Much of the performance enhancement in the Boeing 747-400 over earlier 747 Classics came via three major aerodynamic improvements: wing extension, canted winglets, and a reshaped wing-body fairing, most noticeable at the root of the wing leading edge.
Boeing refined the design of the wing-body fairing in a transonic wind tunnel and because the fairing is external to the main structure of the aircraft, it offered significant performance gains with little to no structural penalty. Additionally, as it’s made of composites, the weight increase over the wing-body fairing used on the 747 Classics in nearly negligible.
Weight savings wherever possible on the 747-400 also translated into improved performance. This is seen in the use of carbon brakes over steel brakes in the landing gear. A host of changes in the materials used resulted in a stronger landing gear in the 747-400 that’s also 1,800 pounds lighter than the landing gear used on the 747 Classics.