Inliners International Forums General Discussion Hi Performance Flow bench day for the 848 head

does a 302 GMC have more mains?...

No, but you can get a better cylinder head with a stock 983 GMC casting than with a Stovebolt.

And a lot more cubic inches. Like 320 with an eighth overbore, more if you stroke it.

and you can get a 12 port head for a GMC, but I wonder how much?...

Dear Dawg;

A lot; probably in the 4-$5,000
range (at least) all is different.

The same basic design; only biger/
stonger/longer/heavier.

Check with Arias, they should be
listed here.

Good luck.

Meanwhile, back to the original subject...
My 848 head's intake ports have been opened up and ported just as it was detailed in the hop up article written by Frank McGurk and published in the May, 1955 issue of "Hot Rod" magazine.
This head was ported in the early 60's and has not been used since the previous owner switched to the later engines when thay became available in 1963.
I purchased the head for my 261, and had it 'freshened' up, (cleaned, new large powerglide valves, springs, etc.) by a very reputable speed shop in my area.
While he was at it, I had three cylinders flowed on his 'SuperFlow' flow bench when he was done. I thought you might be interested in the results.
As freshened, the intakes flowed 200 cfm at .500 valve lift at 28" depression. At that point, the flow 'stalled', indicating port saturation. In other words, increasing the valve lift had no further affect.
Surprisingly, the exhaust was far more restrictive than the intake. As originally tested, the three exhaust ports chosen 'stalled' at 100 cfm, again at .500 valve lift. The graph generated showed a very large dip in the flow at .300 valve lift. All ports tested were very consistant with these results!
Looking closely at the exhaust valve and the combustion chamber, it was noticed that the combustion chamber wall bulges in towards the edge of the valve, before receding away as the valve opened further! The chamber wall shrouds the exhaust valve at exactly .300 lift! This was clearly indicated by the exhaust flow graph.
The speedshop machinist piloted a small boring bar in the exhaust valve guide, and opened up the chamber wall to unshroud the valve, and the ports were retested. Now, the exhaust flow increased to 120 cfm, again at .500 lift, and the large dip in the flow was gone!
The results were 200 cfm intake, 120 cfm exhaust, all at 28" depression, .500 lift, and a smooth flow curve.
I hope that some of you may find this interesting. Toolman

Those numbers are far better than I would have predicted, thank you.
The valve head approaches the relief due to the 16° stem angle, and gets pretty close before the wall flares out again. Is this where you mean?

Those figures also explain why most of the successful hot cams have more exhaust duration (120 ÷ 200 = 60%, far worse than the 75-85% we'd like).
If it can't dump the gas fast enough, leave it open longer.

Panic,(and others),
Yes, that is exactly where I mean. It appears that you can mill that bulge out if you don't go too far. I don't know how far the speed shop went, though. It does look as though you could make the chamber wall the same thickness as the thinest portion, paralleling the water jacket's inner wall. As I recall, that's what my head looked as though he had done.
The numbers were higher than I thought they would be, also. He, (the speedshop operator/machinest) said that the flow would support 300 horsepower, if, one could get the engine to rev that high.
He also agreed that the flow could be better if there was a more gradual short side radius leading into the valve seats. My ports are virtually flat, with a very small radius. Lump ports, anyone?
In any case, it's academic until I am able to solve my rocker shaft alignment problem. Toolman

what a horrible port shape to start with, the roof could use alot of work around the guide boss area, but I'm with the machinist, you almost need to add material on the floor to kinda straighten things out...

Just a guess: 300 hp is extrapolating from the common "2 HP per CFM of intake flow" used for a V8 (and suggested by some flow bench manufacturers), which gives .25 HP per CFM per cylinder or 1.50 HP per CFM for an L6, and in this case 300.
I think it's a bit optimistic since (as said) the RPM is limited by both the stroke length and the inherent design (the port flow rate has been established, but sonic wave tuning isn't practical for siamese ports), but especially by the difficulty in increasing compression using only conventional techniques and commercially available parts.
I would use 1.4, or even slightly less, as a prediction depending on how well developed the engine is. The carburetor area in particular must be fairly large, pulling perhaps as little as 1.0" Hg. A 235 @ 6,000 RPM could use 500 CFM.

Panic,
Of course, you're right. However, the statement was that the flow would support 300 horsepower. The engine, being rpm limited, would not produce that horsepower. My guess is that a 235, or, in my case, a 272 cubic inch 261, should produce about 230 to 245 hp. This is just a WAG. Toolman