 This carburetor was later replaced by the Quadrajet, a spread bore.The term "Spread-bore" signifies the difference in sizes between the primaries and secondaries, using smaller primaries paired with larger secondaries for increased airflow at wider throttle with fuel delivery changes akin to the two-plus-four benefit of Tri-Power but with a single carburetor.The Q jet was not the only thing that gave the top GTO 400" engine and the 428 H-O engines the same horsepower as the 389 and 421.