The SD 455 was available for one more year in the Trans Am. As emissions regulations were increasing, two of the three iterations of the engine were discontinued after 1974, with the final version of the 455 remaining available through 1976. For 1975, Pontiac introduced the new sub-compact Astre, a version of the Chevrolet Vega. This was the brand's entry into the fuel economy segment of the market. Astre had been sold exclusively in Canada since 1973. It was offered through the 1977 model year. 1975 would also be the end of Pontiac convertibles for the next decade. The 1976 models were the last of the traditional American large cars powered by mostly big-block V8 engines. After this year, all GM models would go through "downsizing" and shrink in length, width, weight, and available engine size. The 1976 Sunbird, based on the Chevrolet Vega and Monza's equivalent, joined the line. It was first offered as a Notchback, with a Hatchback body style added in 1977. The Vega Wagon body style was added in 1978, Sunbird Safari Wagon, replacing the Astre Safari Wagon. The Sunbird was offered in its rear-wheel-drive configuration through the 1980 model year. (Sunbird Safari wagon through 1979.) In mid-year 1977, Pontiac introduced the Phoenix, an upscale version of the Ventura which replaced the Ventura entirely after the end of the 1977 model year. Pontiac also introduced its "Iron Duke" 4-cylinder overhead valve engine. It was first used in the 1977 Astre, replacing Astre's aluminum-block Vega engine. The 'Iron Duke' engine would later go into many GM and non-GM automobiles into the early 1990s. The 151 cubic inch I4 and the V8 were the last two engines designed solely by Pontiac. Subsequent engine design would be accomplished by one central office with all designs being shared by each brand. For the 1977 model year, the full-sized Pontiacs received the same "downsizing" as GM's other "B" body cars. The new Bonnevilles and Catalinas continued to be best-sellers, although their styling similarity to the Chevrolet Caprice. In 1981, the full-size Bonneville was discontinued, and the name was reassigned to the "A" body intermediate platform. That left the Catalina as the only big Pontiac, further reducing sales as buyers went for more plushness. The remainder of the 1970s and the early 1980s saw the continued rise of luxury, safety, and economy as the key selling points in Pontiac products. Wire-spoked wheel