The 1965 Skylark Gran Sport was the intermediate Buick Skylark with the Gran Sport option added. Although a 300 cu in (4,916 cc) V8 was already offered in the Skylark, the Gran Sport had the largest engine permitted by GM - a 400 cu in (6,555 cc) Buick V8.
This engine was actually 401 cu in (6,570 cc), but called a "400" by Buick because that was the maximum engine size limit set by General Motors for the intermediate body cars. This engine produced 325 hp (242 kW) and 445 lb·ft (603 Nm) and was known as the "nailhead" engine. Buick sold more than 15,000 Skylarks with the Gran Sport option that first year, and almost as many the next. It was renamed the GS 400 in 1967, and the Gran Sport became its own model in (about) that same year along with a new "400" engine quite different from the famously reliable but becoming-obsolete nailhead engine design that was first introduced in 1953. Sales fell somewhat in the face of increasingly higher-performance and more popular muscle cars from other marques when compared to those from the more stodgy and expensive Buick. Buick, however stepped it up a notch when introducing the Stage 1 option in 1969. This limited production (less than 1,500 cars in 1969) version delivered 340 hp (253 kW) and 440 lb·ft (597 Nm).
The name Gran Sport replaced the GS moniker with the 1973 Gran Sport, and was again revived in the late eighties on the FWD Skylark model with various performance options added.