1973 Darts got new front styling with revised fenders, grille, header panel, and hood. Massive front bumpers were installed to comply with new federal regulations, as well as side-impact guard beams in the doors and new emission control devices. New single-piston disc brakes replaced the more complex 4-piston units offered from 1965 to 1972, though Chrysler did not address the premature rear-wheel lockup that continued to plague disc brake equipped Darts.
Chrysler's new electronic ignition system was standard equipment on all engines, and starter motors were revised for faster engine cranking. The K-frame was modified to accommodate a new spool-type engine mount that limited engine roll to 3°. The upper ball joints were upgraded to the larger B-body units. Along with these chassis changes, the wheel bolt pattern on Darts with disc brakes was enlarged from 4 in (101.6 mm) to the 4.5 in (114.3 mm) pattern common to the larger B- and C-body Chrysler-built passenger cars. Darts with 4-wheel drum brakes continued with the smaller bolt pattern. The standard rear axle was still the 7¼" unit, but the heavy-duty option was now an 8¼" item rather than the previous 8¾" rear axle. Standard rear axle ratios were 2.76:1 with automatic transmission and 3.23:1 with manual, though other ratios were available. Vent wings were deleted from the Swinger but not from the 4-door sedans. A new "Quiet Car" package was available, consisting of extra sound insulation, premium exhaust hangers and an exhaust resonator.
The Demon fastback was renamed Dart Sport in response to Christian groups' complaints about the 'Demon' name and devil-with-pitchfork logo. The high-performance model thus became Dart Sport 340, and 1973 saw styling changes to go along with the name change. The Dart Sport received the same new front end as the other Darts, and its taillights were changed to two lights per side, each with a chrome trim ring. These would remain unchanged through the 1976 model year.
Source: Wikipedia, 2012