With the 1964 introduction of the GTO option on Pontiac's intermediate LeMans, the internecine competition among GM's divisions began. Chevrolet responded with the Chevelle SS 396 big-block a year later, as did Oldsmobile with its 4-4-2. Buick held back, however, and only offered a 300-cid small-block motor in its intermediate Special and Skylark lines for 1965. That changed in 1966, though, when Buick unleashed the Skylark Gran Sport 400, which featured the 325-hp 401-cid Buick big-block "nailhead" V-8 that had been lifted from the Riviera. Bucket seats, a center console, and an optional Warner T-10 four-speed floor-shift transmission gave the new line verified muscle car chops. The line became an actual "series" of its own that year.

Despite the fact that this big V-8 was in its last year, it was prodigious and competitive, even matching the Chevelle SS 396's output. But it wasn't just horsepower that the Buick had in abundance. The torque rating maxed out at a massive 445 ft-lb at a usable 2,800 rpm, compared to 410 ft-lb at a slightly-less-user-friendly 3,200 rpm for the SS 396 engine.

For Buick, this meant the car had a similar top-speed and better acceleration than its primary competition. The other advantage was that the Buick could be equipped with GM's new Buick Super Turbine 400 automatic with three speeds forward and switch-pitch torque-converter (aka the Turbo-Hydramatic 400). The Chevelle 396 would not be so offered until 1967, and without the switch-pitch feature.

Of course, in this era, any small edge was of high importance in racing, whether impromptu street drags between lights, or sanctioned quarter-mile events at drag strips. And for muscle cars, racing meant one thing: straight line acceleration. As tested, the GS 400 would do 0–60 mph in 7.6 seconds. Unlike most other muscle cars of the day, the GS 400 came standard with decent brakes--Buick's famous finned alloy drum brakes, which resisted fading far better than anything this side of front disc brakes.

For 1967, there were two big improvements. Optional Delco-Moraine opposed-piston front power-assisted disc brakes improved braking potential even further, and the "nailhead" 401 was replaced by an entirely new, very modern big-block Buick V-8 of 400 cid. The new motor belted out 340 hp due to better breathing by means of larger valves, allowing for more power at a higher rpm, with peak at 5,000. Torque was still nearly as good as before, with 440 ft-lb. The new engine also went on a diet, weighing in a full 85 pounds less than the 401. This improved handling and steering. A new grille and trim changes made the rest of the package different from 1966.

Reverting somewhat to prior form, a GS 340 was also introduced for 1967, in order to bring some of the "budget muscle" customer base to Buick. This new 340-cid V-8 produced out 260 hp and better-than-competitive torque. This car line was solely available as a two-door hardtop, while the GS 400 could be had as a two-door pillared coupe, two-door hardtop and two-door convertible, as was also available in 1966. The advantage that the GS 340 had was lithe handling prowess, thanks to the small-block's light weight; the engine weighed a full 150 pounds less than the big-block, and 125 pounds less than the Chevy small-block.

The GS was redesigned for 1968, and the second-gen car would carry it through 1972, the end of mainstream muscle.

The 1966 and 1967 Buick Gran Sport and GS is a great choice for muscle car aficionados who are not afraid to drive something outside the ordinary. Prices are generally lower than for other GM muscle of the period, and the Buicks can offer a better driving experience. Production numbers were high so ownership is relatively easy and finding cars for sale is not difficult. Restored examples are not uncommon either, so buyers can afford to hold out for a car that matches their budget and desired condition.