The big-block Chevy’s canted valve heads are largely responsible for the tremendous power capability of the Rat motor and its continued popularity. The original head design is often referred to as having a 26-degree valve angle, although this intake valve angle of inclination is only one of four angles needed to numerically describe the big-block’s valve angles. The exhaust is tilted 17 degrees relative to the deck surface, and both are inclined 4 degrees laterally. This compound arrangement of valve angles gives the big-block head its characteristic “valves pointing everywhere” appearance when the valve covers are removed and is responsible for the early “porcupine head” nickname when the big-block made its debut in the mid 1960s.
Aluminum aftermarket cylinder heads offer tremendous airflow potential to maximize power potential.
Big-block Chevy factory heads are offered in aluminum and cast iron, with either open or closed combustion chambers, and with oval or rectangular intake ports. In the aftermarket, most performance heads are aluminum rectangular port, open-chamber designs reminiscent of the original LS6 casting, although you can also buy aftermarket oval port and cast-iron heads.
Factory heads are classified as either high-performance (rectangular port) or standard passenger car (oval ports). Late-model trucks feature an even smaller oval intake port, frequently referred to as the “peanut” port, on heads that appear to be nearly round at the port entrance. Rat motor heads feature a unique combination of siamesed intake ports mated with symmetrical combustion chambers and equally spaced exhaust ports. As a result of this arrangement, each siamesed pair of intake runners features non-symmetrical left-hand and right-hand ports.
The port on the right side of each pair (as you face the intake flange surface) is longer and directs the inlet charge more to the center of the chamber, and is referred to as the “good” port; the one of the left, obviously referred to as the “bad” port, dumps the air/fuel mix toward the cylinder wall and usually doesn’t flow as well as the right port. This minor idiosyncrasy of the big-block’s design has never been a real problem in terms of limiting the engine’s power potential, until extremely high airflow levels are reached on large-displacement or very-high-RPM engines. On engines approaching or exceeding the 1,000-hp mark (normally aspirated), the use of race-only spread port or Big Chief–style heads serves to even out the port volume, shape, and airflow in all eight intake ports.
Stock iron oval ports from a “206” casting head. Note distance from the ports to the center bolt-hole above the pair of runners
Stock iron rectangular ports from a “990” casting head. Note that the tops of the ports are almost even with the bottom of the center bolt-hole.
Big-block head bolt torque pattern. Stock head bolts get torqued to 70 ft-lbs in three steps of 40, 55, and 70 ft-lbs. Use thread sealer on all bolts that go into the water jacket, which means all blocks except Bowtie and aftermarket. Aftermarket studs usually get torqued to 60 ft-lbs, but check with the fastener manufacturer for specific torque recommendations.
Why didn’t the Chevrolet engineers just make it a mirror-image design like the small-block V-8? Because the big-block has six head bolts around each bore (if you count the hidden bolt bosses on the bottom of two of the intake ports), and that dictates where the intake ports must be placed. Also, it’s possible that the resulting siamesed exhaust ports might run too hot for long-term reliability with the big-block’s higher power levels and resulting increased exhaust flow.
Oval or Rectangular Intake Ports
Much has been said about the differences between rectangular ports and oval ports, and the only fact that everyone agrees with is that each design has its own strengths and weaknesses. All factory high-performance engines featured the larger rectangular port heads, which have higher airflow rates than production oval port heads. However, the larger volume of the rectangular ports produces rather sluggish flow velocities at low speeds, and smaller oval port heads are often a better choice for a daily driver or street and strip car.
When reworked by someone who really knows what to do, oval port heads are capable of providing very good performance up to 600 or more horsepower. However, most high-performance street and full-race big-blocks can still take advantage of larger rectangular port heads. When I refer to the port size as larger or smaller, I’d like to think in terms of port cross-sectional area, but that dimension changes constantly throughout the length of the port, so the most common way to measure port size is to determine the volume in cubic centimeters, just as with the combustion chamber.
The GMPP head (PN 12363400) with 300-cc rectangular intake ports (left) performs very similarly to the oval port head (PN 12363392) with 290-cc intake ports (right), if both have the same compression ratios. The oval port head has 110-cc chambers, and the rectangular port head comes with 118-cc chambers.
Cast-iron late-1960s “206” head with bathtub-shaped 98-cc closed combustion chambers.
Cast-iron high-performance rectangular port “990” head with open 118-cc combustion chambers. Note the lazy-D shape of the chambers.
Cast-iron oval port “215” head with 109-cc semi-open combustion chambers.
While intake port volumes are a valuable guide in cylinder head selection, remember that port volume is not necessarily proportional to port flow, and that just because one design has 340-cc runners and another has 320s, that doesn’t mean that the larger head flows more air or makes more power, although that is usually the case.
In fact, if two heads with different size runners have the same flow bench numbers, you are generally better off with the smaller runner head, especially if low- RPM throttle response and drivability are important. Also, when comparing port volume of spread port cylinder heads, remember that because these heads have raised runner locations, they are longer than conventional cylinder head intake ports, and the port volume is greater due to the extra length. A 400-cc raised runner intake port may actually be smaller in cross-sectional area than a 380-cc conventional intake port. Be careful when comparing apples to oranges.
There are aftermarket oval port heads with about 290-cc intake runners, and small rectangular port heads with around 300-cc ports. I’ll bet you a year’s supply of donuts that the power difference on the dyno is minimal, and you could never tell the difference from the driver’s seat.
Open or Closed Combustion Chambers
Big-block heads all had closed combustion chambers, or bathtub-shaped chambers, when the engine was introduced in the mid 1960s. In 1969 the open combustion chamber was introduced and it offered better air/fuel flow and a better combustion burn in the chamber. The only drawback to the new chamber design was that it was large, around 118 cc compared to closed chamber heads, which had about 101 to 109 cc, so high dome pistons were needed to achieve the same compression ratios as the closed chamber heads. Note that domed pistons designed for open chamber cylinder heads do not work with closed chamber heads due to insufficient clearance. While introduced on high-performance rectangular port heads, the open combustion chamber was soon being used in common oval port heads to lower the compression ratio for use with unleaded gasoline, and the reduced quench area was found to be helpful in reducing exhaust emissions.
Inspect used cast-iron heads carefully for cracks, which are common with highperformance heads that have been used hard. Cracks in the valve seat area (left) might be repaired with new seats, but cracks in the chamber/deck surface (right) make the head unusable.
Mark IV, Gen V and Gen VI Heads
All production big-block heads have similar characteristics, including 7/16- inch rocker studs (except Gen V and Gen VI) and the same seven-bolt valve cover pattern. Early cast-iron and all-aluminum heads used 3/4-inch-reach gasketed spark plugs, while most 1970-and-later iron heads switched to the smaller taper seat “peanut” plugs. Aluminum heads have two additional threaded bosses under the intake runners for additional clamping and better head gasket retention when used with engine blocks equipped with corresponding bosses in the lifter valley. The only OEM blocks with these bosses were the rare ZL1 aluminum blocks, but many modern aftermarket blocks have this added feature. Mark IV, Gen V and Gen VI heads all have the same head bolt pattern, but they are not interchangeable due to different water jacket cooling passages. Most aftermarket aluminum heads feature a universal water passage design allowing their use on Mark IV or Gen V/ VI blocks, but you need to check with the manufacturer to verify this feature.
Gen V and Gen VI heads have different water jacket passages than Mark IV heads, and do not work on Mark IV blocks. (Photo Courtesy GMPP)
All production big-block heads have stamped steel pushrod guideplates under the rocker arm studs for either 5/16-, 3/8-, or 7/16-inch pushrods, but Gen V/ VI heads switched to a non-adjustable valvetrain. Although Gen V/VI heads retained the original valve cover bolt pattern, their matching cast aluminum valve covers were equipped with a durable rubber O-ring in place of the traditional valve cover gasket. All production heads use the same intake and exhaust bolt patterns, which include a bolt-hole between each pair of intake ports. The boss required for this additional bolthole intrudes on the port entrance, so it is deleted on most aftermarket heads because it is not necessary for a good gasket seal.
All OEM big-block heads had an exhaust crossover passage for intake manifold heat and automatic choke operation, as well as supplying exhaust gas for EGR-equipped (exhaust gas recirculation) vehicles from the early 1970s on. High-performance engine builders try to keep the intake manifold as cool as possible for maximum intake charge density, so performance intake gasket sets usually include metal plates to block off this exhaust crossover passage, and most aftermarket heads delete the exhaust crossover provision entirely.
All OEM Rat motor heads were equipped with steel valves that have 3/8-inch diameter valve stems, and the overall length of the intake valves is 5.218 inches while the exhaust valves measure 5.350 inches. Aftermarket heads typically come with 11/32-inch diameter valve stems, and frequently use longer than stock valve lengths that allow installation of taller valve springs that are better suited for use with high-lift camshafts. The smaller-diameter stems not only lighten the valves for better high-RPM performance, but also offer a slight improvement in airflow. OEM intake valve head diameters were either 2.06 or 2.19 inches, and exhaust valve heads measured either 1.72, 1.84, or 1.88 inches. All production heads and most aftermarket heads come with traditional 45-degree valve seats; however, some high-end aftermarket heads are available with 55-degree seats, which offer improved high-lift flow at the expense of some low-lift flow.
Aftermarket Cylinder Heads
The vast majority of aftermarket heads are rectangular port aluminum with open combustion chambers, similar to the fabled LS6/LS7 production heads. However, they are much more than mere copies of the Chevy part, and many offer such advanced features as non-stock valve angles, larger valve head diameters for more flow, more robust valve springs for high-RPM operation, and your choice of various intake port sizes to match your requirements. Some offer raised ports, usually by .100 inch on the intake ports and as much as 3/4 inch on the exhaust ports.
Note that if you plan to use exhaust headers designed for stock heads in a stock engine compartment, you need to check for possible header interference with raised port heads, or have custom headers made. Typical material specs call for the use of either A355 or A356 aluminum alloy, usually hardened to T-6 specs, and most offer beefier construction with thicker-than-stock deck surfaces and port walls, allowing additional modifications by knowledgeable cylinder head specialists.
All production big-block heads had an exhaust crossover passage between the two pairs of siamesed intake ports. The casting cavities above and below the center exhaust crossover are blind cavities that don’t connect to anything. Some aftermarket intake manifolds don’t cover the upper opening, which does not cause any problems.
Many aftermarket heads, such as this Edelbrock Performer RPM 454-R, are rectangular port aluminum with open combustion chambers, similar to the fabled LS6/LS7 production heads. (Photo Courtesy Edelbrock)
Modern competition heads usually have heart-shaped combustion chambers to minimize chamber volume and increase the quench area.
In addition to GMPP, there are an abundance of manufacturers offering high-performance heads for the big-block Chevy including Air Flow Research (AFR), Brodix, Carl Foltz Engineering (CFE), Dart, Edelbrock, Pro-Filer, Racing Head Service (RHS), Raptor (Reher-Morrison Racing Engines), Sonny’s Racing Engines (SRE), Trick Flow Specialties (TFS), World Products, and probably more by the time you read this. See “Aftermarket Cylinder Head Manufacturers” on page 88 for a brief review of these heads. Contact the manufacturer of your choice for more specific information before making your final head selection.
Spread Port Heads
As good as the original Chevy head design was, things really started to heat up when GM engineers got involved in the Pro Stock wars in the 1980s. Because of the Corporate Engine policy, GM competitors were allowed to use any GM family engine in their race cars and the big-block Chevy was obviously better suited to all-out racing than any other GM big-block engine. Pontiac engineers took advantage of this break to create a head for the Rat motor with a superior port design, shallower valve angles, and smaller, more efficient combustion chambers.
The Pontiac Super Duty Pro Stock cylinder head (PN 10045427) featured intake ports that were spread apart to even out the flow differential inherent in the original Chevrolet siamesed port design, and the shallow valve angles required substantially raised intake ports to straighten the airflow path from the port entrance to the valve seat.
From there, Oldsmobile engineers took their shot at creating the best possible drag race head, and introduced the Oldsmobile Pro Stock head (PN 24502585). The Olds Pro Stock head evolved into the GM DRCE, the development of which was spearheaded by a young Warren Johnson, soon to be the dominant NHRA Pro Stock racer of the 1980s and 1990s.
Not to be forgotten, Chevrolet engineers also developed a spread port head, the Chevy Symmetrical Port Head (PN 10051128), with similar design parameters: evenly spaced intake ports and shallow valve angles with miniscule combustion chambers.
The quench or squish area of the cylinder head is the flat surface that hangs over the bore. As the piston comes to TDC the air/fuel mixture is rapidly expelled, creating turbulence in the chamber, which greatly increases combustion efficiency.
D-shaped exhaust ports decrease the flow differential between the floor and the roof of the exhaust ports, and they help to combat reversion.
Today, the latest version of the Olds DRCE is the spec cylinder head for all GM-powered vehicles in NHRA Pro Stock competition, while the Pontiac Pro Stock head has been widely copied by aftermarket manufacturers, resulting in the Dart Big Chief head (paying tribute to the Pontiac Indian tribe), the Brodix Big Duke head, and Edelbrock’s Big Victor head, with more development sure to follow. If you want to make 1,000 hp or more without power adders, “Big” heads are the way to go.
Even though these “Big” heads all share some common design parameters, they are not identical and most require specific matching components such as pistons, intake manifolds, and shaft rocker arm assemblies. All spread port big-block Chevy heads require shaft rocker arms because the pushrods must be relocated around the revised intake port location, and the large amount of rocker arm offset eliminates the use of traditional stud-mount rockers. Most accept the original Pontiac-designed eight-bolt valve covers, although the Edelbrock Big Victor head features a unique valve cover bolt pattern to relocate the bolt-hole bosses for improved rocker arm clearance and valvetrain geometry. This is one of the reasons that fabricated aluminum sheet-metal valve covers are so popular these days they can be quickly produced one at a time or in relatively small batches to fit whatever bolt pattern you want. Besides, they look racy!
What’s Your (Valve) Angle?
Production big-block Chevy heads all came with 26-degree intake valve angles, and that dimension was standard throughout the first two decades of competition cylinder head development, both by Chevrolet and the performance aftermarket. Savvy racers soon realized that a smaller combustion chamber, combined with less dome on the piston, resulted in a more efficient burn and made more horsepower than a large chamber/large dome combination. However, the 26-degree intake valve angle placed the valve seat precariously close to the deck surface of the head and was the limiting factor in how far the head could be flat milled to reduce chamber volume.
The process of angle milling was developed, in which more material was removed from the exhaust side of the deck surface to produce a smaller chamber than was possible with a flat milled head. Angle milling reduced the intake valve angle slightly, usually by about one degree, and it required that the rest of the head be re-machined to correct the intake flange and head-bolt holes, as well as re-facing the tops of the head-bolt holes to present a parallel surface for the head bolt or stud hardware to register against.
The Pontiac Super Duty Pro Stock head was the predecessor to today’s crop of spread port heads like these Brodix Big Duke cylinder heads with reduced intake valve angles and raised runners. You’ll find spread port heads on most 1,000-plus-hp big-blocks.
Progressive aftermarket companies began offering their Rat motor heads “rolled over,” essentially rotating the raw casting before any machining was done to produce an angle milled, small-chamber head while maintaining the original fitment of the part. Today, most conventional design (siamesed intake ports) competition heads are engineered with an intake valve angle around 24 degrees, which is about the minimum valve angle with the stock port locations. Why? Because as the intake valve angle is reduced, the intake port is “bent” more if you start with the stock port entrance. To really take advantage of shallow valve angles, the ports must necessarily be raised to straighten out the flow path, and that is precisely what was done with all of the spread port heads.
Current trends among spread port big-block heads are to have intake valve angles of 18, 14, 12, and occasionally even fewer degrees. While it seems that each new generation of spread port head design offers reduced valve angles, there is a limit to what the engine actually “likes.” A prime example of this phenomenon is that modern competition heads for nitrous or boosted applications typically have an 18-degree valve angle, perhaps because the additional volume of air/fuel requires a larger combustion chamber to contain the intake charge before ignition. Nitrous racers say that the 18-degree heads offer a wider tuning “window” compared to the 14- or 12-degree heads, meaning that if the exact nitrous/fuel/ timing combination is off just a little, the bigger chamber heads are less likely to result in catastrophic failure, usually in the form of burned or collapsed pistons.
With the selection of excellent cylinder heads already available for the big-block, you really don’t need to know anything about porting, CNC-finished combustion chambers, titanium valves, and all that other nasty technical stuff if you don’t want to. Just pay the man and bolt on a set of fully prepped, ready-to rumble cylinder heads and go have fun. On the other hand, I can think of two good reasons to keep up with cylinder head mods: first, if the family budget is already stretched thin with your passion for collecting big-block parts as economically as possible and you want to “tune up” that old pair of heads you just scored at the swap meet. Second, if you can afford to pay the man, you might want to be sure of what you’re getting.
Let’s start at the beginning, the intake port, or runner. The runner entrance is nearly perpendicular to the intake flange face, and goes several inches into the head before making a rather abrupt turn into the valve bowl. Since the intake port is merely an extension of the intake tract, which started in the intake manifold, it is important that port opening in the head closely aligns with the port in the intake manifold. So it is common practice to port match the two parts to the intake gasket being used.
The port exit in the manifold is usually left about .050 inch smaller than the gasket, in order to combat reversion and to account for any slight mismatch when the manifold and heads are assembled. Normally, lay-out dye is painted around the port entrances and a sharp scribe is used to trace the shape of the gasket around the openings. Using a 1/4-inch die grinder and some stones (for cast iron) or cutters (for aluminum), open up the port to a depth of about 1 inch. You can trim a little metal off of the rocker arm stud boss that hangs down from the port roof, but don’t shorten it or remove so much material that its strength is compromised.
With the poppet valves currently used in all automotive four-stroke engines, it has been shown that the valve bowl, or area of the port directly under the valve seat (viewed from the combustion chamber), is critical in setting up the flow around the perimeter of the valve as it opens. As a result, there is a bend in the port as it makes the transition from the entry corridor to the valve bowl, and that bend must be carefully shaped to reduce the air/fuel flow as little as possible while maintaining a homogenous air/fuel mixture. If the turn is too abrupt, the fuel tends to separate from the mixture and pool into large droplets that do not burn completely in the combustion chamber. The port floor at this bend is referred to as the short turn radius, while the roof is called the long turn radius. Because the airflow path is shorter at the short turn radius, flow velocity tends to increase, just as it does over the curved surface of an airplane wing, and many cylinder head specialists widen the port floor at that point to equalize the flow along the short and long turns.
Three-angle valve job narrows the 45-degree valve seat in the head with a 30 degree top cut and a 60-degree bottom cut. Valve seat width should be a minimum of .040 to .060 inch for the intake seat, and .080 to .100 inch for the exhaust. The valve has a matching 45-degree seat and should be narrowed with a 30-degree back-cut to match the seat in the head. These angles are standard, but many cylinder head specialists have their own pet angles that work for them. Note that valve seats of 55 degrees are used in some racing heads to improve high-lift flow, with a trade-off in low-lift flow numbers.
After the port bowl, the inlet charge flows through the valve seat, another critical area in a performance engine and the source of improved performance when done correctly. Obviously, the first job of the valve seat is to provide a gas-tight seal against the valve face to maximize the push against the piston from the combustion process. That is accomplished with a 45-degree angle on both the seat and the valve face. You can improve the airflow through the valve seat as the valve begins to open by narrowing the seat with top and bottom cuts, typically with a 30-degree top cut (the side closest to the chamber) and a 60-degree bottom cut. This is known as a three-angle valve job. The narrowing cuts un-shroud the seat and produce better flow, especially at low valve lifts. More flow means more air/fuel in the chamber, which equates to more power. To complement the narrowed valve seat, the valves are also narrowed with a back-cut of around 30 degrees.
Intake valve seat width on this Brodix competition head measures .040 inch. Note copper-beryllium seats, compatible with the titanium valves used in this engine.
Mark Jones in Colorado Springs, Colorado, preps a lot of cast-iron oval port heads for his crate engines. Note the clean-up of the valve bowls, the area just under the seats. Very little metal is removed from the chambers; just enough to improve exhaust flow. He routinely gets more than 600 hp from his 496- to 505- ci big-blocks with a modest flat-tappet cam and Performer RPM dual-plane intake manifold. He generally chooses either “049” or “781” castings, which are commonly available.
Moving on to the combustion chamber, there is little that needs to be done by the do it yourself head modifier; any material removed lowers the compression ratio and usually results in less power. Certainly professional head porters spend lots of time on the combustion chamber and may make radical alterations to improve airflow or combustion efficiency, but that type of work demands evaluation on a flow bench and plenty of experience in shaping the chamber for the most performance.
High-end aftermarket heads frequently feature CNC-machined chambers that are uniform from cylinder to cylinder, and have a smooth finish to lessen the mechanical bond between the chamber and carbon deposits that occur in every engine. Lightly polishing the chambers on your junkyard refugee iron heads may indeed help to reduce the carbon build-up which can cause pre-ignition, but don’t get carried away; removing more metal than necessary just lowers the compression ratio.
Naturally, if the chambers are reshaped, compression can be restored by milling the deck surface of the heads, and milling the heads for more power is one of the oldest tricks in the book; hot rodders have doing that since the days of the flathead Ford, and it is still a good idea anytime you’re “tuning up” a set of stock heads.
The exhaust valve seat should get the same three-angle narrowing treatment as the intake seat, but it should not be narrowed as much as the intake seat because the wider seat also transfers heat from the valve to the head. Typical widths are .100 inch for the exhaust seat and .040 to .060 inch for the intake. Remember that these specifications are minimums, and the cylinder head specialist of your choice may have his own pet set of dimensions that he has found to work best with his combination.
WANT THE BOOK ON CYLINDER HEADS FOR BIG-BLOCK CHEVY ENGINES?
Like what you’re reading so far? Grab a copy of Tom Dufur’s book, How to Build Killer Big-Block Chevy Engines.
Aftermarket Cylinder Head Manufacturers
There’s no way to list every big-block Chevy head offered by the aftermarket performance industry, but here is a brief review of what’s currently available. Remember that numerical specifications tell you something about a particular head, but trying to compare one head against another based on nothing but specs is often misleading. It’s like trying to describe your spouse by telling weight, height, and eye color. It may get you close, but there’s a lot more to it than just the numbers. Call the manufacturers, and talk to other racers or performance enthusiasts who have real-world experience with the products you are considering.
And don’t sweat the small stuff. The fact that head A made 5 hp more than head B in a magazine shoot-out does not mean that head A is necessarily the best choice for your engine. It just means that it made more power on that engine with that particular combination of parts. Your engine should be a total system of compatible parts designed to work together to produce the power you want under the conditions you experience.
AirFlow Research (AFR)
AFR offers a tremendous selection of traditional big-block replacement aluminum heads designed to fit within the stock architecture siamesed ports with traditional valve locations and open combustion chambers. Their eminently logical approach to marketing big-block heads consists of offering all of their heads with either as-cast ports and chambers, or CNC machine ported for a slight increase in port volume plus consistency in port-to-port and chamber-to-chamber dimensions. All AFR big-block heads feature “rolled-over” valve angles, which are listed as 24/4 degrees (intake) and 15/4 degrees (exhaust); 3/4-inch-thick deck surfaces for brute strength; and 3/8-inch raised exhaust ports.
Most exhaust headers still fit, but you need to check for adequate clearance in your engine bay to be sure. All AFR heads delete the stock exhaust crossover port to reduce intake charge temperature, so they do not work with EGR or hot-air carburetor chokes, and they are not street legal for applications that require those provisions. All AFR big-block Chevy heads have water jackets that are compatible with Mark IV, Gen V, or Gen VI blocks when used with the appropriate head gaskets.
AFR offers two versions of oval port heads: 265-cc as-cast intake runners or CNC ported with 290-cc runners. These oval port heads come with slightly smaller combustion chambers (107-cc as-cast, 112-cc CNC’d) than the rectangular port heads, and the smaller chamber sizes work well with the stock low-compression pistons used in typical 396/454 engines, yielding more compression and more power. Remember that aluminum heads tolerate more compression, without engine-damaging detonation, than their cast-iron counterparts, due to aluminum’s superior heat transfer characteristics.
AFR has four rectangular port head castings with runner volumes that could be classified as small, medium, large, and oh-my-god! As-cast runner volumes are 305, 325, and 345 cc; CNC ported volumes are 315, 335, 357, 375, and 385 cc. Remember that intake port size, like every other component in your Rat motor, must match the intended RPM range and usage. Don’t select the largest head you can carry out of the store unless you’re building a 600-inch stroker motor that is turning 8,000 rpm at the dragstrip. There is an AFR cylinder head that is the perfect match for your engine combo whether it’s a stock short-block engine for your tow-truck, a 600-cube race-only dragstrip warrior, or anything in between.
Brodix Cylinder Heads
Brodix is one of the oldest and most respected names in the business of manufacturing aluminum cylinder heads for the big-block Chevy, and the company offers an almost bewildering assortment of Rat motor heads from mild to wild. Just the conventional siamesed port heads are available in 27 different models, with options available including roller or flat-tappet valve springs, steel or titanium spring retainers, flat milling or angle milling, and as-cast or CNC-machined ports and chambers.
At the modest end of the scale, the Race-Rite heads are designed as straight bolt-on replacements for stock Chevy heads with 26/4-degree intake angles and 17/4-degree exhaust angles, even down to the inclusion of the center bolt-hole between each pair of intake runners. Exhaust port exits are in the stock location for compatibility with standard headers or exhaust manifolds, but all Brodix heads delete the stock exhaust crossover port to reduce intake charge temperature, so they do not work with EGR or hot-air carburetor chokes, and they are not street legal for applications that require those provisions. Brodix Race-Rite heads are available with 270-cc oval intake ports or rectangular ports from 294 to 320 cc, and there are even Jesse James signature-series heads featuring a black hard-anodized finish and embossed Jesse James logos on the ends of the heads. Although aimed at the street performance crowd, the black hard-anodized finish is an especially good idea on any marine engine.
AFR oval port 24-degree head features 100-percent CNC machined 290-cc intake ports and 112-cc combustion chambers with 2.250/1.880-inch valves. (Photo Courtesy AFR)
AFR rectangular port 24-degree head with 100-percent CNC machined ports and chambers. Intake port volumes range from 315 to 385 cc, and the optional black anodized finish resists corrosion in marine applications. (Photo Courtesy AFR)
CNC machined 121-cc combustion chambers on AFR rectangular port heads feature a heart-shaped design for superior flow and combustion efficiency. This shape may require hand fitting of the dome on high-compression pistons based on the original LS6 Chevy. (Photo Courtesy AFR)
The BB-1, BB-2, and BB-3 series rectangular port heads retain the 26-degree intake valve angle but offer more power potential via .600-inch raised exhaust port exits and intake port sizes from 280 to 370 cc. The BB-3 Xtra series heads change the intake valve angle to 24 degrees, and are available with rectangular ports from 363 to 380 cc, or large oval ports from 332 to 365 cc. Don’t confuse these oval ports with stock Chevy oval ports: they are a large oval shape with about the same width and height as rectangular intake ports, intended for serious racers who desire a high-velocity port with good flow potential.
Taking the large oval intake ports to the next level, the Head Hunter series is available with 383- or 395-cc large oval intake ports with an improved exhaust port design, and they retain the 24-degree valve angle. Brodix BB-4 and BB-5 series 24-degree heads feature traditional rectangular intake ports from 340 to 390 cc, but they are raised approximately .400 inch for a better intake flow path and improved short turn radius from the ports into the valve bowls.
For really serious drag racing or other high-horsepower applications, Brodix Big Duke PB 1800, PB 1600, PB 2005, and Man Eater PB 1200 series heads are based on the popular Pontiac Pro Stock head design with raised runners, spread intake ports, and shallow valve angles for reduced combustion chamber volumes. Those heads are offered in 13 different model numbers with intake valve angles of 18, 16, 14.5, and 12 degrees, and intake port volumes ranging from 375 to 510 cc. If you want to live on the cutting edge of big-block racing engine design, Brodix even offers three versions of the PB 5000 series 15-degree heads for use on 5.0-inch-bore-spacing blocks. And yes, Brodix also makes 5.0-inch bore-spacing blocks so you have a nice home for those new heads. Of course, 5.0-inch-bore-spacing engines require custom everything—sheet-metal intake manifold, custom headers, special cam, crank, oil pan, valve covers, rocker arms, etc., but you knew that when you committed to building an 800-ci Rat motor, didn’t you?
Carl Foltz Engineering (CFE)
If you’re looking for the perfect head for your mild street-driven big-block, keep looking: CFE specializes in no compromise, hard-core competition cylinder heads and custom sheet-metal and billet aluminum intake manifolds. The extensive line of big-block competition heads runs from conventional siamesed port BMF (Beat More Fords?) heads that accept traditional Chevy pattern intake manifolds and stud-mount rocker arms, to symmetrical port heads for 5.000-, 5.200-, and 5.300-inch-bore-space mountain motors. If you want to dive into NHRA Pro Stock racing, CFE-prepared DRCE cylinder heads have won multiple Pro Stock national events and championships. All CFE heads are fully CNC ported for consistency in runner shape and volume, and to produce the port shapes that extensive flow bench and dyno testing has revealed to produce more flow for a given application.
BMF heads are available with intake runner volumes of 310, 330, 350, 385, and 405 cc, the last requiring modifications of most cast aluminum intakes to work with the very large intake ports. All BMF heads feature 24.5-degree intake valve angles and intake valve sizes from 2.250 to 2.325 inches, with chamber volumes of 114 to 124 cc. Exhaust valves are 1.880 inches and they accommodate standard Rat motor exhaust headers.
Brodix 383 Head Hunter head with large oval intake ports.
Brodix Big Duke PB 1200 spread port head with 12-degree intake valve angles is fully CNC ported. It features raised oval intake ports, 55-degree seat angles, copper/beryllium valve seats, and 483- cc intake ports with 68-cc combustion chambers.
BMF II heads have additional aluminum added in the rocker arm support area to accommodate CFE’s one-piece intake rocker bar and double-bolted exhaust stands for shaft-mount rockers, which must be used with these heads. The use of .180-inch offset lifters is required to provide adequate pushrod-to-head clearance.
CFE spread port heads are available for standard 4.840-inch-bore-space engines with 11-, 14-, and 18-degree intake valve angles, and they accept industry-standard Big Chief–style shaft rockers, valve covers, etc. A dedicated 18-degree Nitrous Special version can be had in either 4.840- or 4.900-inch bore-space versions. For 5.000-inch bore-space motors, they offer 14- and 18-degree heads with valve sizes so large they only fit motors with a minimum of 4.600-inch bores.
Symmetrical port heads are available for 5.000-, 5.200-, and 5.300-inch-bore-space engines in either cast or billet aluminum with whatever combination of valve angles, port shapes, and chamber volumes are needed for ultra-high-horsepower Pro Mod, truck pulling, and mountain motor Pro Stock racing. Symmetrical ports differ from spread port heads in that every intake runner is evenly spaced, producing eight identical flow paths throughout the engine. This puts every intake port directly on top of one of the head-bolt holes, requiring special head attachment hardware and installation procedures. If you want to make more than 2,000 hp, you have to work at it!
When Dart’s founder, Richard Maskin, cut his teeth in NHRA Pro Stock racing in the 1970s, Pro Stockers had to use real production cylinder heads but you could do just about anything to them. Since the AMC V-8 he was using (he was always a bit of a “Rebel”) didn’t have much to offer in the form of competitive cylinder heads compared to the big-name marquees, he sectioned two pairs of AMC heads horizontally and grafted then together to create a pair of very tall port, very competitive cylinder heads that kept his team in the thick of competition. This innovative approach to racing engine development led to the creation of Dart Machinery. When the Pontiac Super Duty Pro Stock head was introduced, he refined that design and came up with the Dart Big Chief spread port head that held several NHRA Pro Stock records until it was outlawed.
Thankfully, most Sportsman class big-block racers don’t have to comply with restrictive rules and regulations governing cylinder head selection, and Dart’s Big Chief heads have become the standard spread port heads by which all others are judged. In the arena of conventional siamesed port heads, Dart also continues to innovate, offering very competitive 24- and 18-degree heads.
Dart Iron Eagles are budget-priced cast-iron rectangular port heads offered in two runner sizes: 308 cc for small displacement big-blocks and 345 cc for biginch, high-RPM engines. With 2-degree rolled valve angles and standard spacing, they are a direct bolt-on replacement for stock Chevy heads and offer a great bangfor- the-buck option to junkyard stock castings. Like all Dart aluminum heads, PRO 1 24-degree heads are cast from virgin 355-T6 aluminum for its greater strength than standard A356 alloys. They are available with as-cast 310-, 325-, and 345-cc intake ports. PRO 1 CNC heads are fully CNC machined throughout, with 335- or 355-cc intake ports. Heartshaped 121-cc combustion chambers hold 2.300-inch intake and 1.880-inch exhaust valves.
Dart PRO 1 CNC head is available with 335- or 355-cc fully CNC machined intake ports. (Photo Courtesy Dart)
Dart Big Chief spread port head has D-shaped exhaust ports to equalize flow along the short turn in the ports and help to combat reversion. (Photo Courtesy Dart)
Dart Race Series 24-degree heads are offered with rectangular 320-, 360-, or 410-cc intake ports and 119- or 139-cc combustion chambers. The 360-cc versions are available as a solid casting for alcohol-fueled drag racing engines. Oval port Race Series heads with 340- or 370-cc intakes come with 119-cc chambers fitted with 2.250- or 2.300-inch intake valves and 1.880-inch exhausts. The 18-degree oval port heads feature a raised intake port floor and small 102-cc chambers with 330- or 383-cc intake port options.
Dart’s acclaimed Big Chief spread port heads are offered in many configurations including valve angles of 18, 14, or 11 degrees and with oval or rectangular intake ports measuring 402, 421, 433, 440, and 512 cc. Intake valves are 2.400 or 2.470 inches, and exhausts are 1.800 or 1.900 inches.
One of the oldest and most respected names in the high-performance and racing parts business, Edelbrock offers big-block cylinder heads in a variety of configurations specifically designed for the application, whether it is a modest street performance vehicle, a street and strip/weekend warrior, or all-out competition car. Most are traditional siamesed port heads, but the Big Victor is an 18-degree spread port design with several unique features not found in similar heads.
Performer and Performer RPM heads are clearly aimed at the street market, offering bolt-on convenience with solid performance improvements over stock castings. Performer big-block heads even retain the exhaust crossover passage for emissions compliance, and work with EGR and/or exhaust heated chokes. Their 290-cc version of the oval intake port is actually a small rectangular port that fits within the stock oval port window, and is compatible with traditional oval port intake manifolds. Port matching the manifold to this new shape is not necessary, and in fact the mismatch helps to combat reversion.
They retain traditional valve angles and sizes of 2.19/1.88 inches (intake/ exhaust) and are available with 110- cc chambers similar to the semi-open chamber Chevy heads. The Performer High-Compression 454-O has been rolled 11⁄2 degrees to produce a 100-cc chamber that gives a compression ratio of 9.2:1 with 454 flat-top pistons.
Performer RPM 454-R heads have rectangular 315/300-cc (long/short) intake ports and 118-cc open chambers. A revised spark plug location promotes better combustion, but they do not clear high-dome pistons based on the LS6 Chevy chamber.
Edelbrock Performer 454-O oval port head has a small rectangular port that fits within the stock oval port window, and is compatible with traditional oval port intake manifolds. Performer heads retain the exhaust crossover passage in the center of the intake flange for emissions compliance. (Photo Courtesy Edelbrock)
Edelbrock Performer High-Compression 454-O oval port head has 100-cc semiopen combustion chambers that produce a 9.2:1 compression ratio with flattop pistons in a 454. (Photo Courtesy Edelbrock)
Edelbrock/Musi Victor 24-degree CNC large oval port head is fully CNC machined and assembled for 950-plushp potential right out of the box. (Photo Courtesy Edelbrock)
Edelbrock E-CNC 355 rectangular port cylinder heads are designed for very large street Rats and mild race applications. They feature fully CNC machined ports and 110- or 118-cc combustion chambers. They have a 24.5-degree intake valve angle and intake port volumes of 348/356 or 344/352 cc. Stock exhaust port locations provide a no-hassle fit with any big-block header designed for stock applications.
The Victor 24-degree head is competition oriented with revised valve angles and locations specifically for large-bore (4.470 inches or more) big-blocks. Their 340-cc intake ports feed 119-cc combustion chambers via 2.300/1.900-inch stainless steel valves, and exhaust port exits are raised 3/4-inch.
The Victor Jr. 24-degree head is a similar design for smaller bore engines (4.310 inches minimum) with 300-cc runners, 118-cc chambers, and 2.250/1.900- inch valves.
The Edelbrock/Musi Victor 24-degree CNC head is a large oval port member of the Victor 24-degree family, designed in cooperation with noted Pro/street racer Pat Musi. With fully CNC machined 377/367-cc intake ports and 114-cc chambers, it has 950-plus-hp potential right out of the box on current 555- to 582-ci normally aspirated racing engines.
The Big Victor Spread Port 18-degree CNC is Edelbrock’s version of the latest spread port cylinder heads. It has an 18-degree intake valve angle, is fully CNC ported, and assembled with 2.500/1.960-inch stainless steel valves in 92-cc combustion chambers. Intake port volume is 405 cc, and it is treated to a special Hot Isostatic Pressing process which compacts the casting, yielding a much denser and stronger part. The valvetrain configuration was re-engineered in cooperation with Jesel Valvetrain Components to address several problematic areas with other spread port heads. The Big Victor head requires dedicated Jesel shaft rocker arms and unique valve covers, since one of the valve cover bolt bosses in traditional spread port heads interferes with the ideal rocker arm placement.
General Motor Performance Parts (GMPP)
GMPP offers a large number of high-performance heads suitable for your Rat motor recipe. Of particular interest to owners of Gen V/VI engines are the cast-iron rectangular port heads (PN 12562920, casting number 12562934) as used on the 502 HO crate engine. They have 325-cc intake ports and 118-cc combustion chambers with 2.180/1.880- inch valves. Some came with 7/16-inch rocker stud bolt-holes, while most have 3/8-inch stud holes that require special studs to convert to adjustable valvetrain. Aluminum oval-port Bowtie street heads are available in several configurations with 290-cc intake runners, 110-cc semi open combustion chambers, and either 2.19/1.88- or 2.25/1.88-inch valves. It also uses a small rectangular port shape that fits oval port intake manifolds. PN 12363408 is an NHRA-legal replacement for the L88 aluminum head used on 1968–1971 Corvettes and 1969 Camaros (original PN 14011076).
The same GMPP heads used on the 572 crate engines can be yours by ordering PN 12499255 (hydraulic roller lifter valve springs) or PN 88961160 (mechanical roller lifter valve springs). They come with modest 310-cc intake ports that produce outstanding low-end power on the street, 118-cc chambers, and 5/8-inch raised exhaust port exits. (Photo Courtesy GMPP)
PN 12363400 is an aluminum 300-cc rectangular port head with 118-cc open chambers and 2.25/1.88-inch valves. (Photo Courtesy GMPP)
The same heads used on the 572 crate engines can be yours by ordering PN 12499255 (hydraulic roller lifter valve springs) or PN 88961160 (mechanical roller lifter valve springs). They come with modest 310-cc intake ports that produce outstanding low-end power on the street, 118-cc chambers, and 5/8-inch raised exhaust port exits. Valve sizes are 2.250/1.880 inches.
The current “bad-boy” Bowtie race head is PN 12363425 (casting number 14044861), a bare head with 380-cc intakes and valve seats for 2.19/1.88-inch valves. Exhaust port exits are raised 3/4 inch and feature a distinctive vane, giving rise to the popular nickname of the “W” port head. The vane improved the flow numbers by filling in an area of low-flow activity on the exhaust side, but contemporary gearheads have found other ways around that problem; most head specialists remove the vane when preparing a set of these heads.
Pro-Filer Performance Products
Pro-Filer offers its Sniper cylinder heads in siamesed port as-cast or CNC machined versions, plus the Hitman 12-degree spread port head for standard 4.840-inch bore space engines and symmetrical port heads for 4.900-inch and 5.000-inch-bore-space big-blocks. The conventional Sniper heads feature 24-degree intake valve angles and 290- or 320-cc oval intake runners (as-cast), or 365-cc and 385-cc CNC machined rectangular ports with 2.300- or 2.350-inch intake valves and 1.880- or 1.900-inch exhaust valves. The Hitman 12-degree spread port head come with 415- or 470- cc large oval intake ports and 2.400/1.880- inch valves. The 12-degree valve angles allow very small 60-cc combustion chambers for maximum compression with flat top or small-dome pistons.
CONTINUE YOUR CHEVY READING
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