Originally written by Rocky Rotella for 2014 issue of Car Craft magazine. The article can seen at at Edelbrock CNC Round Port.
Edelbrock is keeping the Pontiac performance flame glowing. In addition to the wide array of intake manifolds developed over the years, the company was the first to mass-produce a high-flow, cast-aluminum cylinder head in the mid-1990s, and it has arguably been the most popular aftermarket casting in its time. Edelbrock has since introduced a cast-aluminum Performer D-port in 2011, and in 2014 it revised its original round-port casting to create the Performer RPM CNC. Follow along as we take a closer look at the company’s latest release and learn more about its development from company insiders.
Pontiac Basics
Pontiac purists have coveted the division’s round-port cylinder heads for years. Introduced on the 1968-1/2 Ram Air II, the round-shaped exhaust outlets were intended as a convenience for racers looking to fit tubular headers. According to the late Malcolm McKellar—famed Pontiac engineer renowned for taking the division’s V8 performance to the next level during the late 1950s and early 1960s—internal testing revealed that the round exhaust outlet didn’t actually improve airflow when compared to the typical oval-shaped outlets used on all other Pontiac V-8s up to that point. Instead, the internal-port changes accompanying the new shape were responsible for the peak exhaust flow boost of some 20 cfm at 28 inches of pressure.
The round-port cylinder head was further revised for the 1969 Ram Air IV, where its intake ports were enlarged by 10 percent, increasing peak intake airflow capacity at 28 inches from approximately 210 cfm to nearly 235. The Ram Air IV casting was carried over with minimal changes for 1970. The high-winding, 400-inch mill, complete with a 0.520-inch-lift camshaft and a compression ratio pushing 10.25:1, proved a formidable contender on the street and dragstrip in the limited number of GTOs and Firebirds in which it was installed in those years.
When General Motors mandated a maximum compression ratio of 8.5:1 for its division’s engines in 1971, Pontiac offset the pending performance loss by increasing displacement, and the Ram Air IV evolved into the stump-pulling 455 H.O. The 1971–1972 455 H.O. utilized a modified version of the R/A IV cylinder head with similar intake and exhaust airflow capacity, but its combustion chamber volume increased from 72cc to 111 to accommodate the 455’s now-advertised 8.4:1 compression ratio. When the Super Duty 455 debuted in midyear 1973, its intake and exhaust ports were reshaped to maximize airflow, but the casting retained the same round-port exhaust configuration as well as 111cc combustion chambers.
Where Pontiac performance enthusiasts were otherwise forced to hunt down and modify rare round-port cylinder heads, Edelbrock changed the industry in 1995 with the introduction of its cast-aluminum Performer RPM. Featuring a round-port exhaust configuration and an as-cast intake port capacity of approximately 280 cfm, it continues to sell very well. A new version of the same casting is available with CNC-machined, heart-shaped combustion chambers.
In a time before aftermarket cylinder heads were commonly available, Pontiac performance enthusiasts had no other choice but to seek the original round-port cast-iron offerings for obvious reasons. The desire for a compression ratio of more than 10:1 revealed 455-spec unit’s obvious shortcomings. The 111cc combustion chamber volume was simply too large to attain an appreciable compression ratio on any 400 or 455 without excessive milling and/or the use of domed pistons. The 72cc chambers made the R/A IV head the preferred choice for racers, and once the restoration crowd begun seeking them for their projects, the value of a virgin set shot skyward.
Enter Edelbrock
Edelbrock Corporation in Torrance, California, has been producing high-performance intake manifolds for Pontiac V8s since the 1950s. It had been producing aftermarket cast-aluminum cylinder heads for several years, too, but not where Pontiacs were concerned. “During the mid-1990s, we recognized a large demand for a high-flow Pontiac cylinder head,” says Smitty Smith, Edelbrock’s technical sales coordinator. “We teamed with Ken Crocie of H-O Racing to develop a cylinder head patterned after Pontiac’s desirable R/A-IV casting and introduced our Performer RPM in 1995.”
According to vintage Edelbrock literature, the Performer RPM cylinder head for Pontiac V8 was specifically developed to satisfy the performance requirements of street enthusiasts and drag racers alike. “Maintaining the deck, intake, exhaust, and valve-cover flanges in the stock locations allowed us to offer a cylinder head that would install easily on virtually any Pontiac V8 block from 1962 forward with a bore diameter of 4.06 inches or greater,” Smith says. “The intake ports remained in the stock positions, which allowed the use of any factory or aftermarket intake manifold. In as-cast form, airflow capacity was greater than what most could achieve from any max-ported Ram Air IV casting, and it offered plenty of potential for significant flow increases for those desiring it.”
The document reveals that Edelbrock originally considered D-shaped exhaust ports simply because it recognized that configuration was most common to Pontiac V8s and it might prevent the need for customers to purchase new headers or reproduction exhaust manifolds. Because of durability concerns in the common wall of the center ports at the time, however, it ultimately decided on the round-port configuration, which isn’t susceptible to the malady, and just as Pontiac found, it made fitting headers easy. “It’s interesting to note that alloy composition and casting techniques have obviously reached a point where it’s no longer a concern, as proven with our Edelbrock’s Performer D-port,” Smith adds.
Not to be confused with the Performer D-port (shown), which Edelbrock released in 2011, the new Performer RPM CNC is a modernized version of its original round-port cylinder head design. The D-port casting has a CNC-machined chamber as well, though it’s slightly different than the Performer RPM CNC.
The production Performer RPM cylinder heads feature 2.11-inch intake and 1.66-inch exhaust valves. While original high-performance Pontiac castings were factory-equipped with 1.77-inch exhaust valves, Edelbrock found that with a properly designed port, it could achieve more exhaust airflow than a factory casting despite the smaller valve, and it could improve scavenging and combat reversion in certain applications as well. “The exhaust valve is moved 0.060 inch farther away from the intake valve, which resides in the stock position. The spread allows for the installation of oversized 2.19/1.77-inch valves if airflow demands require,” Smith says.
Intake-port volume measures 215 cc, while exhaust-port volume measures 175 cc. According to Edelbrock’s published information, peak airflow at 0.600-inch lift and at 28 inches of pressure measures approximately 286 and 200 cfm, intake and exhaust, respectively. Having the chance to flow a number of Performer RPM castings our own flow bench, we can verify that Edelbrock’s claim is well within reason.
Pontiac hobbyists quickly found that the Performer RPM was capable of producing 500 hp in as-cast form on a relatively mild 455. Professional builders learned that intake airflow of more than 300 cfm was easily attainable with mild porting, and up to 330 cfm was possible with moderate effort. For maximum performance applications, the Performer RPM is versatile enough that it can provide as much as 370 cfm when widening the ports enough that pushrod relocation is required.
When Edelbrock released its Performer RPM in 1995, the combustion chamber resembled the scallop-shaped, wedge-type chamber that Pontiac utilized. Unlike the stock units, Edelbrock biased the plug toward the exhaust valve, which not only allowed for exhaust port enhancements, it also centralized its position in the chamber for improved combustion efficiency. Edelbrock still offers this cylinder head in 72- and 87cc variants.
The Performer RPM has been an excellent seller for Edelbrock. Since its introduction, the only major change was to those produced after March 15, 2002. “We were using stock-length bolts to fasten the cylinder head to the block in most locations,” says Rick Roberts, Edelbrock’s director of engineering. “We found one aluminum casting boss was located too closely to a water jacket and it wasn’t taking head-bolt pressure well in certain applications. So we revised the boss to make it thicker, successfully increasing its integrity and durability. A new head-bolt kit is required for castings produced after that date, but beyond that, we haven’t needed to make any other changes. It continues to sell very well for us.”
Chamber Chatter
In today’s new-car market, cylinder heads with combustion chambers containing a heart-shaped appearance are commonplace. “The chamber shape tends to improve combustion efficiency,” Roberts explains. “Any wedge chamber, heart-shaped or scalloped, offers a natural motion called axial rotation (or swirl) that sweeps the air/fuel mixture toward the exhaust valve side of the chamber. By creating a chamber shape and locating the spark plug properly to accentuate combustion with axial rotation in mind, we can greatly enhance the combustion process.”
Improved combustion efficiency creates quicker pressure rise within the cylinder. “The faster cylinder pressure rises, the less spark lead that’s required for complete combustion. Past experience with it tells us that 4 to 6 degrees less is common, but there are just too many variables to make a blanket statement. The faster rise also translates into improved piston push, which tends to result in greater torque output. While mixture stoichiometry generally remains the same, a heart-shaped chamber tends to improve the air/fuel-ratio curve because the burn is more complete. So it’s quite possible to run the mixture slightly leaner when compared to a conventional wedge-shaped chamber,” Roberts says.
We knew Edelbrock had been considering modifying its Pontiac Performer RPM heads for several years, but it wasn’t until we spied this prototype in the company’s booth at SEMA in November 2012 that we knew it was a reality. The next several months until production were spent tweaking the design to optimize performance.
The double-quench design typically associated with a heart-shaped chamber tends to maximize combustion by initiating spark where the mixture is densest, allowing the flame to consume as much as fuel as possible in the shortest amount of time. Where production vehicles are concerned, the improved efficiency can also lead to reduced tailpipe emissions, but it’s the other distinct operational advantages—as well as the marketing ability—that pushed many companies into introducing aftermarket cylinder heads with a specially designed chamber of some sort during the mid- to late-1990s. Edelbrock’s first foray into a designed combustion chamber was in 1996 with the small-block Ford offering. The design was so well received that Edelbrock begun updating many popular makes over the next several years.
Shortly after introducing its first designed combustion chamber, Edelbrock found that it could improve upon its design by automating the process using CNC equipment. Roberts explains, “Sand casting allows for some degree of variation and core shift from run to run. The precision machining associated with the CNC process tends to improve flow consistency from port to port on a given cylinder head, as well as from production run to production run. A cast chamber also has some limitations where it interfaces with the valve seat. Our CNC chamber has a machined transition from the valve seat into the chamber, and that tends to improve airflow by a few cfm throughout the entire flow range. We’d also found that in instances where flow can ‘back up’ at high lift on a cast chamber, it is sometimes eliminated with a CNC-machined chamber.”
Much development work went into perfecting the chamber’s shape. Because of CNC machining, consumers are rewarded with a chamber that’s virtually identical from cylinder to cylinder. How is Edelbrock able to ensure that? This digitized program is the answer. (Photo Courtesy of Edelbrock)
Modernizing a Standard
Though its Performer RPM for Pontiac V8 was selling quite well, the company reached the point where it began considering updating its chamber for a combination of reasons. “We released our Performer D-port in 2011 and it featured a modern heart-shaped combustion chamber,” Roberts explains. “The new cylinder head has been quite successful for us and almost immediately consumers begun asking if we could update the round-port head similarly. We recognized the opportunity where we could modernize a cylinder head that historically sold very well for us, and based on similar changes we made to other makes, we knew it would be an upgrade.”
Edelbrock’s new Performer RPM CNC for Pontiac V8 features a fully machined combustion chamber with a pronounced heart shape. It’s easy to see how this chamber differs from the scalloped-shaped chamber of the Performer RPM. Notice that the spark plug has been relocated to further enhance combustion efficiency with a chamber of this shape, which can further lessen total spark lead requirements. Edelbrock offers the CNC-machined chamber in 72- or 87cc volumes.
The Edelbrock team went to work modernizing its Performer RPM. The plan was to make no other changes to the casting beyond the addition of a CNC-machined, heart-shaped chamber. The overall shape of Edelbrock’s new CNC-machined chamber is a proprietary design that was developed internally and continually improved over time. “The basic shape of the Pontiac chamber was inspired by the success we’ve found with other wedge-shaped, angle-plug cylinder heads in our lineup. We then varied it to make it compatible with the typical bore sizes of the Pontiac V8, intake and exhaust valve sizes and locations, and placement of the spark plug. Altering the water jacket to allow for optimal placement of the spark plug was the only modification needed to accommodate the revised chamber.” Roberts reports that it has no effect on cooling.
The fully CNC-machined combustion chamber design essentially eliminates the transition from the chamber to the valve seat, which can not only show a minor flow improvement when compared to a scallop-shaped castings, it tends to better control fuel through the transition as well.
Like the original Performer RPM casting with its scalloped chamber, the Performer RPM CNC is available in two volumes: 72 and 87 cc. While the same basic casting is used for both applications, the larger combustion chamber is achieved by machining it some 0.120-inch deeper into the deck surface, and that places the intake and exhaust valve seats higher in relation to the port floor, which subsequently negatively affects airflow by some 10 to 15 cfm each. “We considered offering additional volumes, but we’ve found that 72- and 87cc chambers are those that most hobbyists are familiar with.”
Edelbrock quickly dispels any notion that the original Performer RPM cylinder head, with its scalloped chamber will be discontinued. “It remains a very good seller for us, and we expect good sales in the future,” Smith says. “Our customers have been using it for more than 20 years and are very familiar with its features and capabilities. Performer RPM castings are available with valvesprings compatible with hydraulic flat-tappet or hydraulic roller camshafts up to 0.575-inch lift. The 72cc castings retail for approximately $1,300, while the 87cc castings sell for about $1,200. And because the 87cc castings are equipped with an exhaust crossover, they are CARB approved.”
When asked how Edelbrock plans to market the CNC-variants of its Pontiac V8 cylinder heads, Smith replies, “There are plenty of hobbyists using our original Performer RPM casting, and they’re satisfied with the quality and performance. While some existing users may upgrade to the new Performer RPM CNC casting for its performance advantages, we really hope to bring in new customers like the Performer D-port did for us, particularly those who are newcomers to the Pontiac hobby and looking for the most modern casting available. We really look for it to help grow the sales of the entire Pontiac engine family.”
The 87cc versions of Edelbrock’s Performer RPM or Performer RPM CNC contain an exhaust crossover, which makes them a smog-legal replacement in all 50 states.
Should you find that professional Pontiac engine builders have limited experience with Edelbrock’s new Pontiac V8 cylinder head, we can report there’s a valid reason. Much of a professional builder’s business is tailoring an engine package for his/her customers’ needs, and that often includes specific cylinder head preparation. Since Edelbrock’s Performer RPM CNC line boasts fully assembled cylinder heads that are ready-to-run, and it doesn’t yet offer a semi-machined (or bare) casting for Pontiac V8, it means builders must start with a fully assembled product and may not be able to reuse some of the existing accessories Edelbrock provides, making it a cost-prohibitive venture for the budget-minded consumer. Edelbrock has recognized this and is actively considering offering its Performer RPM CNC for Pontiac V8 in a semi-machined state and welcomes your comments if it interests you.
When a product is as successful as the Performer RPM cylinder head, altering its current form certainly seems risky, but Edelbrock found from its past experience with other makes in which a heart-shaped chamber was added, new sales resulted. The company fully expects the Pontiac hobby to follow suit, and for those who want the latest technology for their old Indian, the Performer RPM CNC delivers modernity.
We took the opportunity to flow the new Performer RPM CNC on our Superflow SF-110 flow bench. All measurements were taken between 10 and 15 inches pressure and mathematically converted to 28 inches. Our testing reveals that the 87cc casting contains a peak intake flow capacity of 277 cfm and 197 cfm exhaust.
Airflow at 28 inches of Pressure
Valve Lift Int./Exh
0.050” 36/27 cfm
0.100” 66/56 cfm
0.200” 147/115 cfm
0.300” 206/156 cfm
0.400” 235/178 cfm
0.500” 259/186 cfm
0.600” 277/197 cfm
Performer RPM CNC Part Numbers
61515 72 cc with hydraulic roller valvesprings
61525 87 cc with hydraulic roller valvesprings
61519 72 cc with hydraulic flat-tappet valvesprings
61529 87 cc with hydraulic flat-tappet valvesprings