.In Appreciation of John HinckleyComment
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Re: 427 Closed & Open Chamber Pistons
Joe,
The grinding-off of the casting number is a very good point. I have seen
this done ( at the factory ) to modify the 3933163 intake to the 3933198
( open plenum modification ) for ZL-1, L-88, etc..
Now that others here have commented on the commonality of combustion chamber
mods back-in-the-day, I know why I see '842 heads with an open chamber-like
appearance.
My thanks to all.
SteveComment
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minimal increase in power
Because the 842 heads have lousy valve bowls. The 074 bowls are better, and the "D ports" have a vastly better port.
MarkComment
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CAN YOU SAY "BUBBA"
Yes, some intakes had the part numbers ground off and restamped, but this has suddenly become a lot more common since somebody wrote about it in a Chevy by the numbers book! The 198 intake was not just a 163 with the divider removed and the number restamped! There really was a 198 casting! How do I know? I have two original 198 intakes with the full and complete casting numbers (not for sale) and also one factory 163 intake that had the divider removed (and didn't have the casting number restamped).
Please don't misunderstand me, there were SOME intakes that were stamped but NOT a lot. In addition, like 20 years ago, many of those restamps had the first few numbers cast in followed by a cast in small rectangular pad for the final stamped numbers (on BB intakes) not just ground off with a die grinder.
MarkComment
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Re: minimal increase in power
Yes, I agree. I don't think the new open chamber design added as much as a lot of people think it did. Little known is the fact that the new chamber design was actually a result of emissions testing, not HP. It just happened to add a bit of HP in the process. The changes that you mention were probably more substantial as far as HP increase.Comment
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Re: minimal increase in power
the big problem with BBC heads was that 2 intake ports turned into the center of the chamber and 2 intake ports turned into the chamber wall. the open chamber heads helped some on this problem but the new BBC truck engine,8.1 liter, changed the porting so all the intake enter the chamber the same direction for better fuel distribution. we even had cams ground with different valve timing on certain intake lobes to help the difference in how the intake port entered the chamber. installing venturi shaped valve seats into the exhaust on cast iron heads also helped a lot. the later OC aluminum heads had these venturi shaped exhaust valve seatsComment
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Re: minimal increase in power
Clem,
Would these heads be the '077 aluminum "D" exhaust port heads by chance?
SteveComment
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yes
The D port heads are a great improvement over the 074 heads, but you have to get a set that weren't messed with. Bubba usually tended to hog out the ports as big as possible in the 1970's and early 1980's and would really screw up the mixture velocity in the port.
MarkComment
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Re: yes
I actually found an untouched set about a year ago on eBay. The raised
vanes in the center of the exhaust ports are virgin.
SteveComment
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