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OBD Calculated Load is defined as:

LOAD_PCT = [current airflow] / [(peak airflow at WOT@STP as a function of rpm) * (BARO/29.92) * SQRT(298/(AAT+273))]

But when I am in neutral and I open the throttle, calculated load does not change at all according to my OBD scan. It only changes when I am actually moving. I don't see how, according to the definition, that should matter.

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Do you have a source for that formula?

Is it possible that you are reading Calculated engine load value, which would indeed be zero when you are in neutral?

Calculated Engine Load Value

  • Range: 0 to 100
  • Units: %
  • Formula: A*100/255

I did find this page which defines it as:

Calculated Load Value -- Indicates a percentage of peak available torque. Reaches 100% at wide open throttle at any altitude or RPM for both naturally aspirated and boosted engines.

In which case, I would still expect it to return non-zero values when in neutral, as it's not transmission load.

It looks like you're getting the formula from https://github.com/Knio/carhack/blob/master/old/can/pids.py, and if that formula is indeed correct and implemented correctly by the car's computer and manufacturer, then it "Reaches 100% at WOT at any altitude, temperature or rpm for both naturally aspirated and boosted engines", and should indeed be independent of vehicle speed or gear engagement (or lack thereof).

  • That definition is from the SAE standard J/1979, which defines the OBD PIDs. Thanks for your response. – Magnus Anand May 1 '15 at 14:10

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