PMD also had a square-bore 4-barrel at the time, but this was rated at a lower power than the Tri-Power. 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. Aside of the displacement advantage, the new engine had redesigned cylinder heads with different valve angles and larger ports. The different valve angles allowed for larger diameter intake and exhaust valves. There have been many test when a Tri-Power set-up was added to a 400" or 428" engine that they made even more horsepower than a Q-Jet.