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To change secondary opening rates, different calibration springs can be interchanged. Variables in secondary spring selection will include vehicle weight and transmission type. For street cars, the vacuum secondary carburetor works best on midweight or heavyweight cars with an automatic transmission. They are more forgiving than a mechanical secondary (called a double pumper) because they work by sensing engine load. This mechanical secondary carburetor is best on a lighter car with a more radical camshaft. In addition to the mechanical secondary actuation the rear bowl also has another accelerator pump to compensate for the immediate inrush of air that results when the secondary snaps open. This is what is meant by the term "double pumper." A lower axle gear ratio and manual transmission work best with a carb of this type.