big block head question

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
be used!
Dodgem

sackattack84 wrote:
> 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!
>

how many miles on the engine now?


"Dodge-Him" > wrote in message
...
> 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
> be used!
> Dodgem
>
> sackattack84 wrote:
> > 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!
> >
>

Not sure of the mileage, I bought it from a restoration shop nearby and the
previous owner had swapped in the 400.

thinking fresh rings might be good too



"sackattack84" > wrote in message
lkaboutautos.com...
> Not sure of the mileage, I bought it from a restoration shop nearby and
the
> previous owner had swapped in the 400.
>

Possible, although I did a compression check and everything checked out

most engines like this.........if you pull the heads and replace the
gaskets, and it has more than 75,000 or so, the poor thing suddenly starts
using oil!


"sackattack84" > wrote in message
lkaboutautos.com...
> Possible, although I did a compression check and everything checked out
>

That's good to know. Thanks for the advice!

don't want you to suffer from "James bond disease" leaving a blue smoke
screen lol



"sackattack84" > wrote in message
lkaboutautos.com...
> That's good to know. Thanks for the advice!
>