[PepinoBBB]

Hi guys I just wonder if any body knows the difference between 6S and 6X heads. I hear wonders about 6X 's but I have a set of 6 S's. I want to use them on a 65 326 HO engine currently have 140 heads originals. If any advice will be really appreciated. Thank you in advance.

[Brian Baker]

6S heads were used in 1976 production on everything from 350's to 455's so you have to be careful on which ones you get if you intend to use them on a 326. If they're the large chamber variety off of a 455, you do not want them on your 326. Your compression ratio will be so low it'll run pretty crappy.

Unless you can confirm they came off a 350, or you could measure their combustion chamber volume prior to purchase, I would pass on those heads for use on a 326.

[Brian Baker]

Personally, I'd stick with the 140 heads you have right now. They're pump gas friendly (92 octane) and flow comparable to any 6X when coupled with the small bore of a 326.

[steve25]

The 6S heads have the air injection passages that run along the head lenght wise and these are drilled into the bosses on the roof of each exh port runner.

The top of these drilled passages below the valve cover lip is sealed off with a pressed in steel plug.

These heads where only used on CA cars to meet there emissions standards and otherwise they are the same as the 6X casting.

Also if you are running the 1965 4 bbl intake manifold and you switched over to a iron 68 to 74 intake and Q-jet set up, or a performer and Q-jet set up you would pick up more power then if you did the 6S head swap!

Now, your 140 casting heads are not original to your 65, the 140 casting came fitted on the early 1967 250 hp 326 cid motor and are stated to produce a 9.2 compression where as the casting number 22 casting head is the casting number head that would have come on the 1965 285 hp 326 HO motor and would have had a factory stated 10.5 compression .

Note that all of these factory stated compression ratios are at least 1/2 a point higher then the motors had in reality.

Also as stated above your small stock bore size ( 3.720” ) is much smaller then that of even a 350 ( 3.88” )and that in and of itself would greatly limit any possible gain in intake flow that you could be offered by the 6S heads!

Any of the two possible combustion chamber size 6S casting heads you could have would lower your motors compression even more, and the only plus that could be had by running the 6S heads would be its screwed in rocker studs that your 140 castings do not have.

If it where me I would rebuild my 140 castings and in the process convert them over to screw in rocker studs with push rod guide plates.

I would also mill your 140 castings ,030” which would compression wise compensate for the sinking of the valves during the valve job and also help to keep your compression at like 9 to 1.

By the way even though your 140 casting heads are not original to a 65 car, they do have bigger valves then the original casting 22 heads would have!

[steve25]

I ran some numbers in terms of what you could have for a compression ratio and here’s what I came up with .
These numbers assume that your motor is .030” over and I did not factor in any valve notch volume since I do not know what they would be.
This means that if your motor is not .030” over and or your pistons do have valve notches that your compression will be lower then what I am posting here.

Also note that the factory stated compression ratios for any motor are 1/2 point or more lower then real life as I posted up before.

1) 65 CC chamber = 9.66 to 1.

2) 68 CC chamber = 9.36 to 1.

3) 70 CC chamber = 9.15 to 1.
In these 3 calculations I used these factors.

Head gasket volume 9.4 CCs

Deck clearance volume .019”

No valve notch volume as stated,

No ring land volume which would add another 1 to 1.3 CCs in terms of lowering the ratio.

[steve25]

I had more time to run some compression numbers in terms of using the 6S heads you question running.
The smallest factory chamber size you will find these heads in is like 92CCs , and when I run the numbers as I did in the above post you would have a Piss poor 7.4 to 1 compression!

I have no idea how thick the cylinder walls are in a 326 block, but if I where rebuilding the block also I would go right to a .060” overbore in a Heart beat!!

At least at .060” over the motor finally has a bigger bore then stroke which will help out nicely with the hp production.

[steve25]

Further investigation into the original casting # 22 heads that would have been on a 65 326 ho motor show that it would have by factory info have a 60 CC chamber .
A chamber of this size explains the small 1.88” and 1.60” valves used and is the only way that the motor would have even close to a real 10 to 1 compression, no less it’s factory rated 10.5 comp with common production tolerances involved.

[Formulas]

Long time ago I randomly bought junkyard 326 heads for a 350 in the end due to valve and port size I did not use them the ports were way smaller than a 1970 350 2bbl

About the best 326 head upgrade would be the 218 1968 350 head but they are scarce

If I had a 326 I would probably install larger valves open the ports in the original heads in order to keep compression higher mill them a touch and bore the block for a compression bump

I have original 400cid 6S heads on a 462 I popped out the drilled hole plugs for air injection ran a 7/16 tap through them put in 7/16 all thread trimmed flush externally contoured them in the ports and pinned them, on mine only the end cylinders actually got the air

Combustion chamber sizing 6S heads mimic 6X variants for a given cid.

[PepinoBBB]

Hi Guys , Certainly I forget to mention that my block is HO block. And basically just trying to have decent power with 91 Octane Super unleaded gas in California. My car is a Tempest 2 door HT kind of nice. That I use on the weekends. And is running a little sluggish. However I do know that high performance engines need good quality fuel not available very easy in California.

Thank you , Guys your help is greatly appreciated.

[steve25]

The 326 HO block is nothing special, this game Pontiac did a lot , ending in 1976 with the 455 so called HO motor in the Trans Am, which as it turned out was the very same 455 you could get standard in the Grand Prix that year!

The only difference being a tad different advance curve and the duel exh after the cat on the TA cars!