Open Chamber Factory HeadsIn 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.
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.
Aftermarket Cylinder HeadsIf your combination requires more flow than a factory head can support, or if you desire the weight savings of an aluminum cylinder head, an aftermarket head is your only choice. Edelbrock, Indy Cylinder Head, Brodix, and Mopar Performance offer numerous choices that can accommodate your need for power as well as your budget. Choosing the right aftermarket head depends on what you wish to achieve from your combination. Placing a large port, high-flow race head on your stock 383 is not a good choice, and the results will probably disappoint you. Conversely, a budget aftermarket head will certainly restrict that big cubic inch race motor you're building, netting similar, disappointing results. In this section we'll outline aftermarket cylinder head selection by manufacturer and give you the lowdown on which head may be right for your combination.