It was in 1963 that Chevrolet used what was then called the Mystery Motor in their record-setting Daytona 500 stock cars. This new “big block” was a substantially modified version of the W-series (348/409) engine. In 1965, it was released for production as the Mark IV, Turbo-Jet V8.
The Mark IV considerably differed from the W-series engine, especially in regards to the cylinder heads. The previous engine utilized a “chamber-in-block” design. This design did not use an actual chamber that was cast into the cylinder head. Rather, the surface of the head was flat, and the “chamber” was the area at the top of the cylinder where the piston did not travel. With the Mark IV engine, this design was replaced with a more conventional wedge-design cylinder head. This moved the chamber into the cylinder head. One thing that didn’t change was the valve orientation. While the valves continued to use the displaced arrangement of the previous W-series engine, they were also inclined a few degrees so that they would open away from the combustion chamber and cylinder walls. This change in valve placement gave the new cylinder head a significant improvement in volumetric efficiency at high RPM. This resulted in a big increase in power, especially at high engine RPM.
As part of the new cylinder-head design, the spark plugs were relocated in a way that they now entered the combustion chamber at an angle relative to the cylinder centerline. The W-series design placed the spark plugs at a straight-in position. When this new cylinder-head design was established, Chevrolet then decided to create two distinct versions of the cylinder head. If a vehicle was ordered with as “high-performance” with a big block, that engine received rectangular-port heads. More “pedestrian” engines (read passenger car and truck), received the oval-port cylinder heads.
The obvious appearance difference of the ports also has an effect on performance. The rectangular ports are physically larger than those of the oval-port cylinder head, therefore they have higher airflow-volume capabilities than production oval port heads. However, the larger volume of the rectangular ports does have a drawback.
In order to efficiently get the air/fuel mixture into a cylinder for combustion, you must have a reasonable amount of port velocity within the intake port. The velocity of the air/fuel mixture will be greater when traveling through a smaller port, but the small port will typically lack the ability to carry a large volume of air like is required for high-rpm power. Likewise, a rectangular port will have more volume, but less velocity, which could hurt the low and mid-range RPM torque levels. This is why using either cylinder head on the wrong application “could” hinder performance.
In a nutshell, choosing the correct OE head for your big block comes down to what you want your engine to do. Unless you are building a full-tilt race engine (which would probably use an aluminum head anyway), an oval port head might be right for you. If your engine will not spend much time above 6,000 to 6,500 rpm, the added flow given by the rectangular port will probably not be necessary. If you are looking for a set of oval-port heads, look for the following casting numbers; 336781, 353049, 3992241, and 3993820.
If you are planning to build a race-specific engine that will spend most of its time above 6,500 rpm, then getting a set of rectangular-port heads would probably be your best choice—other than using an aftermarket aluminum head. Although we didn’t previously mention it, we must also make note that the rectangular port heads were available in both closed and open-chamber designs (Mark IV and Mark V). Using the closed-chamber cylinder head will have a huge effect on the final compression ratio, and the flow characteristics of the closed chamber led to the introduction of the open chamber design. When sourcing a set of OE cast iron rectangular-port cylinder heads, spend your time look for the following casting numbers; 6272990, 14096188, 6260482, or 3994026.