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I have a 400 thats bored .040 over . The pistons are flat top with 12cc valve releifs how far off am I to be able to bolt on a set of 670 heads and run premium 93 octane . I like the way my engine runs good power for a cruiser it currently has 143 heads on it .I came across an excellant condition ready to bolt on pair of 670's for a deal that is very hard to pass up let me know if its worth the hassle
 

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670 heads are real nice. They will bolt right up. 93 octane is kinda weak though. I AM NOT SURE ABOUT THE 12 cc THING so wait for more replies....
 

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The valve reliefs are pretty standard with flat top pistons, and won't significantly reduce your compression with 670's. 670 heads are one of the better Pontiac heads for high performance use, BUT are more prone to pinging and knock than most other heads. They are a closed chamber design like the earlier 421 heads. They typically cc out at 68 to 72cc. I think with your combo, running the 670 heads, you'd need about 98 octane fuel. Some folks have had moderate success at relieving the 670's a bit to open them up a few cc's, but this is highly specialized work and easily botched. My advice is to run a head with bigger chambers.
 

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Do search for a thicker head gasket. I know a guy that got his compression down to 9.5 with them.
 

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if you buy heads with small chambers for more performance and then grind them out or use a thick gasket arent you back where you started? if you arent racing and you like the way it runs now seems like a waste of time. :confused
 
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