For 1973 the Firebird received very minor changes, the most notable a heavier bumper requiring the grille inserts to be less sunk.  The rear bumper stuck out an inch furtehr as well.  The big news was the introduction of the now famous screaming chicken, the large hood bird on the Trans Am.  In the Engine department Pontiac released one of the most highly sought after engines of any era, Super Duty 455 engine conservatively rated at 290hp. When the Formula was ordered with this engine the hood and scoop from the Trans Am were used instead of teh dual snorkels.