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After the 1974 SD455 was dropped the 1975 Firebird's top performance engine was an 'L78' Pontiac 400 cu in (6.6 L). Pontiac still offered the regular 455 (RPO L75) in its full sized cars, and after a negative public reaction for dropping the 455 engine, it was re-introduced mid-year as an available option for the 1975 Pontiac Trans Am.
Engine selection saw the addition of Pontiac’s largest V8, the 455, available in the L75 325 hp (242 kW) version and the LS5 335 hp (250 kW) HO version. Both the 455 and 455 HO were available as engine options for the Firebird Formula, but the Trans Am received the 455 HO as standard equipment.
Optional engines included a 455 cubic-inch V8 with two- or four-barrel carburetion and respective horsepower ratings of 285 and 325, respectively. All Pontiac engines for 1971 were designed to run on lower-octane regular leaded, low lead or unleaded gasoline thanks to a GM corporate edict, necessitating reductions in compression ratios.
Engine options remained mostly unchanged, however, the L75 455 engine was dropped, but the LS5 455 HO remained as an option for the Formula and standard for the Trans Am. [16] Pontiac advertised the 1972 455 HO as de-tuned to 300 hp, but the engine was unchanged from 1971. [17]
The big-block engines initially used a forged crankshaft with a stroke of 3.975" for the 1965-1967 425 and 400 CID versions; starting in 1968, both the 400 cu in (6.6 L) and the 455 cu in (7.5 L) big blocks used a stroke of 4.25 in (108 mm), with crankshaft material changed to cast iron except in a few rare cases.
Being nearly identical, it too has the distinctive odd-sided hexagonal shape. These engines can be fitted in rear wheel drive vehicles with the right bellhousing and are used in hot rods, kit cars, sand rails and late model engine swaps. All Cadillac Northstar V8s; Oldsmobile Aurora L47 V8; GM 3.5L LX5 "Short Star" V6
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Also announced for the '73 GTO and Grand Am was the Super Duty 455 V8, which was rated at 310 hp (230 kW). Cars magazine tested an SD-455-equipped 1973 Pontiac GTO and chose it as Car Of The Year, yet that engine never made it to production in a GTO or other Pontiac A-body, but would be limited to the Firebird Formula and Trans Am.
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