Complete Cylinder Heads Guide for Big-Block Chevy Engines -Pt 2

Nashville Cat · Jul 27, 2022

Complete Cylinder Heads Guide for Big-Block Chevy Engines - Pt 2

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 valvesprings 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 allout 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, smallchamber 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.

Modifications

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-torumble 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.

Intake Port
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.

Valve Bowl
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.

Valve Seat
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.

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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.

Combustion Chamber
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.

Exhaust Seat
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.