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I recently bought a spare 400 engine for my 71 Grand Prix, an L78 with the 6X 8 heads which have large D shaped combustion chambers. It has 7.7 CR, Jimmy Carter era low compression, and I want to bump it up to the low 9's and put a medium cam in it for some perfomance. I wonder if this is a better way of doing it rather than all of the block and deck milling and all that goes along with that. The engine was rebuilt and bored .030 over with 411NP flat top pistons with valve relief which look great, along with the bores. The existing car engine runs good for its 90k miles with no leaks or oil burning, so replacement isn't the motive as much as some street performance. I am also considering swapping heads on the engines when I install the replacement since the original engine has 10.1 CR.
 

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Grumpy Old Goat Herder
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Your best bet would be to find a pair of 6X-4 heads from a 350. They should give you about the compression ratio that you want. Be sure to cc the combustion chambers to verify that they haven't been modified.

Bill
 
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