%META:TOPICINFO{author="blundar" date="1080583318" format="1.0" version="1.1"}%

Understanding Fuel and Ignition Maps.

Fuel Maps

Below around 80% throttle, the ECU uses the oxygen sensor in conjunction with the fuel tables to calculate how much fuel is needed for your engine. This is called "closed loop" mode because the ECU is in a closed feedback loop using the Oxygen Sensor to steer the Air Fuel Ratio to the desired level. Above the throttle position threshold, the ECU exclusively uses look up tables. The ECU looks through the lookup tables and, depending on engine speed and intake manifold vacuum, selects a length of time that the injectors will stay open. If you are interested in modifying your ECU, chances are that your ECU spends a fair amount of time in the look up table mode (open loop).

I will assume you are looking at an editor like this:

The columns are split up into small segments based on intake manifold vacuum. The left most column is the most vacuum the ECU accounts for (shut throttle plate) and the right is obviously WOT. The OBD0 ECUs have 15 of these columns and use inches of mercury as a unit. The OBD1 ECU's use 10 columns and use millibar (mBar) as a unit. The editor can choose to display this information in two ways. Absolute Pressure or Relative Pressure.

The rows are split up into little rev ranges.

The values in the cells are in what is believed to be milliseconds that the injector stays open. To calculate these values the editors use a formula (in this case: OBD0 Fuel; OBD1_8bit Fuel for OBD1 applications) that combines the raw 8 bit value at the location of the cell in the map with a Column Multiplier to arrive at the value you see in the editor. Some editors allow you to directly manipulate the raw value directly instead of interpreting it as a pulse-width

Ignition Maps

The ECU also does the look up table thing with the ignition maps.

Again, the columns are vacuum and the rows are RPM. The value in the cells is degrees of ignition advance.

Ben Ogle