Chrysler engineers recently invited Automobile to its Auburn Hills, Michigan, technology center to revel in the wonders of its Hemi engine, America's most celebrated powerplant. After a few contentious moments debating whether or not this engine truly deserves the Hemi appellation (the combustion chamber configuration is definitely not hemispherical), we grasped tools and parts to commence the assembly process.
This 5.7-liter pushrod V-8 uses a meaty deep-skirt cylinder block made of cast iron as a cost savings measure. The crankshaft is well supported by two vertical and two horizontal retention bolts per main bearing. The massive twin-plug, two-valve cylinder heads are aluminum and an even larger air-fuel module (aka intake manifold) is injection molded plastic, both to save weight. Computer-controlled actuators located between the heads shut down half of the cylinders when their output isn't needed to maintain speed, a strategy Chrysler calls Multi-Displacement System.
The build process went smoothly with minimal soiling of hands or invoking profanity (in contrast to the normal at-home experience without expert supervision). Our reward for completing the assembly with no spare parts was a demonstration run in a Hemi-powered Chrysler 300C around the greater Auburn Hills environs.
While we've driven, tested, and enthusiastically reviewed Hemi cars and trucks in the past, this run was different. A lap-top-armed engineer accompanied us to document the drive in a special way: this test car was equipped with sensors to facilitate charting speed versus time along with a means of indicating which cylinders were working and which were sleeping.
During our twenty-minute drive, we encountered suburban traffic and a couple of wide-open stretches after stop lights that allowed the free-breathing Hemi the opportunity to show its full-throttle muscle. Even though I kept careful watch for the eight-to-four and four-to-eight mode changes, they are virtually impossible to detect. The transitions are so smooth and buried in the background that only a highly trained and sensitized seat of the pants can accurately and consistently identify them.
Back at base, analysis of the results showed that only four cylinders were on tap during 17-percent of the suburban traffic portion of the trip. In the freeway cruise mode (including acceleration to 70-plus-mph speeds), we needed the full complement of eight energized cylinders only 48-percent of the time. Overall, the 300C motored smoothly on just four cylinders 40-percent of the time.
Our conclusions:
1. The Hemi is a fine engine in part because its combustion chamber is not a slave to obsolete engineering.
2. The Multi-Displacement System works as advertised. Regrettably we have no fuel efficiency benefit to report but letting half the engine rest at times imposes absolutely no noise or vibration hardship.
http://automobilemag.com/news/0408_build_a_hemi/
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