Pro-Stock's Elephant

Chrysler's D5 HEMI Dual Plug Engine

The D5 HEMI was an experimental 426 HEMI engine by Chrysler in the early 1970s for Pro Stock drag racing. The program was a top-secret project, as Chrysler wanted to keep the competition in the dark. Tom Hoover and Harry Weslake, of Chrysler, designed the engine to improve performance at higher RPMs, from 6000 to 9000.  The key features included round intake ports, a wider block, a fabricated tubular intake manifold, dual distributors, and two spark plugs per cylinder.

The block was a specific, heavy-duty racing block designed from the original 426 HEMI. The block will have a casting number 3577430-1 or 3577430-2 with a date of 1-19-70. It used a stronger material —a high-nickel iron alloy with extra metal —and a thicker casting than the production 426 HEMI blocks. A pushrod relief was added for the D5 HEMI heads, and the oiling system was improved, along with adding material between the camshaft journal and a thicker leading saddle to prevent cracking under the extreme stress of higher RPMs —an issue in earlier 426 production blocks.

The D5 HEMI heads were cast from a lightweight aluminum alloy with two spark plugs in a shallower combustion chamber than standard production heads.  The valves were larger than the production heads: 2.40 for the intake and 1.94 for the exhaust. Both intake and exhaust ports were rounded to improve airflow over the production square ports. Aluminum rocker stands were also incorporated. The heads required a unique round tube intake manifold.

The ignition system consisted of two distributors, a dual ignition system that fired all sixteen plugs. This allowed for more complete and efficient fuel burn at higher RPMs with high-octane fuels, within the hemispherical combustion chambers.

 While a debate on whether this system actually made a difference continues today., Some say it didn’t, and just used 8 of the 16 plugs firing, while others swear it was the only way to achieve the extra horsepower needed in the 8,000 to 9,000 RPM range, producing well over 700 hp. The D5 heads were considered innovative technology at the time. They were part of Chrysler's factory-backed efforts in the mid-70s. Teams like the Rod Shop, Sox & Martin, and Dick Landy used this engine.  Although the engine did not dominate for long, the technology learned from the D5 program contributed to the ongoing development of Hemi engine technology in drag racing.

1975 Spring Nationals Rod Shop Nationwise Team

Running D5 HEMI Engine. Video Courtesy of McGrath Keen