Closed Chamber Factory Heads
The closed chamber heads were manufactured and installed on pre-'68 vehicles; popular units have casting numbers ending in 915 or 516. These heads are easily recognizable by their closed, quench-style combustion chamber and are actually a good choice for a performance build. The Chrysler engineers knew the quench area of the combustion chamber was key to power production, and these heads utilize that theory very well. The main drawbacks of the closed chamber heads are relatively small 1.60-inch exhaust valves and the lack of hardened valve seats. An exception is the '67 440 HP 915 casting, which is the first big-block head to incorporate a 1.74-inch exhaust valve, making the '67 440 HP 915 a good choice for a performance build. In fact, an old racers' trick was to swap 915 heads onto a '68 440 HP, which netted some 13:1 compression! Of course, that would never work with today's pump fuels, but the combination does make great power if race fuel is utilized. Cylinder heads with the 516 casting number are virtually identical to the 915s, but were only produced with 1.60-inch exhaust valves.
 This picture shows the 915... This picture shows the 915 casting number as well as the closed chamber design of these heads. The quench-style combustion chamber is ideal for making power even though most units incorporate relatively small 1.60-inch exhaust valves. |  When the 915 head (right)... When the 915 head (right) is compared to the later 452 casting (left), the differences are apparent. Though the 452 head does have the advantage of larger exhaust valves and hardened seats, the combustion chamber of the 915 heads is better suited for making power. |  The small, 1.60-inch exhaust... The small, 1.60-inch exhaust valve (shown here) is typical of most 915 or 516 castings. When compared to the 452 casting on the right, the difference is apparent. Cutting the existing seats, or better yet, having hardened seats installed to accommodate larger exhaust valves is a great way to get extra power from closed chamber heads. |
While the combustion chamber of the closed chamber factory heads is well suited for power, there are some drawbacks to these cylinder heads. First, these heads were manufactured when there was plenty of lead in fuel to keep the valve seats lubricated, so the valve seats didn't need to be hardened. Running an engine with these heads on today's fuel can lead to valves, especially on the exhaust side, becoming recessed into the head, resulting in substantial power loss or even broken or dropped valves. Second, the exhaust valve size limits the flow of these heads and, thereby, also limits their power potential. To remedy these drawbacks, hardened seats and larger valves can be installed, which give these heads the flow potential of later castings and the ability to run on unleaded fuel while retaining the desirable closed combustion chamber. Another issue with any factory steel head is the shape of the valveguide and seat pocket, which limits valvespring choices. The only way to solve this problem is to machine the outside of the guides and the seat pockets to accommodate double or triple valvesprings. Add to the above operations the cost of porting these heads, and you can quickly have as much tied up in your steel heads as the cost of an economical set of aluminum units, so it's hard to justify the cost unless you're restoring a rare car or racing in a class that dictates a steel head.
Open Chamber Factory Heads
In 1968, Chrysler big-block cylinder heads were redesigned to be manufactured with an open-style combustion chamber. The engineers knew this was not the best power producing combustion chamber design, but foresaw tighter emissions standards and substandard fuel so the 906 casting was developed to take the Chrysler big-block into the future. Knowing the performance enthusiasts wouldn't like the new cylinder head design if it didn't make the power of the closed chamber designs, Chrysler decided to upgrade the open chamber heads with larger, 1.74-inch exhaust valves. Intake valve size remained at 2.08 inches. Designing pistons to reach the top of the cylinder (or above) in their performance models gave engines with the 906 heads similar compression ratios to engines with closed chamber heads. The 906 heads were used on every big-block engine between 1968 and 1971 and will make decent power in stock or ported form. After 1971, the casting numbers changed, but the overall design of the ports and the combustion chamber of the big-block head didn't. All of the post-'68 open chamber heads offer similar power potential and work equally well in a performance application.
 Notice how the outside diameter... Notice how the outside diameter of the intake valveguide prevents the installation of double or triple valvesprings. This problem is common to all factory steel heads and can only be remedied by machining the outer circumference of the guide or by replacing the guides entirely. |  This 452 casting shows the... This 452 casting shows the open-style combustion chamber common to the '68-and-up big-block Chrysler cylinder heads. These heads had limited potential due to the open chamber design, but made up for it with larger, 1.74-inch exhaust valves. |  Casting numbers can be found... Casting numbers can be found on the top of the intake runner, as well as the bottom side of the head under the intake ports. The 906 and 452 are the most common open chamber heads, but many casting numbers were manufactured between 1968 and 1978. All of the open chamber heads have similar power potential. |
While the open chamber design limits the power potential of the post-'68 heads, these heads do offer the builder several advantages. These heads are a good alternative if compression ratio would be too high with a closed chamber design. Also, they came from the factory with 1.74-inch exhaust valves, so the extra expense of machining the seats and purchasing bigger valves is saved. It is recommended, however, to install hardened seats in the pre-'74 heads as they suffer from the same limitation as the closed chamber units. In our opinion, the best open chamber head is the 452 casting, which was produced from 1976 through 1978. These castings offer the same combustion chamber design, flow, and performance potential as the 906 castings, but they already incorporate hardened valve seats, making them very tolerant of today's unleaded fuels.
 Factory steel heads offer... Factory steel heads offer decent performance in stock form, but really respond to port work. These 452 castings have been ported extensively and made close to 600 hp on a moderately prepped 440. Mild port work can be accomplished with a template kit from Mopar Performance, but for optimal flow we recommend a flow bench. |  The port size difference is... The port size difference is apparent in this photo. The near head is an unported 915 casting; the far head is a fully ported 452. In ported form, steel heads can be good for around 600 hp on a well-prepped, high-compression race engine, but the cost to modify them may make an aftermarket head a better choice. | |