Cylinder heads are understandably a vital ingredient in any recipe for "porcupine" horsepower pie, so whether the big-block in your Corvette is being built for street performance, drag racing, road racing, autocrossing, or simply holding down the garage floor, the heads must be properly matched to the pistons, camshaft, and intended usage if an engine is to deliver anticipated performance. Countless numbers of big-blocks have had all visions of glory destroyed by misdirected "experts," who either selected the wrong castings, or took a wrong turn while modifying them for maximum power. Such mistakes aren't uncommon because ego and poor judgment frequently win out over common sense.
In spite of the vast array of cylinder heads produced by Chevrolet and a host of aftermarket manufacturers, there are usually only a few models that are truly applicable for an engine of a particular personality. Just as passenger car heads rarely provide the ultimate power levels on a race engine, race heads usually provide less than impressive results when installed on a true street engine. It all has to do with combustion chamber volume, port size and shape, and air flow characteristics.
Big-block cylinder heads cast by GM are most commonly identified by their casting numbers, which may be found in a variety of locations depending upon the particular head. However, the complete casting number is located beneath the valve cover, usually above an intake port. Additionally, either the last three digits of the casting number or the complete number can usually be found beneath one of the intake ports when a head is viewed from the combustion chamber side.
Rather than the complete number, only the last three digits are commonly used when referring to a specific casting. As an example, in machine shops, bench racing parlors, and other houses of high performance, casting number 6272990 is called simply a "990" casting. In the real world, the first four digits are virtually meaningless, as is the actual part number (6260482), which is used only when ordering a head from a Chevrolet dealer's parts department. However, once a cylinder head is out of its original box, a part number is of no use because the only identifying marks on the head itself are the casting numbers.
Back in the day when the big-block was introduced (1965), Chevrolet offered a variety of engines, each designed to deliver a specific level of performance. As fate would have it, cylinder heads were also designed to deliver specific performance characteristics. Big-block (Mark IV) engines originally configured for low and medium performance were fitted with cylinder heads having oval-shaped intake ports; high-performance big-blocks derived increased breathing capacity from heads with rectangular intake ports. The word "Pass" cast into a big-block head identifies it as being of the oval port persuasion; "Hi-Perf" cast into a head denotes rectangular ports. Early models of both oval and rectangular port heads have a bathtub-shape closed chamber; 1971 and later castings are usually of the open-chamber design.
Another characteristic that distinguishes early from late-model cast iron heads is the spark plug hole. The '69 and earlier heads are machined for gasketed 31/44-inch reach plugs (Champion "N" type) with a 131/416-inch hex; 1970 and later heads require a tapered seat 11/42-inch-reach plug with 51/48-inch hex. The aluminum heads installed on production engines continued to be machined for 31/44-inch-reach 131/416-inch hex spark plugs.
A third type of big-block head is the one used on truck engines. These heads have almost round intake ports and very small valves. They're about as applicable as a one-barrel carburetor for a Corvette engine, but some people mistake them for passenger car oval port castings.
In addition to open and closed chambers with either rectangular or oval intake ports, GM big-block Chevy heads are also available in either cast iron or aluminum. As you might expect, all factory-installed aluminum heads incorporated rectangular ports. It wasn't until the Performance Parts division made a serious entry into the aftermarket cylinder head business that oval port "GM" aluminum heads became available.
In 1992, the world of big-block Chevy heads got even more diverse with the introduction of the Generation V engine. Although this was long after the last big-block Corvette was produced, Gen V heads are similar enough to original-style big-block heads that it isn't all that unusual for someone to attempt to install them on a Mark IV engine-something that can't be done without a few modifications.
The term "open chamber" refers to heads with combustion chambers having a nominal volume of 118 cc, while "closed chambers" are nominally rated at 108 cc. In spite of these "official" specifications, real world combustion-chamber volumes vary somewhat according to casting.
Regardless of intake port configuration, all big-block heads produced from 1965 to 1969 had closed chambers. The first open chamber appeared in '69 ZL-1 aluminum heads. Cast iron heads with open chambers first appeared in late 1970 for the '71 model year.
Without question, a big-block will produce more power with open-chamber cylinder heads. In fact, the improved air flow and combustion efficiency offered by the open-chamber design is frequently sufficient to more than offset the loss in compression ratio (which can be over a full point) produced by the increased combustion-chamber volume.
Oval Port Iron Castings
For anything less than a full-tilt, take-no-prisoners race engine, oval port heads are actually the hot ticket. Such a statement may seem to arise from a softening of brain tissue, but it is in fact true. While there's no question that rectangular port heads unquestionably offer greater maximum horsepower potential, it is also true that an extremely aggressive engine, (super-high compression ratio and fender-shaking cam) spun at high rpm is required to take full advantage of the air flow potential.
At less than tach-warping engine speeds, the smaller cross-sectional area of the oval-shaped intake port promotes higher velocities, which results in superior cylinder filling. Depending upon the degree of port rework, this advantage extends up to approximately 6,500 rpm with a 454, and somewhat higher with smaller engines. Above that speed, the ports become restrictive, whereas rectangular ports are just starting to get with the program.
Dyno tests by numerous engine builders have repeatedly proven the horsepower prowess of properly prepared oval port heads. A 454-cid Corvette engine, equipped with a single Holley four-barrel or electronic fuel injection, and the appropriate camshaft and exhaust system, will easily pump out over 615 hp and 540 lb-ft of torque. In milder trim, a 454 will easily push its horsepower curve to the 575 level and put torque over 520 lb-ft.
The oval port casting numbers of interest are 336781, 353049, 3992241, and 3993820, which were originally installed on '71-'74 402 and 454 engines. These castings were originally equipped with 2.06-inch intake and 1.72-inch exhaust valves. For high-performance use, these heads are routinely fitted with 2.19-inch intake and 1.88-inch exhaust valves.
Rectangular Port Iron Castings
The definitive rectangular port, cast iron, big-block head is casting number 6272990, (or 14096188), which was once available through GM Performance Parts dealers as PN 6260482. Originally released in 1971, on 425hp 427 engines, these castings feature open-style 118cc combustion chambers, 2.19-inch intake, 1.88-inch exhaust valves, and Texas-size intake and exhaust ports. This casting is very similar to 3994026, which originally brought exceptional breathing capability to '71 LS-6 engines.
Although there are numerous other Mark IV rectangular port castings, most are of the closed-chamber persuasion, which as previously mentioned are inferior in their ability to produce horsepower. About the only place you'll find Chevrolet head castings designed for Mark IV engines are at swap meets, or through companies dealing with used/reconditioned engine parts or new old stock.
Chevrolet Aluminum Heads
Almost since the inception of the big-block, Chevrolet has offered aluminum cylinder heads that are more appropriate for racing than for mundane service on a passenger car engine. As early as 1967, aluminum castings were supplied on the famous L-88 Corvette engines. As time progressed, so did demands for horsepower and several editions of big-block aluminum heads were released to meet those demands.
In late 1968, a second design L-88 head was released and brought the term "open chamber" to the lexicon of big-block Chevy engine builders. Service replacements were available (but aren't any longer) for both versions, with casting number 3919842 matching up to closed chambers and 14011077 pertaining to open chambers.
Proliferation of big-block aluminum head designs proceeded at a feverish pace when the Bow Tie high-performance parts program kicked into high gear. Along with a myriad of head configurations came an alphabet soup of exhaust port shapes- C, D, W, and O- which can be more than a little confusing.
The 077 head was a ground-breaker because it included a vane in the exhaust port flow that was designed to reduce turbulence. This vane, which is frequently removed by head porters, gives the port a "C" shape as opposed to the "D" shape it would have without the vane. So the same port configuration is alternately described as a "D" port with a vane or as a "C" port.
Bow Tie aluminum heads with rectangular exhaust ports and vanes in the floor are often referred to as "W" ports. However, the "W" may look like a "C" to some people, so the question then becomes is a "C" port a "D" port with a vane or a rectangular port with a vane? Typically, the "C" designation applies to the production heads, while the "W" is reserved for Bow Tie heads.
After the big-block went out of production, inventories of original equipment cylinders dwindled to the point that they became virtually non-existent. GM Performance Parts has introduced a host of replacement heads over the years, but there are only a few part numbers in the current catalog that are suitable for use on a Mark IV cylinder block. None of these are iron castings, so you'll have to look elsewhere if you're trying to maintain the original appearance of a cast-iron-headed engine.
Of the available aluminum heads, casting number 12363391 features oval 290cc intake ports, 110cc combustion chambers, and either a 2.25/1.88-inch or 2.19/1.88-inch valve combination (depending on the part number). Casting number 12363401 is also available in a number of configurations, including one that's NHRA-legal for L88 engines. All feature 315cc rectangular intake ports, 118cc combustion chambers, and- with the exception of the L88 bare casting-2.25-inch intake and 1.88-inch exhaust valve seats. The L88 head has 2.19-inch intake valve seats.
A number of aftermarket companies also offer big-block head castings, and while many of these are designed for "race-only" applications, some are suitable for engines installed in Corvettes that are street-driven or autocrossed. As is the case with GM-produced heads, the key to maximum performance lies in choosing a head that offers the optimum air flow characteristics for a particular set of operating conditions. The head with the largest ports doesn't guarantee the best performance.