Nissan's Infiniti Q45 received high praise indeed during its first year on the market, with most automotive writers mentally parking it in the same lot as more expensive luxury sedans from Mercedes and BMW.

For 1991, the Q45 is offered with what Nissan calls "full-active" suspension, a complex system intended to combine the handling of a small sports car with the boulevard ride of a large luxury sedan.The biggest bump buyers may face is the cost of this new system. It adds a hefty $5,000 to the sticker price of a Q45, which for 1991 starts at $40,000 with a wide array of convenience and safety features already on board.

Also included in that $45,000 price tag is an owner-paid federal gas guzzler tax, doubled under the new tax laws effective Jan. 1, 1991, to $2,100.

An Infiniti Q45 was extensively reviewed early last year, scoring high marks in all areas but fuel economy. This writer even gave it the nod over Toyota's Lexus LS 400 sedan, considered its most direct competitor.

So a 1991 Q45 with full-active suspension was borrowed to evaluate the merits of this system, which Nissan claims to be the first of its kind offered in any automobile sold in the United States.

The idea behind active suspension is simple: give the various suspension components enough versatility to offer both a smooth highway ride and nimble handling between one set of bumpers.

The execution of such a system is somewhat more complicated, however, and involves combining age-old hydraulics with New World electronics. Simply explained, hydraulic "actuators" reside where one would expect to find shock absorbers. The pressure in the actuators are regulated by a network of pumps, accumulators and sensors connected to two 16-bit microcomputers.

One computer regulates pressure to keep the car level from side-to-side during cornering. The second monitors ride height and controls pressure to minimize front-to-rear pitch during hard braking or acceleration.

In other words, actuator pressure is constantly regulated to keep the car on an even plane regardless of driver habits or road conditions.

Nissan engineers insist this system can be programmed to eliminate body pitch or roll altogether. But a small degree has been dialed in to keep the driver more in touch with the car.

Obviously, the benefits of such a system, which adds only 132 pounds to the Q45's 3,950-pound curb weight, are most apparent if directly compared to a Q45 without full-active suspension.

But after several hundred miles of real-world driving without the benefit of contrast, one was hard pressed to feel a dramatic difference.

Although engaged at all times, its presence is more pronounced when the car is driven aggressively, or faster than most traffic flows safely permit. There is noticeably less lean during severe cornering, and slamming on the brakes results in little if any pitching.

As mentioned, one can say without fear of contradiction that the biggest bump buyers face with the Q45's full active suspension will be in their wallets - although luxury car owners are usually well insulated there as well.

But in addition to the extra $5,000 is a 2 mpg penalty on the Q45's already thirsty EPA rating of 16-city/22 mpg highway - premium unleaded fuel required. That bumps the $1,000 federal gas guzzler tax on Q45s without active suspension to $2,100.

The system's main pump also exacts a small penalty in horsepower. But drivers will hardly notice a loss in performance, since the Q45's 32-valve, 4.5 liter V8 engine delivers 278 hp at 6,000 rpm in lusty fashion.

The verdict? Forget active suspension and save the $5,000. Its cost far outweighs the degree of improvement to the already fine suspension found under every Q45.

Maybe that's why Infiniti expects only 15 percent of the 20,000 Q45s sold this year to have full-active suspension.

But don't forget the car itself. Those contemplating a large luxury sedan, whether it be European, American or Japanese, should give the rear-drive Q45 a spin.

It has the sleek lines of a Jaguar sedan, welded into a Rock of Gibraltar feel. Its leather-trimmed interior has an art-deco touch to it, notably in the instrument panel gauges. All controls are well-placed, with the exception of awkwardly arranged buttons on the door panel for the seat adjustments.

A driver's side air bag resides in the steering wheel, which has an electric adjustment for distance as well as height.

Its engine, mated to an electronically controlled 4-speed automatic transmission, idles like a Swiss watch. Yet it is able to deliver stunning acceleration through a raspy exhaust note when called upon. Its anti-lock brakes scrub off speed without fanfare, and were a welcome addition on slippery, snow-packed roads.

The overall level of fit and finish on the test car was impeccable.

For 1991, each Q45 sedan is backed by a 48 month/60,000 mile basic warranty and a 72 month/70,000 powertrain warranty. Also included is a 24-hour roadside assistance program and a free loaner car during routine servicing.