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Since the AFC is intercepting the MAS signal and modifying that signal to allow you to change the A/F ratio, a byproduct of making any changes with an AFC is that the timing curve gets all screwed up. You see, the ECU uses the MAS and engine RPM to determine engine load. Its what they call a volumtric efficiency calculation; where it divides the airflow by the RPM and then you know how much air is entering each cylinder per intake stroke. Well, once the ECU determines how much air is in the cylinder, then it knows exactly how much fuel to deliver to make the air/fuel mixture you are specifying in the fuel map. If you change the MAS voltage, you change how much air the ECU *thinks* is coming into the cylinder and you are playing trickery on the ECU to manipulate fuel. Well, the ECU uses this same calculation to determine ignition timing, since timing is based on the charge density and RPM, which is determined when it measures how much air is in the cylinder. So the ignition timing is dependent on the MAS and RPM, just like fuel delivery.. So, when you're on the dyno and correcting that A/F ratio, all you are seeing is the change you are making to the A/F, but if you were to look at the timing curve, it will inevitably become more and more like the shotty fuel curve you are correcting. The more you fix A/F with an AFC, the more you screw up your timing...
[ ashleypowers.com ]
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