Hey, I have a '75 duster with a 400 big block in it (non original) with
smog heads and an 8.2:1 compression ratio. I would like to raise the
compression ratio, possibly by swapping on a set of '67 915 magnum
castings with a closed chamber design as opposed to the open chamber
design of the later heads. I heard that with the lower compression pistons
in the 400, however, that the combustion chamber design of these heads
makes the combo prone to detonation. Would it be easier to just have the
open chamber smog heads milled to decrease chamber volume? please help!
They will not raise your compression near enough to cause detonation.
The 915 castings are hard to find would have to be milled a further .050
to give you 9.2 to 1 (10 to 1 and more to detonation)
A piston swap would likely be more beneficial and you smog heads will
have flame hardened seats for no lead.
Yes you can mill your heads .060 and get to 9 to 1! (with steel shim
head gasket, or take .015 more if you going composition!) 915s unmilled
will only get you 8.6 to 1 remember to mill at least half the amount
taken off the head on the intake side of head more if steel shim is to
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