For the '75 model year, Chevrolet cancelled the Z28 package and Ford radically redesigned
The 455 H.O. package finally reached production in May 1975. Because of its late release,
The 455 H.O. package finally reached production in May 1975. Because of its late release,
The D-port 455ci cranked out 200 hp and 330 lb-ft of torque and was no different internall
Since the beginning, terms like Ram Air, High Output, and Super Duty have been synonymous with hard-core Firebird performance. But in 1975 the Trans Am became another victim of emissions standards and insurance premiums. If it weren't for the late-year introduction of an optional 455 H.O. package, it may have never survived to become an icon of '70s Americana. Follow us as we explore the optional 455 packages that helped catapult the Trans Am into becoming a mid-'70s American supercar.
The SD-455 was cancelled after the '74 model year, and the D-port 455 was eliminated from the '75 Firebird option list. This meant the 400 four-barrel was the only engine available in the Trans Am that year. But lower compression and a catalytic converter reduced its 225hp rating to a paltry 180. As rear-axle ratios decreased numerically to reduce engine speeds, performance suffered dramatically.
In its May '75 issue, Motor Trend magazine tested a Stellar Blue Trans Am with a 400 four-barrel engine and automatic transmission. Despite its attractive appearance, quarter-mile performance was a sluggish 17 seconds with a trap speed of 85 mph. Considering that just one year earlier a typical SD-455-powered Trans Am was running low 14s at around 100 mph, this new level of performance was an extreme disappointment to true enthusiasts. Without any optional performance equipment, it may have seemed the best alternative was to find a clean, low-mileage, used T/A. This would, however, change during the model year.
The first sign of performance arrived on February 4, 1975, in Car Distribution Bulletin 75-47. It simply stated that a 455 four-barrel engine and four-speed manual transmission would be available on the Trans Am in mid-March. More detailed information came on February 17 when Sales Information Bulletin 75-7 was issued, outlining the contents of an optional 455 H.O. package for the Trans Am. Several more weeks passed before CDB 75-62 was issued on April 14 announcing that option package UPC L75 listed for $150 and was available in all states except California. It added that orders would be accepted immediately, and production was expected to begin around May 12.
News of the high-performance option package spread like wildfire. Although Pontiac had not portrayed the engine as a special, high-performance unit, enthusiasts were envisioning round-port cylinder heads, an aluminum intake manifold, and high-flow exhaust manifolds like those found on '71-'72 455 H.O.s. But once these cars started reaching the public, a standard-production, D-port 455 under a "455-HO"-labeled Shaker scoop led to feelings of great dismay.
UPC L75-455 H.O. And 455 Four-Barrel Performance Packages
The Trans Am shared the same 455 four-barrel engine rated at 200 hp at 3,500 rpm and 330 lb-ft of torque at a lowly 2,000 rpm with other A-, B-, and G-body applications. It had an 067 camshaft with 273/289 degrees of duration and 0.407-inch lift, an 800-cfm Rochester Quadrajet, and an HEI distributor. But unlike those used in other applications, the Trans Am WX-code 455 had a carburetor and distributor that were calibrated slightly differently and a specifically tuned exhaust system for improved performance.
Backing the 455 was a Borg-Warner Super T10 four-speed manual transmission and a floor-mounted Hurst shifter. Carrying UPC M21, the close-ratio gearbox had a First-gear ratio of 2.43:1, and was the same unit available with the 400ci but with an 11-inch-diameter clutch disc to accommodate the additional torque output from the long-stroke 455. Power was transferred to the pavement through a limited-slip, 8.5-inch GM 10-bolt differential with a 3.23:1 gearset. No other transmission or rear axle combinations were available.
The exhaust system included in the 455 H.O. package was comprised of a specifically tuned single-inlet/dual-outlet muffler and dual-outlet chromed splitters. Although most reference sources incorrectly state that all '75 Trans Ams received the chromed splitters, standard '75 models received a unique exhaust tip similar to those used in 1974 but constructed from lighter gauge steel with a small amount of lateral bend to follow the contour of the quarter panel.
A simple "455" decal was used on the Shaker of all Trans Ams equipped with the 455 Four-Ba
Though the engine was essentially the same as the standard 455 found in other models, carb
The Borg-Warner Super T10, four-speed manual was the only available transmission. A floor-
A 3.23 axle ratio was included with the 455 package from either year. It was a considerabl
A specifically tuned, transverse-mount muffler was included with the 455 package from eith
While it may first appear that the 455 H.O. package was geared entirely toward acceleration, a set of semimetallic front brake pads otherwise found in police packages were included to reduce brake fade. These pads were used in conjunction with a specific combination valve for smoother braking action. Vintage tests indicate that panic braking from 60 mph required only 180 feet to come to a full rest. However, due to concerns that not all cars were being assembled with the correct combination valve, a warranty recall was issued for the 455 H.O. Trans Ams to ensure each had received the proper valve.
The package was carried over for the '76 model year. It was, however, renamed "455 Four-Barrel Performance Package" and "HO" was subsequently removed from the Shaker. List price also decreased to $125. And after thoroughly searching through factory sales literature and service documents, it appears that the semimetallic brake pads were eliminated, ultimately leading to the $25 reduction. In addition, every Trans Am for 1976 received the chrome splitters, but only those with the 455 Four-Barrel Performance Package had the specifically tuned muffler.
The lackluster performance of the '75 400-powered cars had magazine writers nervously awaiting their chance to drive a 455 H.O. Trans Am. But since production did not begin until late-year, few magazines had the opportunity to test the Sterling Silver press car. Performance proved only marginally better than its 400ci counterpart, with quarter-mile times in the low-16-second range and trap speeds around 88 mph. Various comments made by the authors, however, indicated that a combination of harsh testing and poor tuning contributed to its meager performance.
Although the drivetrain was unchanged for 1976, performance noticeably improved. The two press cars-one Carousel Red and the other Sterling Silver-accelerated from 0 to 60 mph in about 8 seconds, with quarter-mile times in the mid-15s and trap speeds in excess of 90 mph, which is a much more realistic performance representation. Though not staggering by today's standards, these 2-ton pieces of American iron still held the potential for respectable performance. And with only a few modifications, some of that hidden horsepower could be unleashed.
In its May '76 issue, Cars magazine placed Pontiac performance legend Nunzi Romano of Nunzi's Automotive in Brooklyn, New York, behind the wheel of the Sterling Silver '76 press car. After establishing a baseline, Romano adjusted the tire pressures, opened the hoodscoop, and removed the air filter. These simple modifications reduced quarter-mile time to 15.07 while trap speed increased to 93 mph. Romano then removed the head pipes from the manifolds to bypass the catalytic converter and followed up with an astounding 14.71 at a speed in excess of 98 mph. The article also stated that additional modifications were planned and that the results would appear in the June issue. But readers saw no more.
During a recent conversation with Romano, we asked if he recalled modifying the car for a follow-up article. He said that after the initial test, Ram-Air exhaust manifolds, a true dual exhaust system, higher-ratio rocker arms, and 3.73 gears were installed. The carburetor and distributor were also modified accordingly and the smog equipment was disabled. He stated that performance dramatically improved and that it felt much like an earlier model on the street. But while driving the car to the track for the second round of testing, Romano was rear-ended while waiting at a stoplight, and the car was totaled. No subsequent testing was ever performed.
Not considered a vintage musclecar in the traditional sense, the '75-'76 455-powered Trans Ams became nothing more than used late-models within a few years of their release. As popularity of the W72 400ci soared in the late '70s, many were literally driven to death and unceremoniously parted out or crushed after their tenure. This left few of the 8,439 cars produced in an unmolested state. Most were driven hard or modified, and many are without their numbers-matching drivetrain, yet they remain restorable and offer many miles of driving enjoyment.
As the trend to restore the mid-to-late-'70s Firebirds increases, so does the availability of restoration parts. Once overlooked by many, only now are the '75-'76 455-powered Trans Ams being recognized as affordable, big-cube, restoration candidates. So before writing one off as a low-performance, smog-era vehicle, take a moment and realize how much potential these 455-powered Trans Ams offer. You might just find that the 455 H.O. and 455 four-barrel Perform-ance Packages contain everything you want in your next project.