Big-Block Chevrolet Heads - CHP How It Works
Conventional 24-Degree Big-Block Chevy Heads are Giving Spread-Port Castings a Run for their Money
From the May, 2011 issue of Chevy High Performance
By Stephen Kim
Now this is our kind of war. Over the last decade, hot rodders have watched with glee as the war between cubic inches and cylinder heads rages on. Just as the enormous displacement made possible by today’s stroker cranks and aftermarket blocks push the limits of airflow, cylinder head manufacturers have come roaring back with so much cubic feet per minute that now any short-block seems too small.
The new crop of big-block Chevy cylinder heads can top 500 cfm, more than enough to feed the appetite of a 632ci short-block turning 8,000-plus rpm. Making things even more interesting is that there’s a mutiny brewing within the walk of Rat motor heads. In the not-so-distant past, such astonishing airflow was the exclusive territory of spread-port, Pro Stockstyle cylinder heads. These days, however, big-blocks routinely produce 1,000-plus horsepower with conventional 24-degree castings.
Curious as to how this is even possible, we contacted Jason Neugent of Brodix, David Canfield of Trick Flow, Tony McAfee of Dart, Rick Roberts of Edelbrock, and Tony Mamo of Air Flow Research. In addition to dissecting the state of modern big-block cylinder head technology, we wanted to know the pros and cons of conventional 24-degree heads versus spread-port heads, price differences between the two, the types of specialized components that each require, and which style of heads reign supreme in the horsepower department.
Before getting to the good stuff, we’re obligated to clarify that Big Chief is a term that’s often used generically to describe spread-port heads. In truth, Big Chief is Dart’s brand name for its spread-port heads. Likewise, Brodix calls its spread-port heads Big Dukes. With that factoid out of the way, let’s get to the good stuff.
Jason Neugent: From the factory, stock 26-degree big-block Chevy heads don’t offer the best architecture from an airflow standpoint, but the design has been highly refined over the years. Their strength is in their popularity, and therefore all the off-the-shelf parts that are built around it. On the other head, a 26-degree valve angle isn’t conducive to airflow and not much can be done about it. A common practice is flattening the valve angle to 24 degrees, which improves airflow, torque, and horsepower gains. The combination of refining port shape, moving valve centerlines, testing out different valve jobs, improving combustion chamber efficiency, and utilizing wet flow bench technology has allowed us to create a great cylinder head. The oval ports used on some of our high-end conventional heads have deeper bowls, a taller shortside radius, and better transitions in the corners. Also, bigger valves and flatter valve angles move more air at a faster pace. These refinements, along with advances in CNC-porting, have resulted in conventional-style heads with flow numbers that are comparable to spread-port, Pro Stockstyle heads. Some of the specialized parts needed to run a high-end 24-degree cylinder head are a shaft-mount rocker system, special pistons, and an oval port intake manifold. Fortunately, all these parts area available off the shelf. Shop wisely, and you could save approximately 20-40 percent on the heads and valvetrain by going with a high-end 24-degree versus a Big Duke cylinder head.
David Canfield: The factory 26-degree big-block Chevy head’s biggest flaw is its large combustion chambers. The factory chamber is comparatively large and creates several issuesdifficulty in raising compression being one of the primary drawbacks. Also, the fact that the ports are not mirrored creates unique issues. By changing the valve angle from 26 to 24 degrees, along with increasing the side cant of the valves helps unshroud the valves and increase airflow. Advances in valvetrain technology have proved instrumental, as well as computer modeling technology that has contributed to port design and research. Most castings are designed with extra material to allow for porting techniques that were impossible just 10-15 years ago. Unlike spread-port heads that require an intake manifold, valvetrain, and pistons that are specific to that head design, conventional 24-degree heads maintain a stock-type setup. While this may not offer the same horsepower potential of a spread-port casting, there are serious financial advantages to using a conventional 24-degree head.
Tony Mamo: The greatest weakness in the factory big-block Chevy cylinder head architecture is that there are two pairs of completely different intake ports in each head. Both the port length and the flow are notably different in both of these pairs as well as the architecture and overall layout of the port. The combustion chamber design of the original OEM heads leaves a lot to be desired as well, especially their closed chamber design, which really hampers flow and performance. Its greatest strength is the fact that it’s a large head with a lot of room to work on improving some of its inefficiencies when it comes to its port and chamber design. Also, with the ability to raise the height of the exhaust port in an aftermarket casting, another large shortcoming of the OEM design is greatly improved upon.
The latest crop of large, efficient 24-degree heads like our 385cc castings are certainly encroaching on the airflow and power capabilities of some of the over-the-counter, spread-port heads from 5 to 10 years ago despite the fact that the spread-port design has a huge advantage with a raised intake runner and a much better port layout. How did it happen? Like it always does in this industry: By constantly pushing the envelope on the R&D side of things, and, of course, a lot of sheer determination to produce a better product. The 24-degree conventional head market is still a really hot market, and the time invested in producing a new higher flowing product is very much warranted. Truthfully, it’s hard to believe some of the flow we are getting these days out of conventional, nonraised-runner port designs, and had you asked me just five years ago, I may have said it was impossible to get where we are today.
Tony McAfee: Achieving big airflow, while working around the limitations of the factory big-block Chevy cylinder head architecture, is very challenging. In order to keep costs down with a standard 26-/24-degree head, it must be compatible with the stock valvetrain. Consequently, the trick is to manipulate the port design and cross-section around the stock valve location. While high-end conventional big-block Chevy heads do have raised exhaust ports, the intake ports remain in the stock location. To work around this limitation, the short-turn radius on the intake port is raised tremendously and the plugs are relocated as well. Competition amongst manufacturers has pushed things to the next level, and racers are the beneficiaries. The result is conventional head castings with outstanding airflow that are compatible with off-the-shelf components.
Rick Roberts: Development work on our Victor 24-degree big-block Chevy heads started roughly 10 years ago. At the time, we were working with Billy Glidden on our small-block Ford heads, and he suggested integrating similar design elements into our Victor big-block Chevy heads. Cylinder heads are cylinder heads, and regardless of engine make, the same rules of airflow apply. As such, we moved the intake valve away from the cylinder wall and toward the center of the bore by 1/16 inch to assist in high-lift flow. We also moved the exhaust valve up against the cylinder bore, but in the opposite direction. A factory big-block Chevy head has 26-degree intake valves canted 4 degrees, and 17-degree exhaust valves canted 4 degrees. With the Victor heads, we changed the intake valve angle to 24 degrees with a 5.5-degree cant, and exhaust valve angle was flattened to 14 degrees. As a result, when the valves are opened 0.800 inch or more, the heads of the valves are very far away from the cylinder wall. The exhaust ports were raised as well. These changes, along with relocating the spark plug, yield more efficient combustion chambers. Changing valve angles messes up rocker arm geometry, so we had to redo the stud angles and rocker positioning. Our heads also require guideplates specific to our heads, but they’re inexpensive and readily available. Once racers started using our Victor heads, they gave us input on where they were hitting water during porting, and how we could improve the design. The result is a head that doesn’t require knocking guides out and welding up the seats, which are common practices with many conventional 24-degree castings. With 2.400 intake valves, ported Victor castings can flow 500 cfm, right on par with many spread-port heads.
Tony McAfee: The original port model of the spread-port big-block Chevy heads was designed by Lee Shepherd back in the mid ’80s. He was driving for Reher-Morrison’s NHRA Pro Stock program at the time, and came up with the concept of a raised-runner, spread-port head that addressed the inherent drawbacks of the factory big-block Chevy heads. Unfortunately, he died before he could do anything significant with them. The design was submitted to GM, who then sent it off to Dart for evaluation. Once the prototypes were at Dart, Richard Maskin tapped into his extensive experience in cylinder head design and manufacturing to address the problems that existed with the spread-port architecture. Since the spread-port design radically changed the configuration of the ports and the castings themselves, nothing normal would work and there were lots of valvetrain issues to fix. Lots of people had attempted to perfect the spread-port design up to that point, but Maskin is the person who really got them to work. Spread-port heads are now the standard in Pro Stock, and are used in many amateur racing classes as well.
Rick Roberts: The basic premise behind a spread-port head is to significantly raise the intake ports off the deck, and spread them apart for a more efficient flow path. Combined with a smaller 18-degree valve angle, the result is a significant increase in airflow over a conventional big-block Chevy cylinder head. Back in the early ’80s, several NHRA Pro Stock teams were testing out the spread-port head design. John Callies was the director of motorsports for Pontiac at the time, and he was able to get a head casting legalized for Pro Stock competition. The heads were for big-block Chevy engines, not Pontiacs, but they had a Pontiac part number and logo since they were intended for use in Pontiac-bodied race cars. GM submitted the heads to Richard (Dick) Maskin for his help in sorting out the design. From there, several manufacturers started offering their own spread-port castings, and most these days feature an 11-14 degree valve angle. Edelbrock was one of the last companies to join the spread-port party, but this enabled us to address many of the valvetrain geometry issues associated with this style of cylinder head. We have just about the best 18-degree head you can hope to have. Cylinder heads are almost like women’s fashion, and small valve angles are fashionable these days. It’s not that 18 degrees aren’t capable, they’re just out of fashion.
Conventional vs. Spread-Port Heads
Jason Neugent: Several factors distinguish a spread-portstyle cylinder head from a conventional big-block Chevy casting. Spread-portstyle Big Duke cylinder heads are much taller than your conventional 24-degree big-block Chevy castings. The primary benefit of a spread-port design is that it provides a straighter direction for the air to flow. Likewise, it also allows for equal volume and airflow from port to port, whereas a conventional-style head will have long and short ports. The result is different port volumes and airflow from one intake port to the next. That’s why people often refer to good ports and bad ports when describing big-block Chevy heads. Airflow potential for a max effort Big Duke head is tremendous. We have a new CNC-ported 12-degree Big Duke head that will flow 578 cfm at 0.900-inch lift. Brodix offers spread-port heads in a variety of options, which results in a big range of prices. The Big Duke sets start at a retail price of $3,750, topping out at $8,800. Big Dukes also require special pistons, big-bore blocks, shaft-mount rocker arms, longer valves and specific gaskets, valve covers, and intake manifolds.
Tony Mamo: Obviously, with the same fortitude and dedication to product design as with a high-end 24-degree head, a spread-port head’s flatter valve angle and raised-runner configuration blows it away on paper and when it comes to achieving large airflow figures. It’s all relative, though, and a lot of that potential will depend on the finished volume and cross-sectional area of the head in question as well as the valve job angle. These factors play a very key role in the higher-flowing heads, both spread-port and conventional. Steeper valve angles will always flow more peak airflow if they are designed properly, but there will always be a trade off in the lower and middle lift portion of the flow curve, so beware that steep valve angles are very camshaft and application sensitive. Also, keep in mind that steeper valve angles affect reliability and tend to need a more frequent freshening up of the seat and valve faces because they wedge more under operating conditions. One of the greatest strengths of our 385cc castings is the fact we are getting big peak numbers of 465 cfm with exceptional low- and mid-lift flow as well. The no-compromise, have-your-cake-and-eat-it-too scenario made the execution of that head even more challenging. What makes these heads even more appealing is that they’re still truly a bolt-on piece, no different than installing a much smaller 315 or 335cc head. The only difference with the 377/385 series is the intake valve is about 0.040-inch longer to facilitate the use of an optional triple spring we can supply that’s meant to install at 2.100 (the height of the spring at rest) if so it can’t handle 0.900 lift. Both styles of heads accept stud-mount conventional rocker arms, and the exhaust ports are in the exact same location of the rest of our lineup.
Tony McAfee: Many design factors distinguish a Big Chief cylinder head from a conventional 24-degree big-block Chevy head. To improve upon a conventional head’s limitations, a Big Chief’s casting is much larger physically. This frees up space to move the valveguides substantially closer toward the center of the cylinder bores. Furthermore, Big Chiefs have flatter 18-degree valve angles with different cant angles as well. The intake ports are raised an inch over stock and spread 0.400-inch away from each other. This helps equalize port volume and airflow from one port to the next. When Big Chiefs were first used in Pro Stock, the problem was trying to stabilize the valvetrain with such radically repositioned valves. The solution was Z-rocker arms, which had a 0.900-inch offset on intake and a 0.600-inch offset on the exhaust. If not for Z-rockers, the Big Chiefs wouldn’t have come into existence. In today’s world, however, stability is no longer an issue. Now we use multi-angle rockers with thrust bearings along with offset lifters, so offset rockers are no longer required.
Rick Roberts: One of the biggest downsides of a conventional 24-degree head is trying to work around the stock intake port location. This results in a very sharp turn at the shortside radius. The short-turn radius is a very finicky part of the port, and it requires a delicate touch, especially in applications where the ports can flow up to 500 cfm. The idea is to make the short-turn broad at its highest point, then widening it out and laying it down before making a quick transition into a circular seat. If you saw the short-turn radius on some of the heads these high-end builders putting together, it would scare you. It looks so radical, but it works. It’s possible to hit water in the short-turn area, so we’ve revised the water jacket location on our Victor heads to prevent this. That said, in order to get 500 cfm out of a conventional head, you’ll give something up at 0.300-inch lift, but if you need a head that flows to 0.850-inch lift, you probably don’t care too much about low-lift flow.
Tony McAfee: What’s really pushed the development of high-end 24-degree big-block Chevy heads over the last few years are several amateur racing classes to prohibit the use of spread-port heads and limit displacement to 565 ci. Even so, these motors are producing more than 1,100 hp. Ironically, the idea behind banning spread-port heads was to keep costs down, but as usual, racers with the deepest pockets are winning once again because they’re putting so much money into the heads. Consequently, the cost of building a max-effort motor with conventional heads versus building an engine with Big Chiefs is very close. With conventional heads on a typical 565ci big-block that use standard rockers, stud girdles, and standard lifter locations and pistons, making 1,050 hp isn’t that uncommon using out-of-the-box parts. To get into the 1,100-1,200hp range with conventional heads, however, you usually have to move the valveguides. This can take hundreds of hours of work, and at the end of the day you still can’t match a Big Chief head airflow-wise because you can’t raise the intake ports. All this head work adds up to lots of labor, which drives up costs. These days, it’s just as easy to build an engine with Big Chief heads as it is to build an engine with conventional heads for not that much more money.
Rick Roberts: The reason why conventional 24-degree cylinder head technology has advanced so much in recent years is because spread-port heads are prohibited in many racing classes. One class that stands out in particular is Texas Pro Stock, which caps displacement at 565 ci, and requires conventional heads and a single Dominator carb. These rules were intended to reduce costs, but the exact opposite has happened. While spread-port heads are a little more expensive, by the time you factor in the expense of shaft-mount rockers and titanium valves, the top end package costs $6,000-$8,000 for a conventional-headed engine combo, and roughly $1,000 more for a spread-port top end. You might be able to get away with stainless valves to further cut down on costs with conventional heads, but that’s not an option with spread-port heads because their valves are so big and heavy. Ultimately, you’re not really saving that much money with a set of conventional heads. Using conventional heads to beat someone else who has a big, bad spread-port motor, which should be more capable, is part of the appeal.
Jason Neugent: Although a high-end 24-degree big-block Chevy head can come close to a spread-port head airflow-wise, spread-port heads still have more potential and will make more power. From top to bottom, a spread-port head is just a better design. They have shallower combustion chambers, better ports, and a flatter valve angle, all of which are crucial in making horsepower. With valve angles ranging from 12-18 degrees, air enters the intake ports at a straighter line-of-sight trajectory than with a 24-degree head. Consequently, spread-port heads will make more horsepower in the long run.
Tony McAfee: Even though a high-end, 24-degree head can come close to a Big Chief head airflow-wise, the Big Chiefs will still make more power. That’s because what a cylinder head flows on a flow bench and what it flows once fuel is added into the mix are completely different. With a conventional 24-degree head, air and fuel enter the port below the short-turn radius, are raised up, then turned back down into the bore. With this type of flow path, the fuel tends to separate from the air, puddles down into the motor, and goes back out the exhaust in unburned form. Thanks to its raised ports, a Big Chief head keeps the fuel in suspension much better. So if you have a conventional 24-degree head and a Big Chief head that both flow 500 cfm, the Big Chief head will still make more power.
Rick Roberts: Conventional big-block Chevy heads can come close to spread-port castings airflow-wise, but spread-port heads still have a power advantage. Due to their smaller combustion chambers, in a high-compression application you’ll have a smaller piston dome on a spread-port motor than with conventional heads. A smaller, more gradual dome slope promotes more efficient combustion. Likewise, even if the peak airflow numbers between the two styles of heads are similar, a spread-port head will have superior mid- and low-lift flow than a conventional head. Spread-port heads also lend to more efficient intake manifolds as well, especially if you’re limited to a single carb. If you’re not bound by rules or restrictions, when comparing an optimized spread-port engine combo to an optimized conventional-headed combo, a spread-port head is simply the better way to go. If you’re starting from scratch, why mess around with conventional head and leave horsepower on the table? Comparing like-for-like engine combos with the same durability, you’ll make 100 more horsepower with a spread-port head and you’re not going to have to work it as hard to make that power.
Tony Mamo: A spread-port head design offers similar-flowing ports in every hole, whereas a 24-degree design will always be handicapped by four ports flowing weaker than the other four. Even a good 24-degree head will still usually fall short on the bad ports by about 20 cfm. In fact, I have seen as much as a 50 cfm disparity in the weaker 24-degree designs. Then there is the more consistent cross-sectional area of the spread-port design with less velocity deviation, and also the fact the intake manifold design and layout has a much straighter shot at the back of the valve and ultimately the combustion chamber so it allows for more airflow net to be converted into power. Conversely, 24-degree heads don’t have as sweet a vantage point when it comes to manifold design, but due to the fact that the intakes are typically much shorter, they do fit a lot better in cramped engine bays and other applications where the sheer height of the spread-portstyle motor simply couldn’t be tolerated. Flatter valve angles common to the spread-port design will also have better burn characteristics and take less timing advance to make peak power. Truthfully, it’s a lot of little things that add up to create a situation where the spread-port is a more efficient design. Ultimately, if you’re looking for the most power you can generate with the displacement and compression you’re working with, and you’re willing to spend the money necessary to get there, the spread-port heads won’t disappoint. However, the difference now in 2011 is a well-prepped combo with 24-degree heads might be on your back bumper in the other lane, or worse if he really did his homework. Even just five years ago that simply wasn’t possible! CHP