GM's RSX ZZ632 Big

Nov 27, 2023

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It's the natural order for things to progress over time, but often you need to look to the past in order to build the future. The RSX heads on the new 1,004-hp ZZ632 big-block Chevy crate engine are a perfect example of this. To get the lowdown on this high-flowing head we paid a visit to GM's Performance and Racing Center in Pontiac, Michigan, and met with the guys who made this new big-inch, big-power, pump-gas crate engine possible, including Ron Sperry, the guy the RSX head is named after.

Sperry is a legend around GM and joined the company in 1968 after graduating from the General Motors Institute. His time at GM has mostly revolved around cylinder-head design and in his years at GM he's bounced between some pretty amazing projects, including NHRA Pro Stock, NASCAR, and programs with legendary names like Smokey Yunick, Bill "Grumpy" Jenkins, Junior Johnson, and an Indy 500 program with A.J. Foyt—so you can see he's been there, done that, and more than likely has the T-shirt.

In the 1980s, he began work with the V-8 group at GM and undertook some high-level projects such as updating the L98 aluminum heads for the new Gen II LT1 engine. But what pertains to these new RSX heads started in 1986 when Sperry had the idea for a spread-port, straight-flow head for use in Pro Stock racing. His new head was available by 1988, but by then the NHRA had modified its rules in favor of wider bore-spacing, leaving his new big-block head without a real market. The heads (PN 10051128) stayed in the Chevrolet Performance catalog for years but were mostly used in blown alcohol marine applications.

Sperry was also heavily involved in the development of the heads and intake manifold for the new Gen III LS1 engine that first showed up in the 1997 Corvette. Sperry told us that the mission was to design a simple head with the valves in line with the pushrods.

Related: Everything You Need to Know About LS, LSX, and Vortec Engines

This led to limited space for the intake ports, hence the cathedral port shape. Later the team developed an offset rocker which gave more room for the rectangular port found on the LS3. Sperry stated, "The LS production engines that I worked on took a big step forward in terms of lightweight components." This and other lessons would eventually roll into the design work on the new RSX big-block head.

Not really. Remember that Pro Stock head that Sperry developed back in the mid-'80s? Well, when Director of Performance and Racing Propulsion Russ O'Blenes, Design Release Engineer Chris Cogan, and Dom Lester needed a head to support the new 1,004-hp, tall-deck ZZ632 big-block they were working on, they enlisted the help of Sperry. He had retired from GM in 2008 but was still involved as a contract employee. The first thing Sperry thought of was using his old '80s Pro Stock head as the basis for this new head. "Since it fit on the production engine, why not do an up-level version of that cylinder head for the program O'Blenes was discussing?" recalled Sperry. He continued, "So we needed a new elevated intake flange, which was the biggest part of it, and then reintroduced a lot of the geometry that I did for the last production-block Pro Stock head. We modified valve angles to 12-degree inlet and 8.5-degree exhaust, increased valve sizes, rotated the inlet toward the flange, repositioned the spark plug to bore center, and worked on improving airflow numbers along with CNC programming."

According to Sperry, the head that really influenced the design changes to the RSX head was the Gen V LT head. In addition, the RSX heads' raised and spread intake ports are more aggressive than the production design. Sperry relayed, "The original Big-Block has a pair of what are known as a long and a short port married together with a common wall between them, approaching the cylinder at different angles and lengths, but this one has four equal-spaced ports down the length, identical in length and approach. The flange that the manifold attaches to is elevated upward to give it a more aggressive attack angle of the port. In addition, the raised flange attachments give it a little more airflow advantage." The result is that the symmetrical-port heads can support up to 7,000 rpm. To sum it up, the new RSX head is based on Sperry's mid-'80s Pro Stock head, improved with lessons learned from the modern LS and LT head programs. With the changes they were also able to design the 632 to use a dry intake manifold, with water traveling through the new valley cover and not through the intake manifold. That and the fact that the intake uses O-rings makes pulling the intake for service a snap.

This brings us to our recent behind-the-curtain visit to GM in Pontiac, Michigan, to build one of their new 632 crate engines. The 632s are built in the same place where LT6.R, GT4, Le Mans, and other race engines are assembled, and the 632s get the same hand-built attention. This also applies to the heads, so we thought we would follow Rick Swain through the process. In addition to hand-finishing and assembling two-valve performance heads he does a lot of the flow development with GM's head program and helped work on the final design of the RSX head. Also, if you're rocking a set of Chevrolet Performance CNC-ported LS or LT heads then there's a good chance Swain worked on them. Fun fact: Swain learned the head-porting game working for Roush Racing, where he tried to beat the heads designed by guys like Ron Sperry, and now he's on the other end of the field, happily working with Sperry to maximize head performance at GM.

Some companies try to be sneaky with "assembled in the USA" or "finished in the USA" which is code for "all the heavy lifting was done overseas." In the case of the RSX head, it's truly made in the good old U-S-of-A. GM starts with these raw aluminum castings from a foundry in California.

The raw castings then take a ride in this four-axis CNC machine where they are machined into the basic shape of the RSX head.

Swain explained, "After the heads are machined from the raw casting, I do a complete deburr for safe handling, followed by washing them and installing the bronze seats and guides." This whole process takes a solid two hours to knock out.

At this stage the guides and seats are in place and ready to head back over to the machine shop for CNC porting.

The guides are then hand-honed, the heads washed again, and Swain does a valve job on them before sending them off for final milling. The heads have relatively small 70cc chambers. According to Sperry this allows for high compression with a sump in the piston, which is a benefit since flatter pistons have better burn capability, improving efficiency. It also means that anyone wishing to put these heads on their own big-block will need to watch piston-to-valve clearances.

"When I get them back, I have about one hour of final prep work, which includes final port detailing and finish deburr work before final scrubbing and wash," recalled Swain. After this they are ready for assembly. So, with the guide work, valve job, and all the prep work, there will end up being many hours invested in the heads, not counting the time the heads spend in the CNC machines.

Assembly time! The raised-runner heads have been cleaned spotless and all the critical measurements have been rechecked in preparation for the valves and other parts.

A top-shelf engine deserves top-shelf components. Here, one of the massive 2.450-inch titanium (5.16-inch stem OD) intake valves is slid into place.

In an effort to cut weight wherever possible, titanium was also chosen for the 1.800-inch exhaust valves.

This is a pump-gas street engine, so seals were installed on all the valves.

GM gets all the cool tools, like this pneumatic spring-compressor. When you do the volume of heads that this shop does, every little bit helps to improve productivity.

The ZZ632 RXS heads field these stout ovate wire beehive springs that are designed to handle the 0.780 intake and 0.782 exhaust lift. The springs have a 1.589-inch diameter on the large end and have about 175 pounds of seat-pressure at a two-inch installed height with 410 pounds over the nose. The 10-degree retainers are tool-steel for durability.

And just like that, the heads are done and ready to be sent over to the engine build shop. Each head is given a unique serial number so it can be tracked if there are any issues down the line, and all of the build specifications are kept on file at the build center.

Before heading over to the engine-assembly side of the operation all the heads are given one last inspection to make sure they are good to go. The shop has a high-end flow bench and heads are randomly checked, time permitting, to make sure they are working at the goal of around 475 cfm of airflow at 0.787-inch lift and 340-350 cfm on the exhaust side. But the real story here is how this head flows in the lower lift ranges—we were told the RSX head flows an astounding 375 cfm at 0.400-inch lift! It goes to show that these heads aren't even working hard to support the 1,004 hp of the ZZ632. If you're interested in running higher lift, the head will flow up near 500 cfm when approaching 1.000-inch lift!

As you can see, the air inlet from the intake is large and has a straight shot to the intake valve. Next up we will be building a ZZ632 crate engine for these heads and running it out on GM's dyno to see what GM's biggest big-block crate motor can throw down.

On episode 24 of Engine Masters, the team explores one of the most-asked tech questions: What budget bolt-ons can be used on a stock small-block Chevy 5.7 to get more horsepower? Watch as David Freiburger, Steve Dulcich, and Steve Brule take the cheapest Chevy 350 crate engine you can buy and make mods that gain nearly 130 horsepower! It's a dyno-proven, easy-to-install power combo that will give you the best bang for your buck. Sign up for a free trial to MotorTrend+ today and start watching every episode of Engine Masters, plus much more!