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.