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Old 12-08-2004, 08:33 PM
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Pontiac 400 and 455

I had two engine one was a 1968 400 YE block and #15 heads which was 10.5 to 1 compression 340hp. Ok I bought a 1973 455 YC block and #4x heads which was 8 to 1 compression 250hp.. I just got the 455 back from rebuilding it but I put the Heads off the 1968 #15 heads on 455.. I ask the guy who rebuild it to cc the heads to make shore what cc they were but he for got he said. Does any one know what the compression will be.. This is my frist engine I tore down, Just don't want to engine to blown up... CAN SOMEONE HELP !!!!!!!!!!! Thanks

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Old 12-08-2004, 08:43 PM
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You're gonna have too much compression, around 12:1 or so.

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Old 12-08-2004, 10:35 PM
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Even the 400 running the 15 heads would be tough to keep happy on the street. The 455 will self-destruct unless fed a diet of race gas all the time. The 455 would be most happy on the street with a combustion chamber of about 96 cc's. The 4X 455 heads are 114cc's like you said, and would even be happy on the cheapest regular gas. Maybe sell the 15's on ebay and look around for either 96's, 7K3's, 6X's, or even 4X's off a 400 (98cc). I like the 96's or 7K3's because of the 1.77" exhaust valves, but the 1.66" valves actually seem to work okay on the street.
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