Timing & Octane: '91 200TQ Motronic ECU

["John W. Cooper" <Jacques.Noir@worldnet.att.net>]

Chris, I have a '91 200 Wagon in which I have installed a TAP chip set.
But I  have read considerable material on the Bosch Motronic ECU, and
looked at length at the code/maps in the EPROMS.
What goes on in there is explained in the book "Bosch Fuel Injection &
Engine Management" by Probst, available at some foreign car parts stores.
Basically what happens is that the fuel and ignition timing are controlled
by 'maps' or memory points determined by exhaustive dyno testing.  For each
RPM and load (air travel through the air mass sensor plumbed in right after
the air cleaner) and ambient temperature the ECU looks up the value stored
for that particular situation, adjusts the ignition timing, and the fuel
delivery.  As long as the knock sensors don't send back any signs of
distress, the map stored is always used as a reference for mixture.  But if
there is signs of detonation, the ECU will dial back the ignition advance
to the point where the knock stops.  In extreme cases, the boost will be
bypassed by the waste gate control.
But under normal circumstances, no matter what gas you put in the tank the
ECU will behave just as if you have the max octane recommended UNTIL the
engine knocks.
There is no compensation for high or low octane fuel as long as there is no
knocking; the ECU always assumes the most optimistic situation until told
otherwise.
I hope this helps.