In 1968 and 1969 Buick offered the GS 400 in a convertible and hardtop model. Standard issue of the GS 400 was a 400 cubic inch (6.6L) engine (with likely lower stated horsepower rating to keep insurance premiums lower) of 340 horsepower and 440 ft. lbs. torque, a four barrel Rochester carburetor, dual exhaust, limited slip differential, and the three speed turbo hydra-matic 400 automatic transmission (revered as the finest automatic transmission ever built), with a Hurst "dual-gate" shifter and linkage, located at the front of a center console. Shift pattern was (forward) Park, Reverse, Neutral, Drive, Second, First. A standard manual gearbox was also available. Compression ratio on this engine was a very sensible 10.25:1, which allowed for the use of any grade gasoline to be used in normal driving.
Source: Wikipedia, 2013