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.
Factory Cylinder Heads
When comparing factory Chrysler big-block cylinder heads we can break them down into three basic categories: Max wedge, open chamber, and closed combustion chamber. We won't consider the pre-'64, non-Max-Wedge heads because the rocker shaft and bracket system they utilized is not compatible with modern valvetrain components. Max Wedge heads are easily identifiable by the lack of a heat cross-over passage, large 1.88-inch exhaust valves, and very large ports. Casting numbers for the Max Wedge heads end in either 286, 209, or 518; keep an eye out at the swap meets in the unlikely event you'll run across a set of these desirable heads. While the Max Wedge heads do offer great power potential, they are expensive and hard to find, so unless you're building a Stock or Super Stock class racer, or restoring a factory race car, the cost of these heads is prohibitive for the average build.
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.
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.
Factory Chrysler big-block...
Factory Chrysler big-block heads, such as the 915 and 452 castings shown here, are plentiful and make respectable power even in stock form. These heads are a good choice for a stock engine or mild street engine and respond well to porting, but valve and port size limit their power potential.
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.