I guess my understanding of the Edelbrock power circuit is flawed too. I thought the step-up pistons were pulled down the bores by manifold vacuum. Thus, a spring rated for 7" Hg (silver) would begin raising the metering rods at 7" Hg and springs rated at 5" Hg (orange) would begin raising the metering rods at 5" Hg. In a nutshell: stiffer springs start pushing earlier as manifold vacuum approaches theoretical zero.