Originally Posted by johnracer

I see your point. Perhaps "need" wasn't the right term. If the ditributor doesn't require turning to nail the timing you want, then that'll work. But if you have to turn the distributor your rotor phasing will be off. I'm not "The Professor", but I'll do my best to explain.
First are production tolerances. All mass producers have them. Basically the relationship between the reluctor and the distributor shaft, between the distributor and the rotor, and between the distributor and cap, could all be slightly off. Let's assume that each one was off 2 degrees retarded. That's probably an extreme and unlikely example by the way. Now if you set it up the way you decribed, you'll end up moving the distributor to set timing, and the rotor will be out of phase 6 degrees retarded with the post in the cap. To further complicate matters, all of MSD's timing controls, including the 7al3, retard the timing about 4 degrees just by hooking them up, not activating them. That throws your rotor phasing off another 2 degrees retarded for a total of 8.
Now you throw in a 25 degree start retard and your rotor phasing is off another 12.5 degrees for a total of 20.5 degrees retarded. At this point, it might be easier for the spark to jump to the next post instead of the intended one and bang! It kicks back as if the timing was really advanced.
Sorry for the long post, but I couldn't think of a way to explain it any shorter.
Later,Johnny