The SD-455 castings likely contain the greatest port efficiency of any as-cast factory cylinder head Pontiac ever produced. Flowing around 240 cfm, they remain highly coveted by performance and restoration aficionados, but their massive 111cc combustion chambers limit their use on moderate displacement mills with flat top pistons. This particular SD-455 combustion chamber was modified by a previous owner, and no longer contains the small ridge that otherwise bisects the spark plug hole.
Factory Round-Ports
Pontiac engineers developed a series of cylinder heads aimed at increasing the high-rpm potential of the division's top performance offerings during the late '60s and early '70s. The castings share one common characteristic-round exhaust outlets. With their well-designed port shapes and excellent airflow characteristics, these castings carried the performance torch when there were few options, and they became the basis for many of today's offerings.
The first Pontiac engine to receive the new "round-port" cylinder head was the R/A-II, which replaced the original R/A 400ci containing traditional D-port castings in May 1968. Like their D-port brethren, the R/A-II cylinder heads (casting No. 96) contained 2.11/1.77-inch intake and exhaust valves, an intake port volume around 153 ccs, with airflow peaking around 210 cfm at 28 inches of pressure. Exhaust flow peaked nearly 20 cfm higher than a D-port, however.
Retired Pontiac engineer, Malcolm "Mac" McKellar, tells HPP that, contrary to popular belief, the round-shaped exhaust outlet wasn't intended to increase airflow, but was instead a convenience for racers. "The D-shaped center exhaust ports made fitting headers difficult, and since serious racers typically replaced the cast-iron exhaust manifolds with tubular headers, we changed the outlet shape to make it easier for them. We revised the exhaust port internally at the same time, however, and that's where the airflow increase came from."
The R/A-IV arrived in 1969, and its round-port cylinder heads featured larger intake ports to improve performance. McKellar says that, when designing the heads, focus wasn't on flow numbers, but more so, a combination of port volume and velocity. "I can't recall us measuring actual airflow often. We simply knew that an engine needed a specific volume of air to run at a certain rpm. Our goal for the R/A-IV was 6,000 rpm, so we increased intake port cross-sectional area to a size we felt would allow the 400ci to accomplish that."
Intake port volume of a '69 R/A-IV cylinder head (No. 722) measures around 180 cc, or nearly 30 cc more than that of a typical D-port casting, and as a result, measured peak airflow increased from roughly 210 cfm to 235. The R/A-IV continued as Pontiac's top offering into 1970, and the cylinder head's intake ports were further revised in a successful attempt at maximizing port efficiency. Numbered 614, port volume of the 1970 casting was similar, but airflow peaked near 240 CFM.
R/A-II and R/A-IV castings were utilized on 400ci engines only, and as such, contained combustion chamber volumes somewhere near 72cc to achieve static compression ratios in excess of 10.0:1. When General Motors mandated that compression not exceed 8.5:1 on any of its divisions' engines in 1971, Pontiac combated the resultant horsepower loss with added displacement, and thus the 455 H.O. was born.
Pontiac's maximum performance engines in the late '60s and early '70s included cylinder heads with round exhaust port outlets, and the castings were once highly sought after by performance enthusiasts due to the lack of anything comparable from the aftermarket. Fortunately, a number of options exist today, leaving castings like this No. 197 (1971 455 H.O.) for restoration applications.
"The 455 could make similar power without revving as high as a 400, so we kept the R/A-IV heads, but with a few changes," says McKellar. The '71 455 H.O. castings (No. 197) were extern-ally identical to No. 614, but the massive combustion chambers pushed the intake port floors upward, which not only reduced port volume to around 170 cc, it also reduced peak airflow to around 230 cfm. The intake ports were mildly revised in an attempt to improve low-lift flow for the 455 H.O. in 1972, and the casting was renumbered 7F6. While intake port volume remained around 170 cc, high-lift airflow further degraded and peaked around 225 CFM.
An engine that many consider Pontiac's greatest effort was introduced at a time when other manufacturers were shifting from performance to emissions. The Super Duty 455 was released for production in midyear 1973 as a complete engine package that boasted maximum reliability and 6,000-rpm capability. It contained a host of exotic components to accomplish that.
The SD-455's cylinder heads (No. 16) were similar to past round-port castings, but the intake and exhaust ports were redesigned to maximize port efficiency. To achieve 6,000 rpm with the larger mill, engineers increased intake port volume to roughly 186 cc, and airflow peaked at or slightly above 240 cfm. These castings are generally regarded as the ultimate performance cylinder head ever produced by Pontiac. Today, a usable pair can draw several thousands of dollars.
Edelbrock was the first aftermarket company to produce a high-flow Pontiac cylinder head on a grand scale. Its bolt-on offerings contain 2.11/1.66-inch valves, single valve springs rated to 0.575-inch valve-lift with a typical flat-tappet cam, new pushrod guide plates, and ARP rocker studs. Pricing starts around $1,800 per pair.
Edelbrock Aluminum
Edelbrock is a name most hobbyists correlate with a longstanding history of producing high-performance engine components. Though cylinder heads were among the first products the fledgling company offered decades ago, it wasn't until the mid-'90s that Edelbrock released its first casting for the Pontiac V-8.
"We found a large demand for a high-flow Pontiac cylinder head at the time," says Smitty Smith, Technical Sales Coordinator of Edelbrock Corporation in Torrance, California. "We teamed with Ken Crocie of H-O Racing to develop ours, and patterned it after Pontiac's R/A-IV casting. We specifically retained the round-port configuration because of the wide selection of round-port headers on the market then."
The combined effort produced an aluminum casting with 2.11/1.66-inch valves, an intake port volume of 215 cc, and two distinct combustion chamber volumes-one displacing 72 cc, and the other, 87 cc. According to Edelbrock sales literature, peak airflow in as-cast form exceeds 280 and 270 cfm at 28 inches, respectively-the larger combustion chamber requires that the valve seat be placed higher in relation to the port floor, which subsequently affects airflow.
Edelbrock offers two different combustion chamber volumes-72 cc (shown) and 87 cc. The 72 cc casting (No. 60599) contains no exhaust crossover, while the 87 cc offering (No. 60579) has provisions for crossover heating.
"We offer a semi-machined version of the 72 cc casting, which allows builders to finish them as needed, and either casting is available fully machined, but bare. Our Nos. 60599 (72 cc) and 60579 (87 cc) are ready to bolt on for around $1,800 per pair," says Smith. "We initially thought of the basic casting as a high-performance replacement for musclecar-type applications, but we've seen it used in every type of Pontiac application imaginable, and that's something we never expected."
When asked if there were any foreseeable changes for the casting in the future, Smith replied, "We haven't had to make any changes yet. It flows well and performs to our expectations, and it continues to sell well too. In fact, our records show continued sales growth even after all this time. So when we consider all that, there doesn't seem to be much of a need to at the present time."