Advantages of spark+fuel vs fuel only?

[Clive Apps Techno-Logicals 416 510 0020]

> 
> 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)?


on a street car
if you have light load under cruise
you can lean out the mixture and increase the timing advance to get
approx double the normal A/F ratio
this is quite beneficial to the pocketbook

by changing both spark and fuel at the same time you can get close to the 
optimal engine config for any situation
if you had control over cam timing as well you really optimive the setup
for all conditions

Clive