The two cars in question are the new Saturn Aura and Chevy Impala with the 3.9L Active Fuel Management system. While his impressions of the Aura sound like typical corporate fawning, especially considering few journalists have spent any time behind the wheel of one, Bob's take on the Impala's new powerplant sheds light on what AFM will do for the car's six-cylinder engine.
Like its use in other applications such as the General's V8-powered full-size SUVs, Active Fuel Management will switch off half of the Impala's six-cylinders in certain conditions, which do not include hard acceleration nor idling. Though some larger engines that employ cylinder deactivation can operate with half a motor while idling, that function was nixed for the 3.9L because a trio of pistons pumping makes for a rather rough idle. We'd guess AFM will be most active while crusing at around-town and reasonable highway speeds.
Lutz also points at that the Impala's fuel economy label likely won't change because of the addition of AFM, as the EPA's testing procedure doesn't take into account many situations in which the technology would make the most difference. Because of this, buyers will likely be surprised that their Impala beats the sticker's EPA rating in normal driving situations. In fact, the Impala Bob drove had an average of 29.8 mpg on its trip computer after several thousands of miles of operation.
[Source: GM FastLane Blog]