[drewm]

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

[PAUL K]

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.

[drewm]

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

[PunchT37]

#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.

[Mike S]

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.

[Skip Fix]

Originally Posted by PunchT37

#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.

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.

[steve25]

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.

[NeighborsComplaint]

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.

[drewm]

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?

[steve25]

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!

[dmac]

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.

['ol Pinion head]

Originally Posted by drewm

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

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.

[drewm]

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?

[PAUL K]

Originally Posted by drewm

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?

[drewm]

Originally Posted by PAUL K

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?

[Jay S]

One of the 64 heads started leaking coolant, that is when Hand made the switch to the 6x-4s. Rocky Rotella showed me that particular heads and let me look it over. Very nice looking port work, shouldn’t have been close to hitting a coolant passage. IRC it flowed just over 250 cfm @28” on the intake and slightly over 200 on the exh. The 64s are fairly well known to have smaller port volumes against other d ports, might explain why he had trouble.

I think Hand gained a fair amount of performance with the 6x-4 combo, almost 20 cfm on both the intake and the exhaust. 260 something on the intake and 220s over on the exhaust. There is a lot of flow potential on a 6x if you put in the work. IMHO they are a great head as cast though too, not unusual to see them out flow some of the older heads as cast. I tested one very early 6x-4 from a 76’ 350 that nearly made it to 220 cfm. Upper teens anyway, most of the older d ports I have tested barely made 200-210.

[steve25]

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.

[drewm]

Originally Posted by steve25

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.

[NeighborsComplaint]

Since it's October I'd wait for a Black Friday deal on the Speedmaster heads and go with the appropriate piston dish. This assumes they discount them. You'll put that much $ in the 6X heads anyway

[Jay S]

Originally Posted by pastry_chef

I'm impressed with a person running 11 to 1 compression with cast iron heads on pump gas with a mild cam. I've seen it claimed but how legit?

LOl, I am on a 8 + hr work road trip from the WI/IL border to AL, so being a bored pickup passenger, I will take a swing at that…..

Some could be legit, it is easy to miss estimate the compression though when the engine is near to slightly over 11:1. A few cc’s swings the compression quite a bit, if they didn’t check all the volumes it is a good bet the SCR was over estimated.

We have build some dual fuel propane/pump gas engines with high compression up around 11:1. Octane for propane is well over 100, so we raised the compression to utilize the octane more. They were a little picky on pump gas but not as much as we expected. Never did it with a Pontiac, but I think the size of the cam matters a ton. In the 11:1 small cam scenario I think it is opposite of what people talk about with “bleeding compression”. A very very small cam the engine can’t breath well enough to pump as much compression as one might expect. It may pump a bunch of compression on a compression check but doesn’t use it efficiently at all. If you put really big flowing heads for the cid, with a big CSA and combined it with a small cam I think the same thing would start to show. The octane requirement would raise, but not as much as one would expect.

I have seen that same thing play out on some of the old 60s era high compression 2 bbl engines,. An Pontiac example would a factory 389 2bbl 290 HP. The one we had would run better on less octane than it’s higher HP version but had the same compression.. More cam (still mild) and it made it worse. What you don’t want in a streetcar is so much compression that you run out of head flow for the cams power band trying to get the compression under control.