Advantages of spark+fuel vs fuel only?

[Thor Johnson]

Could someone tell me the advantages of having the
uC control both the fuel and the spark?  Fuel-only
is pretty forgiving, and I'm not sure I can get
the timing perzactly right for the spark not to do
bad things.

Can you do special things if you have the spark
computer know about the fuel computer?

I guess a good first question would be: can anyone
explain the mysteries of BTDC/ATDC and why I want
to advance it more than stock (already know you
need to retard it for knocks)?


TIA,
-Thor Johnson