Not Ranked
I have a Holley 670 as well on a Ford 306 crate motor. I went to long yellow spring and then back to purple. For drag strip I would think soft spring, full throttle and there would be no bog in a 1/4 mile. There is also interplay with pump cam and nozzle as well as jet sizes. It's a balancing act for general use. More is better for drag racing. Why not use your run times and change out spring between runs? (assuming everything else stays the same)
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