Cylinder Head Spotlight: Edelbrock’s Street/Strip RPM XT

When we laid out the plan for our Project MaxStreet Chevy II, our ultimate goal was to produce in excess of 1,000 horsepower from the Edelbrock 555 powerplant that previously motivated our Project Grandma Malibu. In order to do so, we’ve opted to swap our combination over to a centrifugal ProCharger and intercooler setup. But, it’s not just a simple matter of removing all the nitrous pieces and bolting on the blower. Some of our components can be reused, other won’t. One area that we wanted to make a change was the cylinder heads. Here, we’re going to have a close look at the Edelbrock RPM XT cylinder heads that will soon find their way atop our trusty 555.

The Performer RPM XT – or Xtreme – is a cylinder head design from Edelbrock that’s CNC-ported to attain optimal airflow for maximum horsepower and torque. They are intended for use in high performance street cars and light duty racing applications. According to Rick Roberts, Edelbrock’s Director of Engineering, “The RPM XT falls right in line between a racing and street cylinder head. The casting and port design are new and engineered to be as close to perfect as possible, with a fully CNC-ported combustion chamber and CNC-porting in the bowl area for a smooth transition into the valve seats. The intake runner entries and exhaust runner exits have also been partially CNC-machined for increased air flow.”

The engineers at Edelbrock designed the RPM XT to fit it to a variety of applications, with street-driven muscle cars very much a focus. With port cross sections that aren’t too large, and when used in applications such as a 600 horsepower pump gas street engine, they will perform exceptionally well. Many racing-style cylinder heads use ports that have been enlarged past the point where they work well for street use, so the XT line represents a versatile offering for the market.

A Look At Edelbrock’s New Performer XT Cylinder Head

Edelbrock's new RPM XT heads fall right in line between a racing and street cylinder head. The casting and port design are new and engineered to be as close to perfect as possible.

Roberts points to the CNC-machining as being one of the defining elements of the head. “The CNC machining gives this head greater breadth of application. It’s at home in a mildly warmed-over street motor, but if you want to push it with a blower or something of that nature, it’s up to the challenge. I think this head is capable of making over a thousand horsepower with an aggressive camshaft and that type of thing. We haven’t actually demonstrated that, but it has flow numbers that prove it capable of that kind of power.”

The design of the RPM XT line borrows from technology and design cues used in Edelbrock’s renowned family of cylinder heads – such as the full-race Victor lineup and the Performer RPM – and applies knowledge gained from decades of cylinder head development. “Like anything that we do, you always have the experience you’ve gained from all of the other things that you’ve worked on,” explained Roberts. “Much of the trickle down in technology seen in the XT is mainly found in the rather subtle aspects of the port shapes, and not so much in the configuration. That’s simply because we just couldn’t do that and keep it compatible with the stock stuff.”

The RPM XT heads that we’re placing on our 555 features a 25.5-degree by 4.4-degree intake valve angle and a 15.5-degree by 4.2-degree exhaust valve angle, with stock exhaust port locations and modified spark plug angles for added fuel efficiency and horsepower. “The stock intake valve angle on a 1960’s Big Block Chevy head is 26.3-degrees, explained Roberts. “On the RPM XT, we rolled the angle and stood the valve up a degree and a half from what stock would be and did the same with the exhaust. That means all the standard valvetrain will still fit without any problems, but it does give you a little better advantage as far as flow and that sort of thing.”

The RPM XT heads feature a fully CNC-ported combustion chamber along with CNC-porting in the bowl area for a smooth transition into the valve seats.

Raised Exhaust Ports Means Less Header Headaches

One of the most common issues found in many street and strip, high-performance cylinder heads on the market is the location of the exhaust port, which sometimes inhibits the use of off-the-shelf headers. One of Edelbrock’s primary missions with the RPM XT heads was to eliminate that problem. The exhaust port has been raised up an 1/8-inch, which isn’t nearly as much as often found in the racing world, where 1/2-inch above stock on the port centerline is common. “It’s close enough to stock that chances are pretty good that you won’t encounter any problems when fitting headers,” said Roberts. “Whereas, if you raise it 1/2-inch, you’re probably going to have to purchase a custom exhaust system.”

The Big Block Chevy RPM XT heads feature a 25.5 degree by 4.4 degree intake valve angle and a 15.5 by 4.2 degree exhaust valve angle.

The XT line of cylinder heads was conceived primarily as a premium version of the Performer RPM head, which is a popular choice for street applications. As such, the XT heads offer customers the option of choosing something that goes a little beyond the capabilities of the Performer RPM head, with full CNC-porting. “Doing all of the machining in the chamber and the ports, it’s less prone to the cruel reality of trying to do so with cast ports,” explained Roberts. “You can only get them so accurate. There’s much more guarantee of accuracy, and that means you can do things to the port shapes, the valve jobs, and things like that that we know improve flow numbers, and in turn, performance.”

With the RPM XT, Edelbrock has addressed the needs of those utilizing Big Blocks, where there is a variety of bore sizes, such as 4.25″ and 4.60″ bored engine blocks. “We’ve addressed this by offering two different combustion chamber versions of the head. One is a 118cc head for the larger bore engines, and then we also offer one that is 106 cc’s,” says Roberts. “They’re the same port designs – same basic design, same valve angles, same everything. For those who are looking for a more torquey application, such as a daily-driven street application, we’ve also made an oval port version.”

The XT's use stock exhaust port locations and modified spark plug angles for added fuel efficiency and horsepower.

Great Combustion Design Keeps Boost In Mind

Commonly, when designing a set of cylinder heads with boost in mind, there are two primary points to consider. First, with boost, you are in essence shoving air past the intake valve, and intake side isn’t such a pressing design area. However, on the exhaust side of things, the difference in cylinder pressure between the inside and outside world is all that you’ve got going for you. So, making certain the exhaust port is fully up to the task is a critical element.

The RPM XT features large exhaust ports and valves and completely CNC-machined combustion chambers. This allowed the engineers at Edelbrock the freedom to work with the shapes of the combustion chambers to a greater extent than would be possible with cast heads. Any engine, whether using a power adder or not, benefits from good combustion, and while the XT wasn’t fully designed for boosted applications, it’s a very good choice if you’re looking to go that route, now or in the future. “One of the things you have to worry about in boost applications is detonation; you’ve got to be careful about spark advance. That can be cured by just making sure that you have good, efficient combustion,” says Roberts. “We’ve used technology from our knowledge of the Victor head and racing-type combustion chamber shapes to make a combustion chamber shape that we think has a very efficient burn.”

All of the valve train components have been carefully designed for durability, including the springs, retainers, studs, guide plates and other parts.

For these reasons, the XT head exhibits some very impressive flow numbers. Despite the fact that large port flow isn’t quite as important with forced induction, the ability to move air will be of great benefit for supercharged combinations such as ours. Roberts went on to say, “We flowed this head up to .800 valve lift, though we don’t believe that most people will use it up to that much lift. It’s reassuring, because a lot of times on stock-type heads, when you really start to push them as far as big ports and things like that go, they’ll get up to .600 to .700 lift and the port will really stall; not continue to flow. That’s a very common problem, which these don’t have. If you take a look at the flow numbers for racing-type heads, these XT heads are right there.”

Edelbrock uses a material for their valves known as 21-4N (the 4N stands for 4% nickel content), which is a specialty stainless steel, in all of their Performer RPM cylinder heads. This high nickel content gives the valves improved strength and resistance in high-heat conditions, as would be the case when using a supercharger. This is another very beneficial element to the RPM XT that many other manufacturers may not offer as standard.

The Specs: Edelbrock’s RPM XT Oval Port Big Block Chevy Heads

• Combustion chamber volume 112cc
• Intake runner volume 308cc (lh) / 325cc (rh)
• Exhaust runner volume 133cc
• Intake valve diameter 2.25″
• Exhaust valve diameter 1.88″
• Valve stem diameter 11/32″
• Manganese Bronze Valve guides
• Deck thickness 9/16″
• Valve spring diameter 1.55″
• Valve spring maximum lift .700″
• Rocker stud 7/16″
• Guideplate 3/8″
• Pushrod diameter 3/8″
• Valve angle 25°
• Exhaust port location +.125″
• Spark plug fitment 14mm x 3/4 reach, gasket seat
• Made In USA

Many of the other components in the RPM XT, like all of Edelbrock’s products, have been carefully designed and tested for durability, including the springs, retainers, studs, guide plates, and other parts and pieces. Edelbrock performs extensive testing in the development of new products to ensure that every part is of the level of quality the market has come to expect.

All of these elements combine to make the RPM XT line of cylinder heads one of the best new products on the market, and because they’re suitable to virtually any engine combination up to a reasonable level of horsepower, you simply can’t go wrong by bolting a set of these atop your engine. We’re confident that these heads will be just the ticket for our supercharged MaxStreet project.

Keep it tuned right here to powerTV as we’ll highlight the complete engine rebuild by our friends at Pacific Performance, in an upcoming tech piece on MaxStreet, and also have a look at many of the other new components that will find their way into our revamped Edelbrock 555 engine.

Article Sources

About the author

Andrew Wolf

Andrew has been involved in motorsports from a very young age. Over the years, he has photographed several major auto racing events, sports, news journalism, portraiture, and everything in between. After working with the Power Automedia staff for some time on a freelance basis, Andrew joined the team in 2010.
Read My Articles

Hot Rods and Muscle Cars in your inbox.

Build your own custom newsletter with the content you love from Street Muscle, directly to your inbox, absolutely FREE!

Free WordPress Themes

We will safeguard your e-mail and only send content you request.