Buick called its 1986 Grand National’s turbo V6 ‘under-rated’ in its own brochure — then proved it a year later by handing 547 cars over to an outside shop for a transformation the factory couldn’t build itself. The result, badged GNX, put up numbers that worried owners of contemporary Ferraris and Porsches. Officially rated at 276 horsepower, independent testing suggested Buick was still underselling it. Read on to see exactly what changed under the hood.