The road-racing course at Pocono Raceway resembles a writhing snake in freeze frame as it wends its way through the infield of the oval track. Then, it suddenly dumps you out onto the racetrack, at the entrance to a big, banked back turn. You've got your hands full in that reptilian infield section. It is a course that makes it clear just how nimble and athletic the 1991 Infiniti G20 is, and shows off the new, active suspension in the Infiniti's Q45. The G20 is the all-new entry-level car for Nissan's Infiniti luxury division. At $20,000, the sporty sedan generates brisk competition for upscale toys such as the BMW 318is. And it represents the first rung on a ladder that takes the upwardly mobile Infiniti buyer to ever more rarefied automotive air. There is a $23,500 M30 sedan just a rung above it, and the $38,000 Q45 beckoning from its even loftier perch. The road course also reveals just how remarkably the big Q45 handles with its new active-suspension system. (The Q45s equipped with this $5,000 option are the first production cars offered in America with a true active suspension.) It shows how the suspension resists body lean in turns, and how it prevents rear-end "squat" during heavy acceleration and front-end "dive" during braking. The new suspension system is a nifty piece of engineering that controls leanly while allowing a soft, luxury ride. It does this by employing hydraulic "actuators" on each wheel to counter the forces exerted by cornering, braking and acceleration. When, for example, you are going through a left-hand turn, a normal or "passive" suspension would compress on the right side and allow the body to lean to the right. With the Q45's active suspension, the actuators on the right side pump up to oppose that inclination. The system works the same way to counter dive and squat. During heavy braking, the front wheel units pump up to keep the front end from diving. During heavy acceleration, the back actuators fill with fluid to keep the rear end from hunkering down. The suspension, incidentally, is not the only high-tech new option available on the 1991 Q45. The car also can be equipped with a new traction-control system. This setup is similar to the one found in the Cadillac Allante. It employs the same principles, but applies them in reverse. The Cadillac system restores traction by first applying the brakes to the spinning wheel, then diminishing the engine power to it. The Q45 setup initially reduces engine power by restricting fuel flow. Then, if the wheel is still spinning, the system applies brake pressure until traction is restored. The new, entry-level G20 car doesn't have the Q45's embarrassment of technological riches, but it sure is a lot of fun in its different way. In sharp contrast to the big, rear-drive Q45, the G20 is a relatively small (175 inches long) and light (2,535 pounds) front-driver. It is, however, a handsomely designed sedan that boasts a full complement of luxury amenities and a surprising amount of rear legroom for its size. But what is most memorable about the car is the way it drives. With the help of its sophisticated independent, multilink suspension, the G20 corners very adroitly. And no one will complain too much about the red-corpuscle count of the G20's new four-cylinder engine. Urged on by dual overhead cams and 16 valves, this aluminum two-liter hummer develops a feisty 140 horsepower. That's enough to take the car from zero to 60 mph in a sprightly 8.5 seconds, and let it run out to 130. I like the new G20 a lot more than its larger, pricier bedfellow, the Infiniti M30. The M30 (which Nissan has been peddling in Japan as the Leopard for five years) is basically a bore. It is not as good-looking as the G20, and not nearly as much fun.