Smog Head Performance

The cross-sectional area of an intake port (indicated by volume) can play a critical role in the performance of a given cylinder head. Though it might be assumed larger ports always flow better, that is not always the case. In fact, larger ports could reduce the incoming intake charge velocity--degrading cylinder filling, actually resulting in less overall performance. But our experience with stock ports has shown that those with similar cross-sectional area typically flow about the same and produce similar amounts of power. And, after finding the average intake port volume of our five test heads was approximately 153 cc each, we began to wonder if we might see any significant differences in the airflow capacity of each casting in stock form.

To accurately measure airflow and achieve professional results, each head was tested on a SuperFlow 110 flow bench. To reduce the chance of error, the same testing procedures were repeated each time. Every time a new head was placed on the flow bench, it was thoroughly checked for leaks prior to any testing. A radiused inlet was used to smooth incoming air when testing flow on an intake port, while a modified four-tube header flange with 2-inch-long extensions was used when testing exhaust. A valve-opening fixture was used to open the respective valve to a specific lift point, and an airflow reading was taken. The results were recorded at either 10 or 15 inches of pressure (depending on the lift point) and then mathematically converted to 28 inches before being averaged to produce a single average airflow amount. The airflow results are listed in the chart below.

What Did We Learn?
After reviewing the results, we were confident no single casting from the mid-to-late '70s would significantly outperform another. Despite our initial thoughts that the 6Xs would flow marginally better, once it was determined intake port volume was the same as the others, we were not surprised to find similar airflow. Combining these results with those recorded from various projects on the same bench using the same testing procedures, we found a consistent trend. Most D-port heads with 2.11-inch intake valves typically have an intake port volume near 153cc, with peak airflow around 210 cfm at 0.500-inch lift, while heads with 1.96-inch intake valves flow closer to 190 cfm. We also found stock heads show no significant flow increase with valve lift beyond about 0.500 inch lift. And, even if our flow numbers are slightly different from those listed elsewhere, any flow differences we would have measured between castings should be accurate.

Though flowing relatively well, we immediately noticed the large-chambered 6H castings commonly found on the '76 455ci heads appeared to be down on flow at higher lift points when compared to the other heads. A visual inspection revealed that because of the large chambers, the location of the valve seat in relation to the port floor is higher than on heads with less combustion-chamber volume, and it appears short turn flow is negatively affected. With their enormous 124cc combustion chambers, however, it is unlikely the No. 51 and 6H heads would be the first choice for performance applications. But, from our testing, we know that if forced to use them because of class rules or originality, compression notwithstanding, they should not grossly affect vehicle performance from lack of airflow alone.

Two major points to remember about our results are that our heads were bone stock and that any type of modification could affect the outcome. A common practice to increase compression is to remove material from the deck surface of the head, which reduces combustion-chamber volume. Many, however, overlook the fact that any type of valve seat or valve face cutting actually increases chamber-volume lowering compression. Another variable that can affect intake-port and combustion-chamber volume is the addition of aftermarket valves with tapered stems or tulip-shaped heads. And the most unconscious way of increasing port volume (but not necessarily flow) is by grinding on the port with the intention of increasing airflow.

Conclusion
The purpose of our test was to distinguish which unmodified smog-era cylinder heads offer the greatest performance potential in stock form. But, after spending two full days comparing them, we now realize just how similar they are. Though it is not uncommon to find the same cylinder heads we tested measuring more or less in one or several areas, our results appear to be consistent with factory information and results from other knowledgeable figures within the hobby. And knowing any type of modification can affect the results, if there is any question regarding the originality of your heads, we highly suggest you perform your own measurements and compare your results to ours.

After completing the comparison, it may appear combustion-chamber volume is the only major variable between each of the castings tested. We now know heads once considered "smog junk" by many are actually a suitable performance alternative to the high-dollar early castings, which may broaden your selection in the future. But, as the supply of unmodified later castings decreases in future years, knowing similar performance can be achieved from a wide variety of cylinder heads could save you a few bucks on your project. And anytime we can save money on one modification, it only means there will be more available for the next!

*Special thanks to Jim Hand for his assistance with this article.