Quadrajet on a Slant Six

fjr said

The air valves are spring-loaded so that the secondaries stay shut at wide open throttle until manifold vacuum pulls them open. On a 225 CID engine, they will never open fully, although I can hear them starting to open when I’m accelerating at highway speeds.

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But that is true of any vacumn secondary carb including the Holley 8007 (390 cfm) you mention.

FWIW, I ran a 390 Holley on the slant 6 in my Barracuda for almost 30k miles (until I swapped in a 360). The car ran great with this carb. The throttle reponse was great and the car still got 19-20mpg around town and 23-24 on the hiway. It was a fun set-up and gathered a lot of attention with the headers, 4bbl and dual exhaust but still was not what anyone would call a fast car.