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I'm working on a little project where I want to calculate the boost pressure (in bar) which my turbo currently produces. I use a wifi OBD reader that I plug into my car. I saw mobile apps calculating pretty accurate values based on my internal sensors.

Now my question: Which values do I need for calculating the actual boost? I found this forum where one guy explains a Formula. He uses the MAP sensor and a barometric pressure sensor. However I don't know which PIDs they are represented with. Until now I thought the "MAP" sensor is the "intake manifold absolute pressure", but on Wikipedia this values goes only from 0-255. Looks like a pretty unprecise indication.

Can anybody help me identifying the proper PIDs so I can calculate my current pressure?

Thanks already in advance!

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  • Not sure what the "Autotap" that he mentions is, but his formula - well what was explained between them is simple : reading - atmos P = boost P, where boostP is gauge pressure and the reading is absolute pressure.
    – Solar Mike
    Jun 10, 2017 at 17:25
  • Yeah I understand that, but I'm not quiet sure which PIDs I have to read to get the needed values. it's actually the point where I'm getting confused.
    – binux
    Jun 10, 2017 at 17:39

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Calculating Boost from OBD PIDs is pretty straight forward.

Mode 01 PID 0B (Intake manifold absolute pressure) - Mode 01 PID 33 (Absolute Barometric Pressure)

255 doesn't sound like much resolution but it corresponds to 0-36.98psi. Which is a resolution of 0.145PSI.

Another option would be to monitor the raw CAN bus data. Hook up a pot to the MAP sensor to make identifying the ID & Data easy, and repeat for the Baro sensor. Obviously the ECU would have to transmit that data onto the CAN bus and without knowing what vehicle it is I don't know, most manufactures transmit Baro as its used for HVAC and other interior functions but not many transmit MAP

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  • Indeed you're right. Based on the last formula in the forum I calculated a boost value of 1.31bars. I had a MAP value of 230kpa and a barometric pressure of 99kpa. However, it still looks limited to around 1,5 bars. If I have a baro pressure of 99kpa this reduces the maximum value on nearly the half. I know from some diesel cars that they have boost values above 1,6bars. So how could I possibly get such a high value? My car is a european Ford. So we have all the ford specific CANs. I use OBDLink MX for reading which I think is capable of this aswell.
    – binux
    Jun 11, 2017 at 7:35
  • "255 ... corresponds to 0-36.98psi" Just curious how you got this value?
    – pete
    Jun 15, 2018 at 20:41

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