Normally a mild converter, weak rearend gears, and/or a heavy vehicle will call for a smaller carb to retain decent low-end performance. But we know the smaller carb restricts power upstairs. One possible solution: Run the larger carb, but with vacuum (instead of mechanical) secondaries. Vacuum secondaries won’t open until the engine needs the extra airflow. Assuming the vacuum secondaries are properly tuned with the appropriate-tension spring, the engine won’t bog even if you punch the throttle wide-open at low speed. But when there’s sufficient primary airflow to allow the diaphragm to open the secondaries, the engine is ready to accept the extra capacity. Grant says vacuum-secondary carbs work particularly well if running under 3.55:1 gears, the car weighs over 3,500 pounds, or if you have 8.5:1 or less compression.

For just about any other combo, Grant recommends staying with the twin-squirter, mechanical-secondary carb. “If the engine withstands a mechanically actuated secondary without bogging, the rpm will rise quicker and performance will be better than with a vacuum secondary,” says Grant. Note that twin-squirter carbs are much more finicky about proper sizing than the more forgiving vacuum-secondary models, which is why there are more different twin-squirter sizes available in comparison to vacuum-secondary cfm ratings.

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The Thunder Series air valve allows the secondary actuation rate to be adjusted in seconds, without the swapping of parts, for easy and quick tuning of the secondary action. We've had lengthy street time with the AVS in a hot small-block application, and the system really works. The Thunder Series AVS can be had in 650-and 800-cfm flow capacities, and shares tuning components with the Performer Series carbs.

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