6x-4's or 8s?
 

There are many threads on the positives and negatives of these heads, but all things being equal, which bare head would you prefer to start with for a 400 stroker build? The smaller chamber of the 4's or the larger 8's? Or are there other reasons you would choose one over the other?

Thanks
Drew

First priority should be to get the compression ratio were you want it.

After that the -4 would be a better performance head but not by much. It offers slightly better port shape and a smaller chamber.

For sake of discussion, this is for a street car running pump gas.

#4`s. Use the smallest chamber then, dish the piston, if necessary to get your static compression. The smaller chamber head has a longer "short turn" and it unshrouds the valves for better breathing.

I have 6x-8 on my street car. Ported them at home. I shaved the heads down to get 90cc chambers. This was over 20 years ago but I believe they where shaved .060 and intake side was .030.
The car has been 11.8/113 at 3900lbs.

My findings from porting well over 18 sets of of 6X heads over the years with a spread of about 70% of them being -4 and the rest -8 is that the port shapes are the same within casting tolerances.

It's the 6H casting that they had to resort to making the chamber deeper to get up to 124 or so CCs where in the port shapes / depths where shortened.

Due to where the chambers where carved out more in the -8 heads they are a little less shrouded then the -4s, but this added air flow only shows up when the Intake side of the heads are ported a great amount and valve lifts above .600" are used.

All low compression heads that do not use the RA4 type chamber from 71 and up if using 2.11"valves and ported to flow 230 cfm and up could make good use of having the chamber unshrouded as in this photo.

On a 400 the 6x4 would be preferred just based on compression ratio. There really is no significant flow advantage to any of those low compression Dport heads, just chamber size differences. The 6x and 96 are considered top of the heap though. In Pete McCarthy's article "Ultimate Head" published in HPP, he found a set of 5cs that were an anomaly in terms of flow superior to the 6x.

6x8 flows better than 6x4 if you plan to install 1.77" exhaust valves due to less valve shrouding in the larger combustion chamber but you would have to mill them to reduce the chamber volume for your 400.

So, basically if I am looking for a set of heads to work with a stroked 400 (461), and I have a set of 4's and a set of 8's available, I am gathering that the best initial choice would be the 4's due to CC size? No other advantages or disadvantages?

Run the -4 heads and get dished Pistons to achieve your desired compression ratio.
I find no flow advantage on my flow bench to the -4 chamber shape or the -8 chamber shape in regards to either head running the stock 1.66" or the larger 1.77" Exh valve .

The layed back area of chamber enlargment in the - 8 head is more than 1/2" above the Exh valve seat and has no impact on Exh flow!

For a street car running pump gas, getting the CR right should top concerns over flow. I doubt that performance at street RPMs will vary much if at all, but 'street' use can mean different things to different people. And if you already have your pistons, you may have less of a choice, and need to pick based off compression ratio you want to be able to use pump gas, which can also vary. My pump gas can be from 86 to 91 octane, which is enough difference to matter. If you are close to the edge and run low and have limited gas available. I would rather use 91 octane in a car built to use 86 vs using 86 in a can designed to use 91.

Drew, To answer your direct question, in a PUMP GAS (street) cast iron 6x type Stroker build in a 400 block, if the plan was to go with a 4.00" stroke crank, that would allow slightly milled 6X-4's & an undished piston. That noted, if you have a pair of 6X-8's or 5C-7 or 5C-8 heads (with near 100cc chambers), they would be better suited to 4.25" stroke build in a 400 block.

Procuring 6X-4's,mthey will command more $$$ & are typically harder to find than the 100cc chamber '75-78 400 heads. Through the 90's & early 00's, I kept a major Pontiac crate engine builder in 400 6x-4 longblocks. In doing so, I routinely pulled 6X-4's & mated them with '67-early '75 400 shortblocks. Also pulled all the W72 longblocks I could for future sales. In several yards I was literally chunking 100 cc smog era Pontiac heads in the open trunks of cars. Only if I was long on 455 short blocks would I need the few 6x-8's that came in usually on partscar 400's.

I have the time to find either 4's or 8's, so I just wondered which would be a better starting point. Looks like most of Butlers stroker kits are 4.25 cranks so thats likely where I would end up.

There may be a few ported cast heads for sale soon going by a few of the threads.

It would be silly to spend money on a dished piston so you can use the -4 head. It would also be silly to spend money milling -8 heads if the -4's will give you the targeted compression ratio.

That is why your desired compression ratio should be your primary concern.... See post #2

The only difference I noted was the -4 chamber is smaller diameter not a different depth. Mine were 93cc untouched. I smoothed the chamber ridge and milled them 0.050 and they were still 93cc. My 78's original -8s are 97.5cc.

I have a set of ported -8s they smoothed the ridge on that go 105cc.

This has been a fun thread for me. However I can't say that I am any less confused. Maybe if I posed the question another way...you walk into a garage, and the guy has a set of 8's and a set of 4's laying there. Which ones do you take home with you? Maybe there is no right answer?

Which one gets you closest to your desired compression ratio?

Right now, I have a virgin 400 block sitting in my garage. I suppose I am shooting for about 9.5 CR. I realize that the block and heads will need to be machined and built to obtain a good pump gas CR. I just dont know which heads would get me there more efficiently. I am seeing just as many strokers built with 8's as with 4's, so maybe the answer is...it depends?

If your staying with the stock 3.750” stroke of the 400 then the -4s are the only way to go if your working only with 6X castings.

If your contemplating stuffing in a longer stroke crank to gain more cid then there may come a time where the -8s make more sense to avoid having to go for a dished piston.
That’s the bottom line.

Definitely the longer stroke.