Hemi Research and Development

Chrysler used a unique single-cylinder engine to evaluate cylinder head designs with a compression ratio of 7.0:1. The familiar L-head and F-head were tested, as well as a conventional valve-in-head design and a hemispherical cylinder head. Chrysler had practical reasons for wanting an improved cylinder-head design. Combustion-chamber deposits could significantly reduce power over a given period. These deposits affected thermal and volumetric efficiencies, resulting in power losses of as much as ten percent after only 5,000 to 10,000 miles. A combustion chamber with a low surface-to-volume ratio minimized these losses. All indicators were pointing to a hemispherical combustion chamber, which inherently had a high thermal and volumetric efficiency as well as low surface-to-volume ratio.

Hemispherical combustion chambers were really nothing new. One of the first, if not the first, uses of the Hemi head was on the 1904 four-cylinder Welch passenger-car engine. The Hemi head was the design of choice for such famous racing marques as Duesenberg, Stutz, Miller and Offenhauser.

The fledgling aircraft engine industry seized upon the hemispherical cylinder head as the best power-producing design for air-cooled engines. In fact, during World War 11, Chrysler engineers were engaged in developing a V-16 air-craft engine and a V-12 tank engine, both of which had hemispherical combustion chambers. Although neither of these engines reached production status due to the war's end, their performance and efficiency were excellent, impressing the engineers involved.

With some known advantages of a hemispherical combustion chamber, why hadn't it achieved more widespread use in automobiles? There were two basic reasons, Chrysler found. The Hemi head had a reputation for roughness and proclivity for high-octane fuels. Also, the inherent complexity and cost didn't lend itself to mass production. However, Chrysler Corporation had years of experience building relatively high-compression engines. In 1924, Chrysler introduced high compression in its namesake line of cars. Four years later, it turned the automotive world on its ear with the 1928 Red Head engine with an unheard of 6.2:1 compression ratio.

The Chrysler engineering team was up to the task of developing a truly revolutionary passenger-car engine. James C. Zeder was director of Engineering and Research. He was the younger brother of Fred Zeder, who, along with Carl Breer and 0. R. Skelton, left Stude-baker in 1920 at the behest of Walter P. Chrysler to establish the Chrysler Corporation. Working under James Zeder was Ray White, in charge of experimental design; William E. Drinkard, head of laboratory research and development; and Mel Carpentier, in charge of production engine design. These men relished the challenge of working the bugs—real or imagined—out of the Hemi head and making it viable for mass-produced automobiles.

Chrysler began testing a great many passenger-car engines from both domestic and foreign auto makers. Ev Moeller witnessed these engines being tested. He started with Chrysler in 1939 and graduated from the Chrysler Institute in 1941. He was assigned to the air-craft engine program during the war, and in 1947, he was moved to the Engine Development Laboratory.

"We tested every engine in site," Moeller says. "One of the engines that we worked on was the Healey, which was an English small passenger-car engine that had a pushrod-operated over-head valvetrain. It had two camshafts, one on each side of the block. The push-rods came up to operate the overhead valves. It was a long-stroke, Hemi configuration and the thing that was surprising was it was the highest powered and most efficient engine we had ever tested."

Lab testing of the single-cylinder engine with the four different head designs was supervised by John Plainer, who had joined Chrysler in 1931 as a member of the first class of the Chrysler Institute of Engineering Graduate School. Platner worked in Engine Development his entire career with the company, and made his presence felt over the years.

What they found was contrary to what had been doctrine concerning the Hemi head: this cylinder head actually displayed knock-limiting characteristics. To achieve the thermal efficiency of the Hemi head at 7.0:1, the L-head required a compression ratio of 10.0:1 at 1200 rpm, 9.4:1 at 2000 rpm, 8.9:1 at 2800 rpm and 8.5:1 at 3600 rpm. The F-head performed much like the L-head. The overhead-valve head was somewhat better than these two, but it suffered losses of volumetric efficiency and valve durability.

The Hemi head was a superior performer in all respects. With the intake valve closest to the intake manifold and the exhaust valve directly across from it on the other side of the hemispherical chamber with an included valve angle of 581/2 deg., the fuel-air mixture entered, burned and exited efficiently. The Hemi head design also extended valve life and promoted sealing by aiding effective uniform cooling of the valve seats. The engineers found the stability of the valve seats in the Hemi head to be excellent.

With these encouraging results, Zeder's team decided it was time to test the Hemi head on one of its engines. A standard Chrysler straight-six engine was selected and a special double-overhead-camshaft head with hemispherical combustion chambers was designed and built. Bench tests showed a significant performance gain over the standard six-cylinder engine. The engine, with the designation A161, was installed in a Chrysler to test real-world conditions by Wallace E. Zierer.whowas in charge of automobile testing. The car ran effortlessly on the then-standard eighty-octane regular gas. Roughness, which had been a theoretical concern, was absent, thus affirming the use of a Hemi head in a passenger car. It was a research and development milestone for Chrysler.

From a production standpoint, however, the chain-driven double-over-head-valve design was complex and expensive, and serviceability would be a problem. A simpler, less costly valvetrain design had to be worked out. What the team developed was a pushrod-actuated Hemi-head straight-six with a greatly simplified valvetrain at less expense.