Pontiac Trans Am SD-455

Trans Am SD-455 at a glance

A racing name revived for 1973

Pontiac had used the Super Duty name in the early 1960s for 389 and 421 engines prepared for Super Stock drag racing, until GM banned factory racing in 1963. Ten years later the name returned on a street engine developed by a team under Herb Adams of Pontiac's special projects group. Street Muscle notes that the engine was presented inside GM as a warmed-over version of the standard 455.

The plan was broader than the cars that got it. Pontiac intended the Super Duty for the Firebird, the LeMans and the Grand Am, and a 1973 GTO fitted with it was named Car of the Year by Hi-Performance Cars before Pontiac withdrew the option. Certifying the engine for emissions in every model cost too much, so only the Formula and the Trans Am received it. The new EGR system delayed the programme, and the first cars reached buyers late in the 1973 model year.

Inside the Super Duty 455

The SD-455 shared its displacement with the regular 455 and little else. Its block was reinforced with four-bolt main bearings, the crankshaft was nitride-hardened, and connecting rods and flat-top aluminium pistons were forged. Hagerty describes heads that flowed better than those of the 455 HO and the Ram Air IV, and the intake manifold was cast with "LS2" on 1973 engines and "LS2X" on 1974 ones. The compression ratio of 8.4:1 let it run on the low-lead petrol of the time.

The rating changed before most cars were built. Early engines were quoted at 231 kW (310 hp) and 529 Nm (390 lb-ft); changes, mainly to the camshaft, brought the figure down to 216 kW (290 hp) to meet emission standards. That was still 30 kW (40 hp) more than the regular 455 of 1973, rated at 186 kW (250 hp). Street Muscle cites Hot Rod for development engines in GM's laboratories that made close to 600 hp, figures that never applied to a production car.

The EGR episode

The emissions story explains much of the Super Duty's short life. According to Horsepower Memories, Pontiac first fitted a solenoid that switched off the exhaust gas recirculation 53 seconds after a cold start, while the EPA test cycle ran for 50 seconds. The EPA ordered the solenoid removed, and the engines were painted a slightly different colour so inspectors could tell them apart. The same source adds that the EGR requirement cost the engine power.

The shaker scoop carried an SD-455 decal, but it was riveted shut at the factory to meet drive-by noise rules. Street Muscle reports that removing the block-off plate made it functional and was worth about two tenths of a second and 3 mph in the quarter mile.

Production by year and gearbox

Most buyers chose the automatic, and the Formula version stayed rare in both years.

For the 43 Formulas of 1973 the sources disagree: Tran-Zam, citing High Performance Pontiac, counts 33 manual cars, while Street Muscle says 10. In 1973 the Super Duty Trans Am came in Brewster Green, Buccaneer Red or Cameo White, and Pontiac sold 4,550 Trans Ams with the regular L75 455 next to the 252 Super Duty cars. The 1974 car gained a sloped nose, slotted tail lamps and 5 mph bumpers, and 943 of the 10,255 Trans Ams built that year had the SD-455.

Road tests in 1973 and 1974

Period tests showed how far the Super Duty stood from the rest of the 1973 market, though Street Muscle notes a debate over how close the first test cars were to production specification.

Car and Driver's Super Duty reached 60 mph a second sooner than a 1973 Corvette with the LS4 454 and Turbo Hydra-Matic, and covered the quarter mile 0.9 s quicker; it stopped from 70 mph in 69 m (226 ft). Decades later High Performance Pontiac (January 2007) measured 189 kW (253 hp) at the rear wheels of a stock 1974 car on a chassis dynamometer, which the magazine considered consistent with the 290 hp net rating.

Why there was no 1975 Super Duty

Pontiac planned to keep the SD-455 for 1975, but Street Muscle reports that the engineers could not make it meet the EPA's next set of emission standards. The 1975 Firebird moved to catalytic converters, and the 455 HO that returned in mid-1975 was rated at 149 kW (200 hp) and 447 Nm (330 lb-ft), with no Formula version. The 455 left the Trans Am after 1976, and the 1977 Can Am carried on with the smaller 400. The whole span from Ram Air IV to WS6 is told in our Firebird Trans Am hub, and the corporate rival Car and Driver measured it against is described in the Corvette C3 history.

SD-455 today: what to check

With 1,296 cars in two years, the engine is the first thing to verify on any car sold as a Super Duty.

• The VIN carries the engine code X for the Super Duty; a different code means the car left the factory with another engine.
• Original intakes are cast "LS2" (1973) or "LS2X" (1974), and the shaker decal reads SD-455.
• Manual-gearbox cars are the most sought after: 72 Trans Ams in 1973 and 212 in 1974.
• Pontiac Historic Services documentation is the usual proof for a car's original engine and options.
• Hagerty listed values from US$97,700 to US$175,000 for a 1973 Trans Am SD-455 in November 2017.

The SD-455 is part of the Pontiac family that started with the GTO, and it closes the net-power chapter of the American muscle car guide.