That's a Great Question, and RustyRatRod, Rumble, and many others would be better info for you, However, " phasing" the distributor (not dizzy, RRR, lol) is setting the rotors tip, to the plug tower. For guys like us, it's not worth the hassle, IMHO, but when you get your engine Mods dealt with, then a " curved " unit will kick it in, hard, depending