 For the '75 model year, Chevrolet...  For the '75 model year, Chevrolet cancelled the Z28 package and Ford radically redesigned the Mustang into a semicompact. The only players left from the glory days were the Corvette and Firebird Trans Am-but emissions took a drastic toll on the performance of the T/A's 400 engine. Pontiac issued Sales Information Bulletin 75-7 on February 17, 1975, detailing the contents of an optional 455 H.O. Package available on the Trans Am. It included a 455ci engine, a four-speed manual transmission, 3.23 rear gearing, a special exhaust, and semimetallic front brake pads. |
 The 455 H.O. package finally...  The 455 H.O. package finally reached production in May 1975. Because of its late release, only 857 units were produced. Each received "455-HO" decals on the Shaker scoop, even though a standard-production, D-port 455ci engine was under hood. |
 The 455 H.O. package finally...  The 455 H.O. package finally reached production in May 1975. Because of its late release, only 857 units were produced. Each received "455-HO" decals on the Shaker scoop, even though a standard-production, D-port 455ci engine was under hood. |
 The D-port 455ci cranked out...  The D-port 455ci cranked out 200 hp and 330 lb-ft of torque and was no different internally than those used in other applications. It included a cast-iron intake manifold with EGR, an 800-cfm Rochester Quadrajet, and an HEI distributor. In stock trim, a 455-powered Trans Am was a solid mid-15-second performer with plenty of potential. |
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.
History
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.