Large intake ports make factory...
Large intake ports make factory Chrysler cylinder heads, such as the popular X casting shown here, good performers for mild to moderate applications. With port work and proper camshaft selection, we've seen these heads make upwards of 400 hp on a moderately prepped 360 short-block.
Factory Chrysler Cylinder Heads
The Chrysler LA (light A) engine began production in 1964 as a derivative of the poly 318, and since that time many cylinder head variations were produced. For the purpose of this article, we won't go into the many different casting numbers assigned these heads, but rather break them down into a few basic categories and concentrate on the ones best suited for mild, moderate, or race applications. The two basic types of early cast-iron production cylinder heads are differentiated by their port and valve sizes.
Early factory heads
Early 273 and most 318 engines utilized small port castings and are most easily identified by their 1.75-inch intake and 1.50-inch exhaust valves. The larger port 340 and 360 heads came with 1.60-inch exhaust valves and either 1.88- or 2.02-inch intake valves. For a mild street application or restoration, the small valve/small port cylinder heads will work fine,, but power will always be limited. We don't suggest the small port heads if you're planning to make any more than 300 hp because the machining required is cost prohibitive. Also, while outwardly the '64 and '65 273 heads have the appearance of the later 273 and 318 units, they may not be interchanged without also changing intake manifolds due to the intake bolt holes being machined at a different angle. While we don't seek out the 273/318 heads for performance builds, we do keep our eyes open for the older 273-4bbl. engines as they had mechanical valve gear that can be used in the later cylinder heads.
The main drawback to the X,...
The main drawback to the X, J, and 915 castings is the open-style combustion chamber as shown here. These heads do provide good flow for a stock casting, but more power can be attained by using heads with a closed, quench style, combustion chamber.
The 340/360 large port cylinder heads are much more desirable factory units for a performance build. These heads are most often found with the 1.88-inch intake valves, but were also produced with 2.02-inch intake valves in 1968 through 1971. The notable "X" and "J" heads (named for an X or J cast into the head near the spark plug hole) fall into the large port category, as do cylinder heads with "915" as the last three digits of the casting number. AAR and TA E-bodies with the 340 6-barrel option, while sharing the same casting as other X and J heads, were machined for relocated push rods and required special valve gear. It's rare to find these heads today as they were very limited in production. Regardless of the casting number, the large port heads all offer similar potential and are the best of the early small-block cylinder heads. The drawback to these heads, however, is their open-style combustion chamber. In mild applications the X/J casting heads work great, flowing enough air to easily support most street engines and mild bracket race combinations. If more power is required, the later swirl-port or magnum heads would be a better choice.
Pictured here are the swirl...
Pictured here are the swirl port (closest), magnum, and Edelbrock Performer RPM cylinder heads. Notice the closed combustion chamber of the swirl port head, which greatly increases its power potential. Even with relatively small valves, the swirl port design head will make more power than X or J castings simply due to the better combustion chamber design.
Swirl Port Heads
Chrysler's small-block cylinder head design changed very little until 1987 when the 302 casting (last three digits of the casting number), commonly known as the swirl port, was introduced. While the 302 casting, as well as the later 308 casting, were most commonly found on low-performance 318s, they shouldn't be overlooked for a performance build. Even with small 1.75-inch intake and 1.50-inch exhaust valves, the design of this head's heart-shaped quench-style combustion chamber makes it a good choice for mild to moderate performance. An engine with these heads in stock form will outperform an engine with stock X or J castings simply due to the improved combustion chamber design. Additionally, these heads have hardened valve seats, which make them compatible with unleaded fuel. The swirl port heads were utilized through 1991 (302 casting) and 1992 (308 casting) and respond well to port and bowl work, offering better power potential than all of the earlier small-block heads. Another nice feature of these heads is they utilize valvetrain and intake manifolds common to early small-blocks, so parts are readily available. Simply bolt these heads to your small-block and enjoy the extra power they provide.