Head work

[Southtowns27]

I finally got a set of 791's for my block. What are the biggest valves I can stick in these heads? How much porting and where can I do to them? Any other thoughts or tips on improving these would be really appreciated!!

[Southtowns27]

Anyone?

[bubbletop61]

Southtowns;
Check your PM's

[SteveD409]

If its a 348 block then I think you can put larger valves than your bore will safely use. Hipo 409 valves get mighty close to side walls or cutouts.

SteveD

[Southtowns27]

I'm planning on boring this thing almost as far as it will go. I read somwhere about 2.06 / 1.74 valves...do these fit? Anyone have any input on porting? Thanks in advance :cheers

[CDNpontiac409guy]

Southtowns... for the do-it-yourselfer...
In order of priority:

(1) Unshroud and blend the bowl area under the valves. Do not flatten the short radius... just make it smooth.

(2) In the exhaust pocket under the valve... mill away at the bulky sides of the exhaust valve guide bosses. Make them narrower, and smoother.

(3) Raise the roof of the intake port from the intake opening... as far as you can reach ( about 3"-3 1/2" ). You can use a set of large port intake gaskets as a sort of template. Also, I would widen the intake port on the short side... giving a more direct line to the valve opening.

(4) Raise the roof of the exhaust ports ( particularily the outside ports )... but not too extensively at the top of outlet face. What we're trying to do is give the exhaust a smooth even arc to travel through into the header tube. You CAN open up the entire exhaust port, but again, don't lower the short side ( bottom ) radius.

(5) Cutting for larger valves. These heads WILL accept 2.06" 1.735" valves with no problem. If you have a stock cubic inch ( 348-358 CID ) 9.5:1 compression engine, that you will have a modest cam in, and will likely be coupled to a Geritol transmission, there is absolutely no need to install larger valves than original.

If you've upped the compression rato, and/or cubic inches... then the bigger valves would be a good idea.


Just my thougts :p
:cheers

[Southtowns27]

Thanks Aubrey! I wish I knew as much as you about these things... I'm definatley going to need bigger valves. When all is said and done, this should hopefully be an 11.5:1 427cu.in. "348" with a huge cam, dual quads, and a 4 speed. :) I'm going for all out power and not caring much about streetability. I want to be able to drive it back and forth to the track and to cruise nights and shows...that's about it.

[threeimpalas]

Originally posted by CDNpontiac409guy
(1) Unshroud and blend the bowl area under the valves.

(2) In the exhaust pocket under the valve... mill away at the bulky sides of the exhaust valve guide bosses.

(3) Raise the roof of the intake port from the intake opening...

(4) Raise the roof of the exhaust ports ( particularily the outside ports )..


Any pictures of 348/409 heads that have had these steps done to them that we all can use as a reference?

[Skip FIx]

Aubrey what about that extension of the short turn that goes almost to the back of the bowl in the intake? Or it there water hiding there? Never seen another intake bowl with that there. Its on the common wall. If I can figure out my new digital I'll take some pictures.