was renamed 428 HO for 1968. 455 HO 1970 The 455 HO designation made its debut in 1970; Rated at (depending on which vehicle it was installed into) & of torque, it differed from the regular full sized car 455 by large valve heads with smaller combustion chambers, and a larger camshaft. The 1970 '455 HO' was a conventional "D" port engine. It was rated at 360 hp @4300 RPM in the GTO and 370 @ 4600 RPM in the Grand Prix and other full-size Pontiacs. Ram Air induction was optional in the GTO, though power ratings were unchanged. Late in the model year the Ram Air 455 HO was made an available option for those GTOs with the optional "The Judge" package. 1971 The "455 HO" moniker took on a whole new meaning with the introduction of the 1971 model year; Intended as a low compression progression from the previous years Ram Air IV engine, all 1971 455 HO engines used a heavy duty 4 bolt main block, round port cylinder heads (casting #197; with 8.4: compression), "Ram Air" style exhaust manifolds, and a two-part aluminum intake manifold. The 1971 Pontiac 455 HO was Pontiac's first engine to receive a special Rochester Quadra-jet carburetor with specific jetting. The 1971 455 HO was rated at @ 4,800 rpm and of torque @ 3,200 rpm (gross). The 1971 455 HO was available in the Firebird (optional in Formula; standard in Trans Am), the GTO (standard with "The Judge" package), and the 2-door LeMans, LeMans T-37 (including GT-37), and LeMans Sport. Ram Air induction was optionally available with the 455 HO in the Firebird Formula(standard on Trans Am), GTO (standard with "The Judge" package), and 2-door LeMans Sport when the T41 Endura Styling Option was also ordered. 1972 The 455 HO moniker was again carried over, this time as a near-exact repeat of the 1971 offering, the only changes were the carburetors (they used a conventional unit this year), and the head castings (casting #7F6). According to GM mandates horsepower was now rated in net figures as opposed to gross, so on paper the 1972 455 HO appeared to have a significant drop in power, but in fact it was very much the same engine, and performance figures reveal this to be true. The 1972 455 HO was rated at @ 4,000 rpm and @ 3,200 rpm. The 1972 455 HO was