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Old 10-21-2003, 06:30 PM
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Pontiac heads

Have a 1972 Firebird with 350 2 barrell, single exhaust. Upgrading to 4-barrel 600cfm, level 2 cam, headers, dual pipe and headers. Existing heads only have 4 bolts for exhaust on each head. No "boss" to drill, so no headers. Bought set of 1977 6X heads that were on a 400, port matched, new valves, Intake larger than 350. Exhaust slightly smaller. Combustion chamber visually appears very similar but have not cc'd. My book says 350 compression ratio is 8.1-1. Question: 1. Will the 6x head work on 350? What will this do to my compression? Should I mill head? Any suggestions? Thank you.

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Old 10-21-2003, 06:38 PM
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There is actually two different 6x heads one from a 400 and the other from a 350. They have large and small combustion chambers, small one was on a 350 and large one came on the 400.

Theres an easy way to tell which one you have but I cant remember, something to do with casting number though.


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Old 10-21-2003, 06:54 PM
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Years ago I raced a 73 Pontiac Ventura with a 71 350 in it. The heads I went after were off a 68/69 350 the chambers were 62cc at the time the smallest made. I don't have the casting number anymore but thats what I would tell you to do is a lot of research on the casting numbers then find the smallest chamber and you could increase the ratio that way. Plus milling a little off, maybe another .030 could help just make sure you have the valve clearance.
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Old 10-21-2003, 08:11 PM
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answer to your first question is yes the heads will fit on you 350. however since the 6X heads you bought came from a 400 they probably have the number 8 castings. these have around 100cc chambers. the 6X-4 heads have around 90cc chambers. 400's in some trans ams got the 4's.

now comp ratio of 6X-8 heads (the ones you have most likely) on a 400 is 7.6:1 so needless to say these heads on a 350 will lower the ratio even more. and if you do by chance have the number 4's, they originally came on 350's and few 400's got them. the 4's on a 400 had 8.2:1 and on a 350 had about 7.8:1.

i have the number 4's on my 400 right now with flat tops. the heads have been shaved .030 and my block has been decked. i have roughly 9:1.

you can safely mill the heads down .060. i forget how many cc's you'll lose. i think like ten or so. but if you mill them over .030 you will have to get the intake shaved and have different valve train components, etc.
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Old 10-21-2003, 10:44 PM
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On Pontiac heads the reduction is one cc per .006 milled. There are also 6X heads that were offered on a few 455's and the chamber size on them was 115cc's. What is the casting number on your present heads? If you want a high compression 350 the best heads to use are casting #48 Ram Air III heads but these are hard to find. These are the heads that were used on the 69 350 HO engines. You can also use 16,62,13,670,12, heads, most of these heads have 72-75cc chambers and would yield around a 10.50-10.75 compression ratio.
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