(AEM EMS only) Setting MAP Pressure in gaugeDesigner

The AEM EMS is very unusual compared to other engine management systems when it comes to transmitting MAP (ie. boost) pressure over telemetry. Most systems transmit MAP as a unit like PSI or KPA. The AEM EMS transmits MAP as “Engine Load” where the range of the MAP sensor is scaled over 0-100%. This adds an extra step when setting up telemetry products like the gaugeART Video Gauge Adapter.

For digital gauges:

  • Set gauge to “Digital”.
  • Set Input Type to the MAP sensor that you are using.
  • The gauge will automatically reflect the output from the EMS (within .1-.2 PSI).

For circular or bar gauges:

  • Circular or bar gauges are more complex because the gauge has to match the range of “Engine Load” you wish to display. For example: if your engine has a 4 BAR MAP sensor, the full range of the gauge (0-100%) would be 1 BAR in vacuum and 3 BARs in pressure or -14.6 – 0 PSI and 0-43.5 PSI. This is transmitted as 0-255 bytes from the ECU where 0 = 0% engine load and 255 = 100%. If your gauge is -14.6 – 0 – 43.5 PSI, this would mean that you’re seeing the full range of the data from 0-255 and the gauge should be configured as shown below.

  • Due to the unusual way AEM does MAP pressure, things are more complex when you only want to see a portion of the range of the “Engine Load”. For example, if you’re using a 4 BAR MAP sensor, and you want to display only positive pressure (0-30 PSI for example) you would be displaying only about the middle of the range of the sensor from 0 – 30 PSI instead of -14.5 PSI to 43.5 PSI.
  • To calculate what byte range is required, first, calculate the byte scale (full range of the sensor over 256 bytes).
    byte scale = (full range of sensor / 256 bytes)
    byte scale = 58 PSI / 256 = .227
  • Next, calculate your minimum byte (where you want the gauge to start) as
    minimum byte = (minimum display value – minimum sensor value)/byte scale
    minimum byte = (0-(-)14.5)/.227
    minimum byte = 14.5/.227
    minimum byte = 64
  • Next, calculate your maximum byte (where you want the gauge to start) as
    maximum byte = (maximum display value – minimum sensor value)/byte scale
    maximum byte = (30-(-)14.5)/.227
    maximum byte = 44.5/.227
    maximum byte = 196
  • Use the minimum and maximum byte in the window as shown. Now, your displayed range of the sensor will match the sensor value.