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Complete Cylinder Heads Guide for Big-Block Chevy Engines - Pt 1

Nashville Cat

Veteran Member
Senior Member

Complete Cylinder Heads Guide for Big-Block Chevy Engines - Pt 1​


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.

Aluminum aftermarket cylinder heads offer tremendous airw 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). Latemodel 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 righthand 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 raceonly 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 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.

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.

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 bigblock 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.

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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.

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Cast-iron late-1960s “206” head with bathtub-shaped 98-cc closed combustion chambers.

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Cast-iron high-performance rectangular port “990” head with open 118-cc combustion chambers. Note the lazy-D shape of the chambers.


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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 differentsize runners have the same flowbench 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 raisedrunner 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 highdome 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.

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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.

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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 valvestems, 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-inchdiameter valvestems, and frequently use longer than stock valve lengths that allow installation of taller valvesprings that are better suited for use with highlift 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.

 
I never knew 1.84 exh. valve was ever in a BBC head, learned something new and would have probably ordered those when I was having a set of 1969 "063" heads rebuilt instead of the 1.88 valves
 
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