Buick's popular Century has a new look for 1989 along with a new optional V-6 engine. Its traditional General Motors shape has been softened by the subtle rounding of the upper body lines. Perhaps not as soft as a Claes Oldenburg sculpture, but certainly enough to give the car a different appearance.

At first glance, and perhaps even two or three, one isn't really aware of just how extensive the changes have been. In addition to a new roofline and rear window treatment, the car also has a new grille, front-end panel, body-color bumper, fascia, flush composite headlamps with wrap-around park and turn lamps, rear quarter panels, deck lid and full-width tail-lamps with center back-up lamps. Obviously, not just the old facelift treatment.

An interesting result of these changes is that the rear body curves inward. However, this is not apparent unless the car is viewed from above, or, what is known in the trade as a ''plan view.'' Not many people really view cars from above but it is a quick and revealing way to see just how aerodynamic and appealing the overall design is. All that is really needed is an overpass; preferably one on a low-volume back road, not the Thruway.

Anyway, the '89 Buick does have an entirely new look. However, you may have to look for it.

The four-door Century (test car supplied by Kelly Buick, State Road, Emmaus) has a wheelbase of 104.8 inches, length of 189.1 inches, width of 69.4 inches, height of 54.2 inches and curb weight of 2,729 pounds. It has an EPA volume index of 113 cubic feet (97 passenger compartment, 16 trunk) and is rated as a mid-size (110 to 119 cubic feet).

It is an appealing size for a family car, which is no doubt one of the reasons for its popularity. Front seat room is spacious with head room measuring 38.6 inches and maximum leg room at 42.1 inches. With the front seat extended fully aft, rear seat passengers will still have 35.9 inches of leg room. And even with the new roofline, there is 38 inches of maximum head room in the back seat. The trunk measures a generous 16 cubic feet and is laid out for length.

The interior, as should be expected from a mid-size Buick, is nicely furnished. Even though the test car was a Custom and not the more expensive Limited model, one needn't feel deprived. Upholstering, carpeting and trim are all pleasant to the eye and touch. A nice touch is a storage armrest. Typical of many General Motors cars, the Century Custom will not strain the mechanical ability of anyone looking at the instrument panel. There's a speedometer and a gasoline gauge. Can't get much more basic than that. (The Limited adds volt and temperature gauges and a trip odometer.)

Way back when, Buick always had a reputation for a smooth riding car. Perhaps just to show it is keeping up with things, another new item on the Century this year is Buick's Dynaride suspension system, which, according to Buick, ''provides a very significant improvement in ride and handling over the 1988 car.'' Well, I'm not quite sure just how significant an improvement this is but I will say the test car rode exceptionally well for its size and handled very decently.

Dynaride is based on the car's four-wheel independent suspension - MacPherson struts up front, trailing arm twist axle with track bar in the rear - which is just about the way it was last year on this front-wheel drive car. Dynaride works by combining deflected-disc shock absorber valving with other tuned components. So, you not only roll down that highway of life in your Century, you do it smoothly.

However, for those who consider a smooth ride boring, there's always the Gran Touring Package, which tunes the suspension the other way. Sort of the choice between a family sedan and a sports sedan.

Never really a mild-mannered car (with the optional engine), the Century now has a new power plant that turns it int a very responsive performer. This is the 3.3-liter/ 204-cubic-inch overhead valve V-6, a replacement for the optional 2.8-liter and 3.8-liter V-6s. If nothing else, it does narrow the need to make a decision.

This engine features multi-port fuel-injection, friction reducing hydraulic roller lifters and is rated at a healthy 160 horsepower at 5,200 rpm and 185 foot pounds torque at 2,000 rpm. This is a lot of snap for a car weighing in at 2,800 pounds, so it shouldn't come as a revelation that performance is very good. Just lay on that accelerator and it moves: from a start, uphill, passing or whatever else you have in mind.

(Last year's optional engines were rated at 125 horsepower for the 2.8- liter and 150 horsepower for the 3.8-liter.)

The test car was equipped with the optional four-speed automatic (a three- speed automatic is standard) which worked just dandy with the 3.3-liter. Not only for performance but for fuel mileage. The test car averaged 16 miles per gallon for city driving and 27 mpg over the highway. All on unleaded regular.

The standard engine in the Century is the tried-and-true 2.5-liter/151- cubic -inch four. Rated at 98 horsepower at 4,800 rpm and 135 foot pounds torque at 3,200 rpm, the old Iron Duke should provide adequate though far from startling performance. Later in the model year, however, an updated version of this engine, the Tech 4, will replace the current engine. The Tech 4 is rated at 110 horsepower at 5,200 rpm and 135 foot pound torque at 3,200 rpm. Performance, of course, will be a little better.

For what it offers, the Century is a fair-priced car. Base for the Century Custom four-door is $12,429 and standard equipment includes a three-speed automatic, power brakes, power steering, tinted glass and AM-FM stereo with seek and scan and clock.

The test car had options totaling $2,269, a destination charge of $450 and a bottom line of $15,148. Among the options were: air conditioning, $775; 3300 V-6, $710; cruise control, $175; wire wheel covers, $199, and front seat with storage compartment, $187.

The Century is protected by a one-year basic warranty, a 6-year/60,000- mile powertrain warranty and a 6-year/100,000-mile rust perforation warranty.