Smog Head Performance

It seems that Pontiac cylinder heads cast between 1967 and 1972 are typically the most sought after for restorations or performance.

As the supply of unmodified castings evaporates and the prices skyrocket, many hobbyists are forced to use cylinder heads from the mid-to-late '70s. While these heads can still be found in junkyards today, they too are becoming scarce. It's only a matter of time before they disappear altogether.

With such a wide variety of castings produced during that period--each with a specific application--it can be difficult to determine which may work in your combination. And, if you're like us, you may wonder which "smog-era" heads offer the most potential. Follow along as we explore original applications and compare several cylinder heads from the '70s. You might discover an affordable alternative without sacrificing performance.

HISTORY
Though performance took a back seat to emissions for much of the '70s, Pontiac was still producing powerful combinations such as the Super Duty 455 and the W72 400. Beginning in 1973, the 1.66-inch exhaust valves commonly associated with low-performance engines in earlier years were used in every application but the SD-455. The procedure for casting cylinder heads changed as well. The new controlled casting process tightened the casting tolerances, producing a more consistent product. And, according to former Pontiac engineer Skip McCully, though initial cost was higher, overall quality significantly increased, resulting in fewer errors and making controlled casting more cost effective in the long run.

While several different castings were produced between 1973 and the end of production in 1981, this article's primary focus is performance potential in stock form, so our discussion is limited to those heads with 2.11/1.66-inch valves and no A.I.R. As noted in the December '03 article "Heading for Controversy," Pontiac engineers eliminated pressed-in rocker studs on all engines assembled in early May 1973. Prior to that point, only those applications with the potential of running at higher engine speeds received threaded rocker studs. But, from that point on, the pressed studs were eliminated completely. In an attempt to streamline production and reduce overall material costs, threaded rocker studs were used on every head regardless of application.

As was also mentioned in that story, beginning in 1973, each cylinder head (except the 265/301ci) was marked with a secondary application stamp denoting its original application on the vertical accessory boss just left of the center exhaust ports. This stamp, many times followed by an "H" in early years, signified such variables as intake valve diameter and combustion chamber volume and was a way for workers at the engine plant and service technicians in the field to identify the heads. It took researchers nearly 30 years to accurately decode all these stampings. Because of their diligent work, today there is rarely any question about the identity and approxi- mate chamber volume of any head cast from 1973 and on. Based on factory literature, the chart below shows the original application of each casting, along with its respective application stamping.

Wanting to find which of these heads offer the mostperformance potential, we gathered several different castings for an in-depth comparison. Since the same basic casting was used in a variety of applications and it would not be practical to The Comparison
obtain one of each, we narrowed down our selection to those we felt would most commonly be used in performance applications today, giving us an idea of how they might measure up. While other heads from the same era may offer satisfactory performance in any given combination, we limited our testing to 4X-7H, 5C-8, 6X-4, 6X-8, and 6H-6 castings.

Each of our examples came from friends or local junkyards, where the heads were removed from their original application. Once disassembled and cleaned, the heads were visually inspected to ensure they were original and unmodified and the better of the pair used for our test. Any carbon deposits accumulated on the stems of the original valves were removed with a wire wheel and checked for any signs of nonoriginal cutting. The valves were then reinstalled into their original head. To perform the most complete comparison possible, we measured several areas that could potentially affect performance when installed on an engine. We not only measured the combustion chamber and intake port volumes, but we also measured airflow capacity on a flow bench.

Measuring the combustion chamber and intakeport volume was relatively simple. A flat plastic sheet sealed with white lithium grease covered the test area, while two graduated measuring syringes of different volumes were used to measure and inject liquid into the respective port or chamber. To ensure comparative accuracy, we used the same AC Delco R45TS spark plug while measuring each combustion chamber, making sure it was fully seated each time to reduce any variance from improper plug depth. The measurements recorded for each respective chamber and intake port were averaged to produce single volume amounts. The average combustion-chamber volume varied greatly, but our results, which are shown below, were close to the specs shown in Pontiac engine application charts and MVMA specification sheets.