Try to recall some of the industry's more notorious powertrain miscues.

It didn't take long to invoke the name of General Motors Corp.'s 1981 V-8-6-4 engine, did it? The V-8-6-4 resides on the engine infamy list somewhere in the general vicinity of GM's and Volkswagen AG's '80s-vintage diesels.

The V-8-6-4 employed an intricate valve-deactivation mechanism to disable combinations of cylinders, effectively turning the V-8 - under certain driving conditions - intoa 6-cyl. or even a 4-cyl. engine. The idea was to save fuel, running the V-8, when lightly loaded, on fewer cylinders. The end result, unfortunately, infuriated customers: The V-8-6-4 often displayed genuinely wretched driveability.

Looking back, the V-8-6-4 was an almost laughable experiment. Except that modern, high-speed electronic engine controls - along with a renewed interest in "environmental consciousness" - have yanked the underlying ideal of the V-8-6-4 back into vogue. Automakers have established that efforts to use less fossil fuel should be perceived as more than just a hobby.

Suddenly, the idea of deactivating cylinders apparently isn't so silly after all. Last month, Mercedes-Benz launched its all-new '99 S-Class, the model range designed to represent the pinnacle of Mercedes' technical might. And guess what? The S-Class's 5L DOHC V-8 is fitted with an optional cylinder-deactivation system. It's 1981 again.

There are, of course, some differences. First, Mercedes says the system won't be offered on U.S.-specification cars, at least for now. And the Mercedes V-8 is designed to simply cut back to 4-cyl. operation; there's no intermediate step to 6-cyl. power. Most significantly, the electronics governing the Mercedes V-8 are gigaflops ahead of what was available when Cadillac courageously attempted selective cylinder deactivation.

Early information from Mercedes says the system can improve fuel economy by about 7% in city-cycle operation and as much as 15% on a steady 56-mph (90-km/h) cruise. The company swears driveability is in no way compromised.

Cylinder deactivation technology is scorching along with other automakers, too. Sources say Chrysler Corp., soon to be linked with Mercedes, has penned a similar system into a 2002 truck V-8 program - a program that may simultaneously resurrect the vaunted "Hemi" performance name for its combustion-chamber design.

Porsche also plans to presumably emphasize valve deactivation with a performance twist when it introduces its own system on an engine in late 1999. The system is a Lotus Engineering design, the parts made under license by a German components manufacturer.

How the systems "perform" is crucial. Eaton Corp. designed and developed the original valve deactivation system for Cadillac's V-8-6-4. Keith Hampton, chief engineer with Eaton's Engine Components Div., says the V-8-6-4 met fuel economy expectations of the time, but admits the engine overall had "systems issues." It was the first year GM used engine management computers and an early era for volume-production fuel injection, both of which are necessary for selective cylinder deactivation.

He asserts the Eaton system had a single-model-year shelf life because GM was developing a new V-8 for the '82 model year and the new engine didn't need cylinder cutout to achieve fuel economy targets.

Mr. Hampton maintains those first valve deactivation systems lacked the ability to synchronize air/fuel intake with cylinder deactivation - a dilemma largely solved by sequential fuel injection.

Eaton's new-generation system improves fuel economy by 10% to 20% and hydrocarbon emissions are also reduced, although Mr. Hampton can't estimate by how much. He expects Eaton's system to be used in a production vehicle in 2003.

"The preferred approach we see is to allow the intake event to occur to bring in air and fuel and to combust it and leave it (uncombusted exhaust) in the cylinder," he explains.

Once that's done, "You disable the exhaust valve; it keeps the cylinder warm and keeps high pressure in the cylinder, which prevents any leakage of oil past the O-rings," he says. "Then you synchronize the exhaust events with the engine operations. When you go up a hill (or require similar power) you reactivate the exhaust valve and allow the trapped charge to get out. Then you start up all eight cylinders again."

You'll find a similar theory in the Lotus-developed system: The mechanism consists of a bucket tappet with inner and outer coaxial sections, and a camshaft with three lobes per valve. The center tappet bears directly onto the valve stem via a hydraulic lash adjuster, and is vertically aligned with the central circular cam lobe. A flanking pair of identical cam lobes drive the outer section, held in contact with the cam face by internal springs.

With normal valve operation, the tappet sections are locked together by two opposed radial pins incorporated in the outer section and hydraulically thrust inward against return springs to register against the inner tappet. In this operating mode, the two outer cam profiles depress the complete, 3-piece tappet and hence the valve.

For cylinder deactivation, the locking pins are released, allowing the two coaxial sections to slide relative to one another, with the outer one still cam-driven but now incapable of transmitting lift to the valve.

Switching to deactivation occurs in the engine cycle at the end of the combustion stroke, deliberately leaving hot exhaust gases trapped in the cylinder - a practice precisely like that of the Eaton cylinder-deactivation system.

Lotus says the system also promises reduced frictional losses, since deactivating valves reduces the total load of the valvetrain. Speed shouldn't be a worry, either: the system has been reliability tested up to 6, 000 rpm.

Eaton's Mr. Hampton says the drive to meet CAFE standards has sparked renewed interest in valve deactivation. But the success of valve deactivation ultimately rests with the consumer:

"It has to be transparent to the end user," Mr. Hampton asserts.

In 1981, that's one promise Cadillac's cylinder-deactivation system - an innovation now proving to truly be ahead of its time - couldn't provide. - with David Scott in Britain