'69's came with purples. '70 up had yellows. i did some dragstrip test once and for every step up (stiffer) the car went about a tenth slower in the 1/4 mile. this was with a near stock 69 coronet, 3.23 gear, torqueflite. my car gave the best e.t. with yellows. purples will give a little more torque. plain springs went almost 3 tenths slower than yellows. if you have a serious bog i'd look at float level too low, main jetting and idle circuts too lean, low fuel pressure, or lazy timing. stiff springs may only mask another problem. there is a 6-pak tune-up presentation that often is tossed into the suggestions when tune-up questions are asked on moparts. beware, its pure garbage!