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Old 04-20-2018, 10:01 PM   #1
JIMBISK
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Closed Chamber VS Open Chamber BBC heads

I recently replaced the aging 350 in my 71 gmc with a BBC 454, peanut port heads, ZZ502 intake, mild cam and headers. low 8:1 compression with the current 121-122 cc heads. I have a set of oval port 063 heads with a 100.9 cc chamber closed chamber or a set of 113cc open chamber 241 casting heads from a 1972 LS5. I can't decide which set of heads to use to pick up performance. I've heard the open chambers while less compression than the closed chambers flow considerably better. But is it enough to warrant the difference between 9.4:1 (closed chamber) and 8.8:1 (open chamber). Any help on this topic would be great. also going from an SM465 to a 4L80E when doing the head swap.
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Old 04-20-2018, 10:32 PM   #2
ZEKE68
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Re: Closed Chamber VS Open Chamber BBC heads

Are you sure that the pistons you have will work with the closed chamber heads? It also depends on how radical the cam is. More overlap means you can get away with more compression. A mild cam with 9.5 cr and iron heads may be too much compression for pump gas. There are dynamic compression estimating programs on some internet sites. I am running open heads with 20cc dome pistons which give around 9.4 static cr, I am running a 224/234 @ .05" cam with 110 LSA. Have not hit the road yet so I can't tell you how it does.
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Old 04-20-2018, 10:42 PM   #3
JIMBISK
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Re: Closed Chamber VS Open Chamber BBC heads

I am not 100% sure what cam is in the engine, bought it as is from a friend. Doesn't seem to be stock but its definitely not roudy by any sense of the word. The current pistons are just stocker flat tops, which is why I am trying to boost the compression up just a bit. Odds are I will be switching out the cam as well during the process so I am also open to suggestions there. Just want to rid myself of these peanut ports and hopefully pick up a little power with the help of compression.
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Old 04-20-2018, 10:53 PM   #4
OBS454
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Re: Closed Chamber VS Open Chamber BBC heads

While the appeal of more compression is tempting I prefer the open chamber design for the flow characteristics. Do you have flat top pistons? That is very important too. Do you know what the cam profile is? I would check that first to see which would work better with your two options. Lots of info out there on potential combos so find out what you have first then make some decisions. Also, using a metal shim style head gasket can help with increasing compression slightly (re-torque after warm up is in order if memory serves). I run a carburetor and HEI and the heat my 454 makes causes pre-detonation if I put in too much timing (no computer/knock sensor) due to pump fuel. More compression would exacerbate this condition. If you have ECM controlling timing this would be less of an issue and more compression could be compensated for.
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