was made of cast iron with five main bearings, rather than the relatively fragile cast aluminum block used by the 2.3 L Vega engine. (Even with the cast iron block the Iron Duke only weighed about 20 pounds more.) The 2.3 L engine's belt-driven overhead camshaft was eschewed in favor of an overhead valve design with timing gears. Specially-designed bolts that stretch slightly farther than a conventional bolt were used to secure the intake and exhaust manifolds to the cylinder head, to allow slight movement while maintaining the seal of the gaskets in order to prevent cracking the manifolds as they expand with heat. A two-stage, two-barrel carburetor with electric choke was used to improve performance in cold starts, while heat shields incorporated underneath the carburetor and between the intake and exhaust manifolds were used to prevent heat soaking the gasoline in the carburetor thereby improving performance in hot weather. Recognizing that cars with four-cylinder engines equipped with air conditioning tended to experience drivability issues in hot weather, other improvements were made including a cut-off switch that shut the compressor off at wide open throttle and a delay incorporated into the air conditioning's circuitry to prevent the compressor from engaging on until twelve seconds after the engine was started. Early applications The Iron Duke's first applications were in the 1977 Astre and Sunbird subcompact cars, replacing the 2.3 L Vega engine, and in the compact Phoenix. As these cars were originally designed for Chevrolet engines the Iron Duke also used the Chevrolet bell housing bolt pattern, instead of the Buick-Oldsmobile-Pontiac V8 pattern. The following year use of the engine expanded to the Sunbird's Chevrolet and Oldsmobile twins, the Monza and Starfire. For the 1979 model year, the engine was extensively redesigned. The original reverse-flow cylinder head was replaced by a crossflow design, a new two-barrel carburetor called "Vara-Jet" was introduced, the distributor was relocated, and the size of the oil pan was reduced. The only parts carried over from the 1978 engines were the connecting rods. Peak power increased to 90 hp. For 1980 the Iron Duke engine was redesigned to be mounted transversely, to suit the new front-drive General Motors "X-body" cars. The bellhousing bolt pattern was revised to match that of the new 60° V6 engine. GM also began selling the engine to American Motors Corporation (AMC) starting with the 1980 model year. It was the base engine