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The 1971 Pontiac 455 HO was Pontiac's first engine to receive a special 800 cu ft/min (23 m 3 /min) Rochester Quadra-jet carburetor with specific jetting. The 1971 455 HO was rated at 335 hp (250 kW) at 4,800 rpm and 480 lb⋅ft (651 N⋅m) of torque at 3,200 rpm ( gross ).
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.
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
In mid-March 1971, a second design change for the GT-37 switched the striping to a reflective sword-style stripe sometimes referred to as the 1971 ½ GT-37. 1971 also afforded the customer the opportunity to choose any of Pontiac's optional V8s such as 350, 400, or 455 cubic inch models, including the 455 High Output (HO) engine. The 455 had ...
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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.