by Allpar
The 2005 Chrysler 300C and Dodge Magnum RT were the first high-volume, modern production vehicles in North America to feature fully-functioning cylinder deactivation; at Chrysler, they were followed by the 2006 Durango (Honda came out with a cylinder-deactivating V6 later). The MDS (“multiple displacement system”) seamlessly alternates between fuel economy in four-cylinder mode, and power in V-8 mode. Owners receive the powerful benefit of the Hemi engine with the fuel economy that they would expect from a less powerful V6 engine. Most test drivers cannot tell the difference.
"The MDS was part of the engine's original design," said Bob Lee, Vice President Powertrain Product Team, Chrysler Group. "This resulted in a cylinder-deactivation system that is elegantly simple and completely integrated into the engine design. The benefits are fewer parts, maximum reliability and lower cost."
This system should triumph where the Cadillac 4-6-8 failed because of the speed of modern electronic controls, the sophistication of the algorithms controlling the systems, and the use of electronic throttle control. The HEMI will be able to transition from eight cylinders to four in 40 milliseconds (0.04 seconds).
The system deactivates the valve lifters. This keeps the valves in four cylinders closed, and there is no combustion. In addition to stopping combustion, energy is not lost by pumping air through these cylinders.
Fuel economy gains go up to 20 percent under various driving conditions, with a 10 percent (or so) aggregate improvement, without any change in customer experience—drivers will receive the benefit without changing their driving habits or losing power.
Don Sherman's article in Automobile noted described a test of the MDS. They found that only four cylinders were used during 17% of the suburban traffic portion of the test, during a full 48% of the freeway test which included "over 70 mph" speeds.

Overall, they found that the engine powered down to four cylinders about 40% of the time. Non-enthusiast drivers may experience even more savings. As with most testers (including us), Don was generally unable to detect the changes from four to eight cylinders and back again.

Chrysler engineer Cole wrote: "The modern HEMI always shuts off the same four cylinders. In our duarability test cycle (150,000 customer equivelant miles driven at the 95 percentile, meaning that only 5 percent of our owners are more abusive than our testing), we have not found any adverse wear patterns." (Chrysler PR materials boasted of “over 6.5 million customer-equivalent miles through development and durability testing.”)

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