![]() A 170 horsepower, 400 CID V8 was available, as were two 455 engines. The base V8 was a 350 CID engine that produced 150 horsepower, which was also down 10 from the previous year. The base models came with a 250 CID six-cylinder that was now rated at 100 horsepower, down 10 from the previous year. Other than the SD-455, most engine choices carried over from the year before, though nearly all saw a horsepower loss due to other new federal standards. The new egg crate grille also distinguished the cars from previous year models. New federal regulations required a 2.5 MPH bumper, and this would be upped to 5 MPH the following year, which would see an even more drastic change in the look of the Firebirds. Other than the “screaming chicken,” which would come to help define Trans Am, the most noticeable change to the exterior of the car were the revised bumpers. Just under 300 Firebirds got the SD-455 in 1973 – most were in Trans Ams while a few Formula 455 models got them as well. And that is the net rating, making it easily the most powerful engine available in any pony car in 1973. Most agreed the actual output was around 370 horses, if not more. And the 310 horsepower rating, according to many experts, was definitely an understatement. ![]() This makes it all the more shocking that the 455 Super Duty, or SD-455, was introduced this year. By 1973, due to the new requirement that horsepower had to be expressed as a net rating instead of a gross rating, and other government regulations, performance in pony cars was on the decline.
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