Hemi's Back For DC's Heavies

CHELSEA, MI — The storied Hemi engine's back from Chrysler Corp.'s past, but this time it's thanks to a little help from its friends. Chrysler's merger friends, that is, at DaimlerChrysler AG's Mercedes-Benz unit.

The claim to fame for the all-new '03 Hemi Magnum V-8 — to be launched this fall in DC's all-new Dodge Ram heavy-duty pickups — is its unique hemispherically shaped combustion chambers. But an equally engaging design feature is the twin-sparkplug arrangement that makes for outstanding driveability, power and low emissions.

The twin-spark is prominent in Mercedes' modular V-6 and V-8 engines. In the spirit of oft-discussed synergies, Chrysler Group engineers liked what they saw in the Mercedes twin-spark system, says Robert Lee, director, RWD Engine Engineering-Powertrain Product Team.

But bringing in the twin-spark layout wasn't easy, says Lee, noting that the Hemi's basic design and development already was well under way before the notion arose to enhance it with the twin-spark system.

“They (Mercedes engineers) showed us a lot of data,” says Lee. The figures convinced Chrysler engineers that the job of adopting the dual-plug ignition was worth the extra effort. Lee says that in addition to its contributions to power and low emissions levels, the twin-spark system allows an extremely smooth and stable idle speed of just 500 rpm.

The 5.7L Hemi Magnum V-8 develops 345 hp at 5,400 rpm and 375 lb.-ft. (508 Nm) of torque at 4,200 rpm. The block is cast iron and the heads are aluminum. It employs a basic 2-valve-per-cylinder layout with pushrod-activated valves.

Lee says his team studied a variety of valvetrain designs that included 3- and 4-valve combustion chambers and dual camshafts in the block. The 3-valve setup was deemed “almost impossible” to incorporate into the hemispherical combustion chamber, and dual cams for each cylinder bank (four camshafts altogether) seemed too complex. Lee boasts that one of the Hemi's most endearing aspects is its relative simplicity.

“I was looking for airflow,” says Lee of the final Hemi design. “Get airflow and you can make power.”

“Power” to Lee's team meant two valves per cylinder — and the hemi combustion-chamber shape from which the engine gets its name. Although the original Chrysler Corp. used engines with hemi heads prominently in NASCAR racing, the Hemi that enthusiasts came to revere didn't make it into street cars until 1966, in Chrysler's intermediate-body models; the 429 Hemi engines were gone by 1971. The '03 Ram heavy duties are not even the first Chrysler-badged trucks to enjoy Hemi fitment: Chrysler produced a Hemi V-8 for pickups in 1954, a 241 cu.-in. (3.9L in today's vernacular) for the 2.5-ton K-series.

For now, Lee says that the dual-plug system for the Hemi does not follow Mercedes' practice of “staggering” ignition to more-precisely manipulate combustion in relation to engine operation. That feature can be incorporated later, he adds, but to do so requires a larger amount of engine-management software development that currently isn't necessary.

One problem that cropped up after the decision to adopt the twin-spark design for the Hemi: The basic architecture of the Hemi head meant that the two spark plugs had to be situated slightly offset from one another, which adds a step in the machining of the head. “That cost a couple hundred thousand dollars,” laments Lee.

Lee says DC has built a new facility to build the Hemi Magnum at its vehicle and engine production site in Saltillo, Mexico. Capacity is 400,000 units annually, which suggests future usage beyond the needs of the Ram heavy-duty pickup program.