Our Hemi engine building challenge is in full swing, with the invited builders in the thick of assembling their combinations for the shoot-out to come. Chrysler's Hemi is more popular than ever, as the legendary status of these powerplants just continues to grow. What's the key to the Hemi's magic? It's not the crankshaft, pal-it's all in the heads. Though Hemi-configuration cylinder heads had been toyed with by various manufactures since the early days of the automobile, it was Chrysler that embraced the design and put them out in the kinds of numbers that got people's attention. The early Hemi of 1951-1958 set the precedent, but it was Chrysler's limited-production iron elephant-the 426 Hemi-that solidified the legend. Whether the venue was the street, or virtually any form of sanctioned racing in which it competed, the Hemi was supreme in its domination.
Chrysler had enjoyed great success with the early Hemi, however, production costs and complexity as the primary corporate V-8 powerplant saw it give way to the more modern big-block wedge. The wedge offered a very practical balance of output and manufacturing efficiencies, however, when it was time to step the power up beyond what was practical with the wedge at the time, Chrysler once again turned to the Hemi. The edict came straight from the top, with Chrysler's chief executive officer at the time, Lynne Townsend, leaning on engineering to come up with an engine package that could shatter the competition from other manufactures. Chrysler's engineering team looked at the track record of the early Hemi, and the power that combination was capable of, and set out on a course of adapting Hemi heads on a slightly revised wedge block. The eventual outcome of this effort was the famed 426 Hemi.
The 426 was closely related to the wedge big-blocks that preceded it, retaining much of the wedge motor's architecture in the bottom end. The differences were aimed at increased strength and reliability, as well as revisions necessary to accommodate Hemi cylinder heads that defined the new powerplant. The Hemi heads featured domed combustion chambers with the intake and exhaust valves set in an opposed orientation across the cylinder bore. The deep dome allowed for large valves set at an angle of 58 degrees, and a centrally positioned spark plug. With the intake and exhaust valves set at opposite extremes of the cylinder head bank, each row of valves was operated by an individual rocker shaft. This arrangement resulted in a wide cylinder head and the wide Hemi valve covers, pierced with a provision to route an ignition secondary lead to the spark plug buried within. The characteristic look defines the Hemi.
Contemporary standard wedge heads were capable of intake port flow of 200-215 cfm, and the big-port race Max Wedge heads, which immediately preceded the Hemi, delivered in the 230-240-cfm range. In contrast, the Hemi intake ports, breathing through large 2.25-inch valves, supported a massive 300 cfm of flow volume, and, with modifications, could be urged to the 400-cfm mark. Contributing to the Hemi's flow, the head design offered a much improved approach angle from the port into the cylinder, and, as is inherent in the design, the valves move away from the cylinder wall as they open, minimizing the flow-restricting effect of bore shrouding. Hemi heads are born to flow.