[67mustang302]

The secondaries get their signal from the same place as the fuel, the carburetor barrel. There's a little hole inside the barrel that's a vacuum port for the secondary diaphragm, the faster air flows through the barrels, the lower the pressure in the barrels, the stronger it draws fuel and pulls on the secondary diaphragm. As far as what rpm the secondaries open, that depends entirely on the carburetor and engine combination(and vehicle setup as well). The rpm range that Holley lists on those springs is true, but only for the engine they tested with the carb they tested and the way they tested it. If you put a smaller carburetor on the same engine the secondaries will open sooner from a stronger signal, if you put a larger carburetor on then the secondaries will open later from a weaker signal. More engine for the same carb size opens quicker and so on and so forth. Yes, secondary opening is limited mechanically by the primary throttle position, but the rate at which the secondaries open and the point at which they open depends on the particular setup.