Editing Pontiac V8 engine (section)

===455===
The 428 was increased to {{cvt|456.12|CID|cc|0}} in 1970. Again, its bore was expanded, this time {{cvt|0.03|in|mm}} to {{cvt|4.1525|in|mm}}. The stroke increased to {{convert|4.21|in|mm|1|adj=on}}.<ref>Pontiac Registry</ref> It was rated at {{cvt|360|hp|0}}, but the Grand Prix with the same specifications was rated at {{cvt|370|hp|0}}.<ref>{{cite web | url=https://www.carfolio.com/pontiac-grand-prix-428-67078 | title=1969 Pontiac Grand Prix 428 specifications &#124; technical data &#124; performance &#124; fuel economy &#124; emissions &#124; dimensions &#124; horsepower &#124; torque &#124; weight }}</ref>

The gross horsepower ratings of this era were dubious, with engines rated higher or lower in output for advertising, political, or insurance purposes.<ref>{{cite web | url=https://www.hagerty.com/media/archived/horsepower/ | title=Muscle Car Horsepower – How Exaggerated Was It? | date=13 August 2013 }}</ref> Though listed as slightly less powerful than some high-performance iterations of the 400 (such as the {{cvt|366|hp|0}} Ram-Air), the 455 had {{cvt|500|lbft|0}} of torque, {{cvt|55|lbft|Nm|more|adj=pre}}.

The engine was available in all full-size Pontiacs. An HO version could be ordered in the [[Pontiac GTO|GTO]], as GM had lifted its restrictions on offering engines larger than {{cvt|400|CID|L|1}} in mid-sized cars (resulting in the {{convert|454|cid|L|abbr=values}} Chevrolet Chevelle, {{Convert|455|cid|L|1|abbr=values}} Buick Gran Sport, and {{Convert|455|cid|L|1|abbr=values}} Oldsmobile 442).

For 1971, Pontiac introduced another High Output '''(HO)''' version with standard internal parts, a reinforced block with four-bolt main bearing caps, and improved cylinder head design with {{Convert|1/8|in|mm|taller|adj=mid}} intake ports and special round exhaust ports for better breathing, yet still making just {{cvt|335|hp|kW|0}} gross (or {{cvt|310|hp|kW|0}} in the more accurate SAE Net system). Standard in the [[Pontiac Firebird|Firebird Trans Am]], it was still a rare engine.

In 1973 a further refined and even stronger version, the Super Duty (SD) engine, was planned for Firebird, Grand Am, Le Mans (GTO), and Grand Prix models. The SD-455 used round-port cylinder heads similar to those used on the 1971 and 1972 455 HO, with specific "LS-2" intake and cast-iron exhaust header manifolds. Still, it was the strongest American engine offered that year. Pontiac's initial plans were to use a camshaft with specs identical to the 041 Ram Air IV camshaft, but testing showed emissions, while technically compliant, did not have "cushion" to satisfy Pontiac's emissions engineers, to avoid production variation leading to any non-compliant engines being produced and sold. Though a change of camshaft would delay production, Pontiac decided to change camshaft, employing one with identical timing to the 744 camshaft, which had been used in the 400 Ram Air(aka Ram Air III) engine with manual transmissions. As a result, peak horsepower dropped from 310 to 290, though torque increased from 390 to 395&nbsp;lb. ft. After the SD-455 was certified with the new cam, it was production-ready but was then further delayed by the EPA mandating Pontiac recertify all of its engines when the EPA discovered that part-throttle emissions exceeded the limits. Thus in March 1973 Pontiac eliminated a timing solenoid that gave full vacuum advance and disabled the EGR valve in high gear after a period of time that happened to exceed the running time of the EPA testing cycle. Once corrected the SD-455 was recertified and was released for production in late April 1973. Given how late in the model year it was, Pontiac decided not to incur the costs of recertifying the SD-455 for other models other than the Firebird. Ultimately Pontiac produced only 295 1973 SD-455 Firebirds (252 Trans Am, 43 Formula). For 1974 another 1,001 (943 Trans Am, 58 Formula) were built, after which the SD-455 was discontinued.

An evolution of the RA IV and H.O. designs, the SD-455 was a much improved engine. In addition to the more refined cylinder heads, block casting reinforcements in the lifter galley and main bearing oil pan rail area, it had forged connecting rods with larger {{convert|7/16|in|mm|adj=mid|-diameter}} bolts. Made with a provision for dry sump oiling, it truly was a racing engine, detuned for use in passenger cars.

The 455 was used through 1976 when it, as with many other large displacement engines, was discontinued as manufacturers moved to smaller, more efficient models, even in their full size car lines.