Rat's Head SoupMike Myhrvold, Virginia beach, VA: I have been a reader of your magazine for more than 25 years. As magazines come and go, Car Craft has remained the best, and I should know, I have subscribed to all of them. Now that the niceties are completed, I have a request.
I have a set of big-block Chevrolet cylinder heads casting number 3993820 on which I have been trying to get specifics, such as port volume, for some time. I was wondering if you could add them to your cylinder-head database for testing or just throw me a bone. I know I should have measured them myself when I was building the engine, but they were assembled, and the short-block was calling my name. They came on a '72 402 engine (that's now a 414ci) and have 119cc combustion chambers. The engine is installed in a '70 Chevelle. Looks damned good if I say so myself. I have installed 2.19/1.88 valves in them, but they originally came with small valves. No one else can find the info.
Thanks in advance. Don't worry, I will still subscribe even if I don't hear back from you.
Jeff Smith: We don't have access to a set of those heads, Mike, so we can't measure the port volume, but we can come pretty close. These heads were used in '71 402 and 454 big-blocks, and as you mentioned, are oval-port castings. Stock valve size for these heads is 2.06/ 1.72, so you were wise to open them up to the larger 2.19/1.88-inch diameters. According to Alan L. Colvin's book Chevrolet by the Numbers: The Essential Chevrolet Parts Reference 1970-1975, this head had redesigned intake and exhaust valve seats, and the spark-plug seat was changed to a tapered design as opposed to the older gasket-sealing style. According to Colvin, who had unprecedented access to the GM archives for his information, the combustion-chamber size was 113 cc-not 119 cc as you said. That will be worth some additional compression. But if you measured the chambers at 119 cc, then believe that number, since production variations were common.
As for port volume, we can only go by similar heads, since Chevrolet never published port volumes for any of its heads. This is a marketing-generated number created to quickly identify different aftermarket heads. The problem with port volumes is they don't tell you the whole story about an intake port. This is especially true with Rat motors because there are two good ports and two bad ports in each head. The good ports aim the intake charge toward the center of the cylinder, while the bad ports shoot the intake port more toward the cylinder wall.
We looked at several heads in our cylinder-head database, and the closest is the 049 head that also is an open-chamber, oval-port head. This head measured 253 cc and also came with the same smaller valve sizes. Our estimate would be that your heads will measure very close to this same volume.
Hopefully, you opened up the throat area underneath the valve seat when you added the larger valves to these heads. Generally, the throat area underneath the intake will be roughly 75 percent of the diameter of the valve-so a 2.06 valve would have a throat diameter of around 1.55 inches. When the machine shop increased the valve diameter to the larger 2.19-inch intake, the throat area became restrictive at roughly 70 percent of the diameter of the valve. Most cylinder-head porters we talk to agree that 85 to 90 percent throat diameter compared with the valve diameter is a good number, and under no circumstances should the number exceed 91 percent. Using a conservative 89 percent, that means the throat diameter underneath those new 2.19-inch intake valves should be 1.949 inches-a solid 0.040 inch larger. I realize the heads are already on the engine, but the next time you have an opportunity to increase valve size, take a look at the throat diameter as well. It might really be worth increased flow that will pay off with more horsepower. For information on how to do pocket porting correctly, check out Car Craft's past porting how-to stories, like the Vortec porting story on the Car Craft Web site under Technical Articles.