Advantages of spark+fuel vs fuel only?

[Clare Snyder]

Thor Johnson wrote:
> 
> 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
To make it REAL simple,spark advance is required to get maximum recovery
of the combustion pressure by the piston. Too early, and the pressure
acts on the piston on the way up. Too late, a large amount of energy
goes out the exhaust. This means advance is required for economy. Retard
is reqired for power. This is not strictly true, so hold on the flames,
for you guys who are more technical. An engine set-up without vacuum
advance will run just fine, but will be hard on gas. An engine without
centrifugal (or mechanical) advance will be either extremely hard to
start, low on  power, or hard on gas, or a combination.

As for tying the ignition and fuel management together, it just helps
get more power/less polution/ better economy all at once. The two brains
can work together to optimise advance and mixture for all conditions.
Separate systems cannot be as accurate in control, but can be better
than an old carb and point distributor.
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