Impala sixes built. After the short run of early-1972 six-cylinder Impalas were built in the fall of 1971, the six-cylinder engine and three-speed manual transmission were relegated to low-line Biscayne and Bel Air sedans. For most of the 1972 model year, all Impalas were V-8 powered and came standard with Turbo Hydramatic transmission, power steering and power front disc brakes. The standard Impala engine was now the 165-net 350-cubic-inch Turbo Fire V8 with optional engines including the 170-horsepower, 400-cubic-inch Turbo Fire V-8 (the base Caprice/Kingswood Estate engine), 240-horsepower, 400-cubic-inch Turbo Jet V-8 (rated at 215 horsepower on station wagons) or the 270-horsepower, 454-cubic-inch Turbo Jet V-8 – these were the engine offerings in 49 states; in California the only engines offered were the 165-horsepower 350 and 170-horsepower 400 Turbo Fire small-block V8s due to that state's much stricter emission regulations. The Astro Ventilation system was redesigned for 1972 with the vents relocated from the trunk lid to the doorjambs for improved reliability and efficiency over the troublesome 1971 system that was a major source of complaints from owners to Chevrolet and its dealers. 1973 1973 Impalas featured a larger, shock-absorbing front bumper due to new federal mandates which required impact protection. The chassis was tweaked for better roadability, and new square taillights were mounted in the carry-over rear bumper. With the convertible now moved to the top-line Caprice Classic series, (continuing through 1975) Impalas were now all closed body styles for the first time in the nameplate's history and the station wagon known as the Kingswood since 1969 rejoined the Impala series. A 145-horsepower 350 cubic-inch Turbo Fire V-8 engine, Turbo Hydramatic transmission, variable ratio power steering and power front disc brakes were standard on all Impalas. Optional engines in all models included a 175-horsepower 350 Turbo Fire V8 with four-barrel carburetor and 245-horsepower 454 Turbo Jet V8; while the Impala Custom coupe, Sport Sedan and station wagons could be ordered with the 165-horsepower 400 Turbo Fire V8 that was standard on that year's Caprice Classic series. All engines were offered nationwide in 1973. In 1973, General Motors equipped approximately 1,000 1973 Chevrolet Impala Sedans with their experimental Air Cushion Restraint System (ACRS), which was an early prototype of an airbag system. ACRS employed a driver's side airbag that was housed in a unique four-spoke steering wheel, as well as a passenger side airbag. Due to the passenger side airbag,