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    Position of Air Temp sensor in Plenum

    On the MX5 I am considering using one of these Bosch integrated air temp and MAP sensors: https://www.efisolutions.com.au/pres...4-bar-map-sens

    There is a convenient flat area on the back of the BP intake plenum just below the brake booster port where the sensor could easily be mounted:


    However, this is right above the 4th cylinder runner, and furthest from the air stream through the intake. So it wont be as fast to respond to IAT changes.

    I notice from this video that the same IAT/MAP sensor is just located mid plenum by GM:


    So it cant do all that much.

    The real question however, is how much does fast IAT compensation make a difference? I would think that most of the time IAT would be relatively small changes over longer periods as the IC heat soaks or air/track temp increases.

    Will it make a significant difference popping it at the back of the plenum?
    Chris
    ------
    The new nugget
    I never saw a wild thing sorry for itself. A small bird will drop frozen dead from a bough without ever having felt sorry for itself. - D.H.Lawrence

    #2
    The ECU corrects for air temp using Kelvin and a linear equation. The difference between 30 C and 40 C isn't much when you put it in to perspective like that. 33% change in C is only 3% in Kelvin.

    The ecu uses the ideal gas equation or Boyle's law to correct back to 25 C within the volumetric efficiency calculations. The user or correction tables typically remain at zero for fuel otherwise you are doubling up on correction.

    I like to have the sensor before the throttle plate, so it's away from the heat soak of a hot metal plenum.
    Jason Broadhurst

    Someone once asked me if they could use my mower. I said "sure, so long as it doesn't leave my yard"

    Comment


      #3
      It's still in the "primary chamber", it's not like you're putting the sensor in a single cylinder's runner, right?

      Comment


        #4
        Originally posted by Jason Broadhurst View Post
        The ECU corrects for air temp using Kelvin and a linear equation. The difference between 30 C and 40 C isn't much when you put it in to perspective like that. 33% change in C is only 3% in Kelvin.

        The ecu uses the ideal gas equation or Boyle's law to correct back to 25 C within the volumetric efficiency calculations. The user or correction tables typically remain at zero for fuel otherwise you are doubling up on correction.

        I like to have the sensor before the throttle plate, so it's away from the heat soak of a hot metal plenum.
        Thanks for the details on how it works internally in the ECU. I thought it would work similarly and hence perhaps not worry so much about air temp.

        The nice bit with the Bosch sensor is that it combines MAP and IAT anyway, so only one set of wiring and not much more than the standard IAT job. But if I cant get it in an optimal position I may as well use the internal MAP sensor and revert to IAT in the IC pipe pre throttle.
        Chris
        ------
        The new nugget
        I never saw a wild thing sorry for itself. A small bird will drop frozen dead from a bough without ever having felt sorry for itself. - D.H.Lawrence

        Comment


          #5
          Originally posted by aerobrick View Post
          It's still in the "primary chamber", it's not like you're putting the sensor in a single cylinder's runner, right?
          Yeah. In the main plenum.
          Chris
          ------
          The new nugget
          I never saw a wild thing sorry for itself. A small bird will drop frozen dead from a bough without ever having felt sorry for itself. - D.H.Lawrence

          Comment


            #6
            Adaptronic has a far better heat soak calculation for plenum mounted IAT's so maybe consider your original plan. If it were any other ECU I wouldn't put it in the plenum.
            Jason Broadhurst

            Someone once asked me if they could use my mower. I said "sure, so long as it doesn't leave my yard"

            Comment


              #7
              Also check the type of IAT. If it's a small bulb style with thin legs to the body, heat soak will not effect it as much as the older monolithic style sensors (think ECT sensor looking)

              Ideal


              Not ideal
              Jason Broadhurst

              Someone once asked me if they could use my mower. I said "sure, so long as it doesn't leave my yard"

              Comment


                #8
                you also want one with a fast reponse time so when it does get heat soaked the temp comes down in a sensible time as the airflow is increased
                E30 325is M20 3.1L

                Comment


                  #9
                  Its the Bosch sensor above https://www.efisolutions.com.au/pres...4-bar-map-sens

                  Certainly not a monolithic unit. And should probably be ok.

                  Might just T the wiring in the loom so Incan always swap the IAT to a pre throttle IC pipe mounted unit later on if need be.
                  Chris
                  ------
                  The new nugget
                  I never saw a wild thing sorry for itself. A small bird will drop frozen dead from a bough without ever having felt sorry for itself. - D.H.Lawrence

                  Comment


                    #10
                    Weld a section of flat plate into the intake pre throttle body?

                    Comment


                      #11
                      Originally posted by Paul M View Post
                      Weld a section of flat plate into the intake pre throttle body?
                      MAP sensor...
                      Chris
                      ------
                      The new nugget
                      I never saw a wild thing sorry for itself. A small bird will drop frozen dead from a bough without ever having felt sorry for itself. - D.H.Lawrence

                      Comment


                        #12
                        I mounted one of those sensors in the plenum of my 2zz, the IAT response is quick enough, you can see the air temp change with airflow change
                        https://1drv.ms/u/s!AtAw9ijYShwIkf9qogfMFFOWqc78OA

                        Comment


                          #13
                          for reference, 10 degrees is 3% density change at normal ambient temperatures (which means a 3% fuelling change). any sensor will head soak up to the plenum temp (which is basically the head temp) with the engine off.

                          you can overcome the heatsoak with startup enrichment using a table with inlet air temp and number of cycles since start, and balance that with the number of cycles taken for the sensor to cool down from the ingested air. even the open sensors are quite slow at very low mass flows (idle) so the engine cycle axis works well to match the amount of air that has gone past it.

                          beware the bosch combined IAT and MAP doesn't have great resolution in vacuum (only goes down to 30kPa absolute) so isnt really suitable for very high vacuum engines. i'd put it before the throttle body as you'll peg the sensor on high RPM throttle lift if you were going to run one.

                          Oo___oO

                          Comment


                            #14
                            you also get heat soak with engine running but little airflow. its not just on shut down
                            E30 325is M20 3.1L

                            Comment


                              #15
                              Originally posted by burn is weird View Post
                              beware the bosch combined IAT and MAP doesn't have great resolution in vacuum (only goes down to 30kPa absolute) so isnt really suitable for very high vacuum engines. i'd put it before the throttle body as you'll peg the sensor on high RPM throttle lift if you were going to run one.
                              Only 30kpa? So -10psi? That really isnt much at all.

                              Sounds like the standard Honeywell cage sensor plus the onboard Haltech is the go then.
                              Chris
                              ------
                              The new nugget
                              I never saw a wild thing sorry for itself. A small bird will drop frozen dead from a bough without ever having felt sorry for itself. - D.H.Lawrence

                              Comment

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