"If you are reluctant to trade top-end performance for all-around driveability, there are several cheats that will allow a larger-than-normal (for the given application and combo) carb to do a satisfactory driveability job. One important method is secondary actuation.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."
