Despite rising fuel prices and fears of an oil crunch, Americans' lust for big vehicles with gas-gulping V-8 engines is unlikely to abate anytime soon. Ironically, the typical driver rarely uses much of the horsepower under the hood. Sedans today can produce upward of 300 hp, yet as little as 30 hp is enough to maintain a highway cruise. So unless a full-size car or truck is passing at speed, climbing a hill or towing a trailer, its eight-cylinder engine runs only half-heartedly--and inefficiently: operating at less than full load lowers fuel economy.

Power-train engineers at DaimlerChrysler and General Motors have perfected an affordable way to smoothly morph V-8 gas guzzlers into V-4 fuel misers and back again as required. Laying off the accelerator pedal shuts down unneeded engine cylinders, which allows the remaining ones to operate at higher thermal and mechanical efficiencies. The result is 6 to 20 percent better fuel economy, depending on how the vehicles are driven.

Automakers have tried this solution in the past. Unfortunately, the first mass-production attempt--the ill-fated 1981 Cadillac V-8-6-4 engine--suffered from rough transitions between full and partial engine loads. Subsequent systems were more nimble but costly, limiting them to luxury cars such as recent Mercedes-Benz S-Class models. Since then, the necessary electronic throttles and computers have gotten much cheaper.

The two car makers' current schemes are similar. In GM's case, the system engages when a computer determines that an eight-cylinder engine can satisfy a driver's torque demands with only four, explains GM engineer Allen Rayl. It then deactivates every other cylinder in the firing order by disabling their engine valves, which control the intake and exhaust cycles. This job is accomplished by applying hydraulic oil pressure to collapse special telescoping lifters--usually rigid components that operate the engine valves by transferring motion from the rotating camshaft. Decouple the camshaft from the valves, and combustion halts. Thus, when running at low torque, the engine does not need to pull against as high a vacuum to bring in fresh fuel and air and to eject the exhaust products. Hence, the engine does not have to work as hard.

In road tests, vehicles demonstrated mostly imperceptible transitions from eight cylinders to four and back again, thanks to electronic throttles that act to produce the same torque whether all or half the cylinders are firing. Meanwhile passive countermeasures--"hydraulic" engine mounts, enlarged mufflers, flexible couplings--successfully mask the noises and vibrations created when the engine runs at different speeds.

Despite its current connection with big gas hogs, "cylinder deactivation really gives you the best fuel-savings bang for the buck," claims Alan Falkowski, Chrysler's development team leader. That is, of course, short of building smaller, lightweight vehicles that many American motorists disdain. Almost all other fuel-conserving technologies--advanced variable valve-timing systems, diesels, direct-injection gasoline engines and the current customer favorites, electric hybrids--bring with them greater mechanical complexity or higher costs, or both.

Notably, the number of engines incorporating the new cylinder-shutdown technology will top that of hybrid electric vehicles (such as the Toyota Prius) within a year or two. And by sheer numbers alone, the total gallons of gasoline conserved by the new power plants will soon thereafter overtake the amount ever saved by hybrids alone. Further, the trend extends to V-6 engines: Honda has installed the technology in six-cylinder 2005 Odyssey minivans and hybrid Accord sedans, and GM will do the same for some of its 2006 standard models. According to Rayl, GM expects to ship around two million cylinder-shutdown systems annually by 2008.