IAA Frankfurt with new six-cylinder petrol and diesel engines and the V8 petrol engines enlarged to 4.2 and 5.5 litres. The 5-litre V8 was carried over. The visual changes included the smoother bumpers and side claddings, revised "Gullydeckelfelge" (German for manhole-cover wheel rim) alloy wheels, and deeper front bumper with integrated air dam. The W126 generation was replaced by the W140 in 1991. Styling From 1973 to 1975, Mercedes-Benz designers worked on the successor to the W116. After several design concept were presented, the final design for W126 was approved and frozen in 1976. The design work for coupé began immediately after the approval and was finalised in 1977. Design patents were first filed on 3 March 1977 and later on 6 September 1977 at the United States Patent Office. Compared to its predecessor, the W116, the W126 featured improved aerodynamics with a drag coefficient of Cd 0.36 for the sedan/saloon and 0.34 for the coupés. Models The pre-facelift model range (1979–1985 for sedan/saloon and 1982–1985 for coupé) included the 280 S/SE/SEL, 300 SD (North American market only), 380 SE/SEL/SEC, and 500 SE/SEL/SEC. The revised second series (1986–1991) with petrol engines included 260 SE, 300 SE/SEL, 420 SE/SEL/SEC, 500 SE/SEL/SEC, and 560 SE/SEL/SEC. The diesel-engined facelift version for the North American market only included the 300 SDL (the first diesel S-Class with long wheelbase) and then the 350 SD/SDL (the first diesel S-Class to be available in both wheelbase lengths). Safety Use of high-strength low-alloy (HSLA) sheet in its construction to reduce the weight without compromising the structural strength and integrity. Anti-lock braking system (ABS) (first appeared in its predecessor, the W116) was standard on 500 SEL and 500 SEC and optional for the rest of model range until 1985. The ABS eventually became standard for the entire model range during the second series (1986-1991). Seat Belt Pretensioners enabled all seat belts (with exception of rear centre shoulder belt on sedan/saloon) to tighten when the car sensed conditions that could lead to an accident such as sudden, forceful braking or sudden deceleration during the collision. Crumple zones front and rear which absorbed impact energies, reducing passenger injury. Fluted taillamps, the design pioneered on the R107/C107, was carried over. The fluted design maintains the visibility of taillamps when the dirt accumulates on the outer edges. Driver's airbags were introduced in 1981 as an extra-cost option for all models and engines. From