Down underneath, where the greasy driveline bits strain in doing their part to convert reciprocal motion into rotary motion, there's a quiet revolution under way: The mechanics of driving all four wheels are advancing.

New Venture Gear, the 1990 joint-venture of General Motors Corp. and former Chrysler Corp. transmission/driveline expertise, has made quite a name for itself and its myriad 4-wheel drive and all-wheel drive (AWD) transfer-case applications. NVG is likely to enhance its reputation with its latest "spin" on all-wheel drive - the hydro-mechanical progressive transfer case.

In the first application of the technology developed under license from Asha Corp., NVG supplies its NV 247 AWD transfer case in the '99 Jeep Grand Cherokee; Jeep dubs the unit Quadra-Trac II. James L. Lanzon, NVG's vice president-engineering, says the system for the Grand Cherokee was under development for roughly three years.

NVG's Quadra-Trac II transfer case does away with the Jeep's former viscous-coupled transfer unit - and that's the "revolution" part. Until now, viscous couplings - fluid-filled transfer units that enable the driveline to progressively transfer and apportion drive torque between front and rear axles - were the linchpin of just about everyone's AWD system. From Subaru to Steyr, Volvo to Volkswagen, viscous technology represented state-of-the-art.

In one swoop, NVG's progressive transfer case surpasses viscous technology by employing a multi-disc clutch pack acted upon by hydraulic pressure. When there is a speed difference between front and rear axles, the action causes a gerotor pump to build the hydraulic pressure that is applied to the clutch pack, progressively equalizing drive torque between the slipping and non-slipping axles. Needless to say, it's all hands-off automatic.

NVG engineers say the most critical improvement is in response time. Ronald Frawley, chief engineer- transfer cases, says, "It's all in response time. The new unit essentially works in two-wheel drive until wheel-slippage is detected; then reaction time (to send drive to the other axle) is about 40 milliseconds. A viscous coupling can take two or three seconds to do the same thing."

There's more. NVG says viscous-technology transfer units are expensive. Although company execs decline to say what their new progressive transfer case costs, they do claim they've got a price advantage over viscous units.

Mr. Frawley says the clutch-packed progressive transfer case affords other performance advantages, too. For one, it integrates seamlessly with antilock braking; viscous-coupled diffs have to allow "freewheeling" under braking to avoid potential conflict between trying to drive all four wheels and trying to brake wheels that may be working with different coefficients of friction. He also says the interplay between the gerotor pump and the progression of clutch lockup allows the unit to more easily tailor performance characteristics to a wide range of applications.

Probably not entirely by coincidence, Volkswagen AG and its Audi division have fostered a remarkably similar technology. The guts of VW/Audi's new system - to be marketed as "4motion" by VW instead of the former "syncro," with Audi continuing to use its industry byword "Quattro" name - also rely on a oil-bathed clutch pack developed by Haldex in Sweden. But instead of using purely mechanical means to supply the hydraulic pressure that adjusts the clutch pack, the Haldex unit employs electro-hydraulic controls commanded by software developed by longtime Austrian AWD specialist Steyr-Daimler-Puch.

Audi and VW have pushed ahead with their new system because of the rationalization of models onto common platforms.

Audi's initial application is for the TT coupe, which employs the same platform as several VW Group vehicles - all of which have transversely mounted engines. Audi, of course, wanted the TT to offer its hallmark quattro, but Audi never had to engineer a transverse-engined car with quattro until the platform-sharing TT. Thus the TT carries the Quattro name, but its Quattro system is in reality quite different than the Quattro AWD fitted with Audi's traditionally longitudinally mounted engines.

VW and Audi claim benefits similar to NVG's new AWD transfer case, including reduction of understeer and the ability to handle tires of different circumference.