Variable Displacement

We've all heard the phrase "not hitting on all cylinders." That used to be almost as bad as "being a bubble off plumb," or "not all his oars in the water." But in today's automotive lexicon, not hitting on all cylinders represents an innovative yet intrinsically simple concept for improving fuel economy with no compromise in performance.

Terminology
The new terminology is cylinder deactivation, also called variable displacement. GM refers to its system as Displacement on Demand (DOD) on its midsize SUVs. A similar approach used by Chrysler in the Hemi V-8 is called Multiple Displacement System (MDS), which is available on the company's trucks, SUVs and some passenger cars.


Whichever the acronym, in simplest terms the system switches from eight to four cylinders at cruising speeds. When full power is required, at acceleration or under full load, the vehicle operates on all eight cylinders. In our driving experience, the transition from eight to four cylinders (or four to eight) is seamless. With the MDS, it takes place in an imperceptible 0.04 seconds.

Best of Both
According to various manufacturers, fuel savings range from 8 to 25 percent, depending on the driving conditions. For example, on a Ram 1500 pickup powered by a 5.7-liter Hemi, rated at 345 horsepower, the gas mileage is just a tad less than the company's base 4.7-liter engine. According to vehicle reviews, the MDS engine in lightweight cars nearly doubles gas mileage. The 2005 MDS-equipped Jeep Grand Cherokee, a vehicle which has never been known for good gas mileage because of its overall weight, offers 14 mpg city and 19 mpg on the highway—two mpg less than the 3.7 V-6 Grand Cherokee engine.


Drivers get their cake and eat it, too, with no sacrifices in performance. This new technology delivers the same horsepower and torque when it's needed. The advantage of DOD (or MDS) is obvious: the driver has access to the full towing and payload capacities when needed, but also the fuel economy of a smaller displacement engine when the vehicle is not under full load.