Okay, I am going from memory so take this all with a bag of salt. The standard 440 valve gear was good for about 5800 rpm. Chrysler wanted to raise this to about 6200 rpm. So they installed heavier springs on the Six-Pack motors. Heavier spring...more cam load....a low taper lobe provides more contact area and maintains engine durability with the higher loading. Again, I'm going from memory and I only have two functioning brain cells left.