73-87 Chevy _ GMC Trucks > Engine/Drivetrain

A note about carburetor flow capacities

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Greybeard:
Ultimately, Rich has it nailed. The QJ on most engines will very seldom even get the secondaries open a quarter of the throw in most street scenarios. Pulling a trailer is where they shined. The more the load, the better they worked. But the air flow really has to build (on a stock carb) to get the secondaries to open.

I have seen two distinct QJ's of the same CFM. One has very small primary venturi(s), the other has huge primary venturi(s). Holley spread bores are the same configuration small in front big in back. Talk to old-time circle track racers and they will tell you that a four barrel sucks, a two barrel is the way to go for performance. The faster the air flows the better throttle response, that is why keeping the primary venturi small helps. It will allow higher air flows (better throttle response) at low RPM's where it's used most often. Then as the engine needs the air, the secondaries will open. Tuning this transition in a QJ is something I could never do very good so I used Holley's with mechanical secondaries more often than not. However, my '74 Monte Carlo had a 454 with a QJ that ran like a raped ape (for it's size and weight). It would run consistent 14.995-15.195's all day every day in the quarter mile brackets. Slow by most folks standards but speed was not the goal, consistency was. Besides, the car weighed 4,700lbs and was as aerodynamic as a cinder block.

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