Well... I see we have some differing opinions of the state of this battery.
For openers, 427sharp is correct that no battery will have ZERO voltage drop under the load of the starter. I read somewhere that a good battery will drop to about 9.5 to 10.5 volts under normal conditions. If it goes below nine volts, then it considered time for replacement.
Second, the alternator will NOT come into play until the engine is running and turning enough revs to get the alternator to put out a "charging" voltage. (Alternator has to supply more voltage than the terminal voltage at the battery)
So..... if the MSD requires 12.5 volts to work, how does it start the car? My guess is that it will supply a spark.... enough to start the car...... but not at a level that it can output if supplied by 12.5 volts or greater.
If the MSD troubleshooting manual says that it needs 12.5 volts to work (and will not work AT ALL at 12.4 volts or below ) then give me back my simple mechanical points and coil.
I still say the problem is elsewhere and not related to this battery.
- Jim -