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1973 Facts and Interesting Notes
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Super Duty 455 Engine Option
For 1973 the now legendary Super Duty 455 engine was offered for the first time.  This engine, conservatively rated at 290 hp, is arguably the best engine Pontiac ever produced.  Only 252 found their way into Trans Ams for 1973 and another 43 into Formula Firebirds. 
The 455 HO Engine Replaced by a Standard 455
For 1973 the round port 455 HO engine was now replaced with a D port 455 engine in Trans Ams (also available in Formula 455).
The Screaming Chicken Hood Decal
Finally in 1973 the large hood bird, now synonymous with the Trans Am, found its way onto the option list.  Listing for $55.00 the hood bird was available in 3 colors:  Black/Yellow/Orange on Buccaneer Red cars, Green/Light Green/Black on Brewster Green cars, and Blue/Light Blue/Black on Cameo White Cars.
Trans Am Bumper Decal
Buyers who opted not to purchase the large hood bird decal received a smaller bird decal on the front bumper as standard.
New Grille for 1973
New 5 mph bumper requirements forced the grille insert to be pushed more forward on 1973 Firebirds to accommodate 4 inches of urethane foam in the center impact area and rigid steel reinforcements across the top and bottom.  The grille itself received a larger "egg crate" pattern making the 1973 model more easily distinguishable from the 1970 to 1972 models.
The Shaker Hood Scoop
The Trans Am shaker hood scoop would no longer be opened by a solenoid to ingest cold air into the engine.  The scoop would now come permanently closed from the factory to meet federal noise emissions standards. 
Formula Shaker Scoop
The 43 Formula 455s that received the Super Duty 455 engine received the Trans Am's shaker hood scoop rather than the standard dual snorkel hood scoops that came standard on the Formula.

Formula Twin Snorkel Hood
Whether the Formula Hood Scoops could be mad efunctional from the factory is a controversial topic.  Both The Fabulaous Firebird by Michale Lamm and Firebird, America's Premiere Performance Car by Gary Witzenberg say that all hood scoops were closed beginning in 1973 but option 634/WU3 listed Ram Air Hood as available for 400 and 400 equipped Formula's.  It is not unusual for Pontiac to cancel options if regulation were involved as it was for the Trans Am (Trans Am could not meet Federal noise emissions standard with a functional scoop) but in this case it was probably not.  Pontiac parts manauls show Hood part number 490661 as RAM AIR EXC SHAKER HOOD - W/FUNCTIONAL SCOOPS as available up to 1975. The same parts manual shows RAM AIR Decal part numbers (black or White) available until 1974. It is tehrefore determined that the 634/WU3 option was valaid for 1973.

Note that since the Formula with the SD-455 engine shared Trans Am's shaker hood scoop the SD-455 could not be ordered with a functional scoop.  The Trans Am with a standard 455 engine would have a closed shaker as well so it is possible that a Formula ordered with the optional 455 engine and functional scoops may have had more horsepower than a TA.

The Rear Deck Spoiler now Available on Base Firebird
The Rear Deck Spoiler was now available on the Base Firebird this year but still not available on Esprit.
The Rear Spoiler Bird Decal
The Rear Spoiler Bird Decal appeared in 1973 for Firebird and Formula and was available in either Black or White.  The Trans Am continued to use the Trans Am logo on the rear spoiler with larger letters this year.
New Custom Interior Design
The custom interior was redesigned for 1973 and now featured the popular "horse collar" seat design.
Additional Carpet Colors Available.
Pontiac offered only four different colors for 1973 interiors so perhaps to offer more diversity in 1973 only you could order carpets in Red, Orange, or Bright Blue.  Option 611/75F allowed you to order Red Carpet with either a Black or White interior.  Option 612/97F allowed Orange Carpet and option 614/24F allowed Bright Blue Carpet, both available with White Interiors.  These options are extremely rare and unique to 1973 only. 
 
Trans Am Column Shifter
Trans Ams order with automatic transmissions and without the D55 front console received column mounted shifters.  Even the Super Duty 455 was equipped with a column shifter if ordered without a console.

(Note:  the steering wheel shown is the standard Firebird steering wheel and is not correct on a Trans Am)
White Walls on Trans Ams
Ordering option VJ/P85 to get Steel Belted Radial Tires on your Trans Am would have delivered your car with white wall tires.