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Old 07-05-2016, 01:56 PM   #1
mrsaturn7085
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Default Secondary (Spec C) IAT sensor calibration

Due to my OCD, I decided to characterize resistance curve of the secondary IAT that comes on the GD-series Spec C EJ207 engine over the weekend.

I used a combination of precision heat gun (100 to 130 deg C), water soak w/immersion (lab) thermometer (30 to 90 deg C) and GpN calibration data (-40 to 20 deg C) to get the following list. Took well over three days to do all of this while keeping the battery above 12V, boiling and cooling water, and capturing test data.

Hopefully this will save others some time:

Temp. (deg C) | Voltage (V)
-40 | 4.740
-30 | 4.608
-20 | 4.474
-10 | 4.321
0 | 4.117
10 | 3.835
20 | 3.465
30 | 3.036
40 | 2.621
50 | 2.218
60 | 1.859
70 | 1.523
80 | 1.236
90 | 0.997
100 | 0.806
110 | 0.675
120 | 0.548
130 | 0.444
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Old 07-07-2016, 09:32 AM   #2
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Deff nice to see you took the time to do this!
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Old 07-07-2016, 10:44 AM   #3
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You, Sir, are a Golden God.
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Old 07-11-2016, 10:54 AM   #4
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Just a quick question, because I'm a noob to this sensor.

I DO have this sensor on my intake, but it appears that my rom (carberry) is using the MAF IAT sensor instead.

How are you guys using this IAT? Are you splicing into the 5 MAF wires and connecting the IAT wires to this IAT? Seems illogical, since the IAT IS wired up, but I haven't been able to find much info.

Trying to figure this out before I remove my intake to drill and tap for the GM sensor.
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Old 07-11-2016, 01:17 PM   #5
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I have rewired my MAF plug with a breakout cable. The three MAF wires are still connected (pass-thru) while the two IAT wires are routed to my intake manifold. The IAT on the MAF is effectively disabled, and the MAF is only used for reference on my standalone system. The break-out cable does give me the ability to pop my OEM ECU back into the car in less than a minute, however.

Parts I purchased were:

Visconti Tuning "WRX/STI Speed Density Kit"
APEXi IAT harness, p/n 415-XA03

The APEXi IAT sensor for the FD (RX-7) is the only sensor I have found that matches the original Subaru secondary IAT w/M12 threads. I do NOT know if the APEXi FD sensor is the same resistance curve, however. I am sure you could build this harness for less than I paid... I just couldn't find the connectors I needed outside of these two kits.

I also needed to do some light trimming on the APEXi connector; the Spec C manifold w/DBW throttle & adapter plate is a hair too close to the sensor to allow the connector to plug in, unmodified.

Last edited by mrsaturn7085; 07-11-2016 at 01:23 PM.
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Old 07-11-2016, 03:28 PM   #6
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I also have a IAT break out cable from carberry's flex site...

If you're saying I could just use that IAT instead of the MAF IAT by splicing into those wires (which I already have a connector for, with the spec C harness) I should be good.

How do you think the stock spec C sensor would compare to something like the GM open element sensor?
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Old 07-11-2016, 03:40 PM   #7
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Yes, you can break-out the wires and just run the new sensor.

The IAT fueling comp. is very small (within 6-10%) unless IAT values are swinging wildly (and if you're properly set-up, they shouldn't) and the OEM sensor was used in GpN competition without issue. Things like intercooler water spray help stabilize IAT on these cars, so keep that in mind. I've used the GM sensor in the past - it's marginally faster but much bigger, physically.

As always, log data and make adjustments when needed. IAT compensation follows the ideal gas law - really easy to plug these values in.

Last edited by mrsaturn7085; 07-11-2016 at 03:46 PM.
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Old 07-11-2016, 04:05 PM   #8
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Subbed, thanks!
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Old 07-11-2016, 09:51 PM   #9
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This is good information. I really didn't want to drill and tap the intake manifold with a perfectly good sensor already in place. I'll have to take a look and see how it turns out when I have it in the garage next. Thanks for the help.

Quote:
Originally Posted by mrsaturn7085 View Post
Yes, you can break-out the wires and just run the new sensor.

The IAT fueling comp. is very small (within 6-10%) unless IAT values are swinging wildly (and if you're properly set-up, they shouldn't) and the OEM sensor was used in GpN competition without issue. Things like intercooler water spray help stabilize IAT on these cars, so keep that in mind. I've used the GM sensor in the past - it's marginally faster but much bigger, physically.

As always, log data and make adjustments when needed. IAT compensation follows the ideal gas law - really easy to plug these values in.
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Old 07-12-2016, 11:45 AM   #10
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Quote:
Originally Posted by anjuna View Post
This is good information. I really didn't want to drill and tap the intake manifold with a perfectly good sensor already in place. I'll have to take a look and see how it turns out when I have it in the garage next. Thanks for the help.
Keep in mind the Spec C manifold is the only one with the sensor, and the OEM MAF-based IAT is pre-turbo. Unless you have an EJ207, it still takes some work. Drilling and tapping the non-Spec C manifold is FAR easier than installing a Spec C manifold properly on an EJ255/EJ257.
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Old 07-12-2016, 10:56 PM   #11
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Quote:
Originally Posted by mrsaturn7085 View Post
Keep in mind the Spec C manifold is the only one with the sensor, and the OEM MAF-based IAT is pre-turbo. Unless you have an EJ207, it still takes some work. Drilling and tapping the non-Spec C manifold is FAR easier than installing a Spec C manifold properly on an EJ255/EJ257.
I do have an ej207 spec c. I saw the IAT. That's what started my first question of how he was seeing this IAT in the tune
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Old 07-13-2016, 12:31 AM   #12
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Ah my mistake - sometimes I neglect to read the name behind the new post and thought it was a new person asking a question!

I am frankly not sure how the OEM ECU reads the secondary IAT (or if it even does); though the JDM 2008-2014 FSM seems to indicate that the OEM ECU *did* use both, I have my doubts.

The reasons that I think the OEM sensor may not actually do anything on the OEM ECU are two-fold:

1. The GpN ECU (2008+) used flying wires on the PCB to connect the secondary IAT trace to another input. I have not seen the inside of the GD-series GpN ECU... but I have a strong hunch that it is similar in design.
2. The OEM autowash thresholds (spray above 20 deg C) are considerably lower than the values used in the GpN ROM (spray above 40 deg C) - which seems to indicate to me that they were pre-turbo values.

The IAT would have needed to be present (on the OEM motor) to be used in GpN competition however, due to restrictions on wiring harness modifications.
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Old 07-23-2016, 10:47 PM   #13
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For those of us using inferior units and carberry, i did a linear reg on your values and converted to F



0.21 297.2935328
0.36 273.9494092
0.52 251.3847744
0.67 232.2879207
0.83 213.9664537
0.99 197.6066466
1.14 183.91017
1.3 170.898696
1.46 159.3777659
1.61 149.7842577
1.77 140.6809218
1.92 133.0587854
2.08 125.7395015
2.24 119.0880139
2.39 113.3030757
2.55 107.4453157
2.71 101.7349513
2.86 96.35407649
3.02 90.41121129
3.17 84.48772121
3.33 77.61583061
3.49 69.98679582
3.64 61.97635148
3.8 52.33247738
3.96 41.36177344
4.11 29.69551398
4.27 15.58825275
4.42 0.627302022
4.58 -17.37602355
4.74 -37.69251073
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Old 07-25-2016, 12:26 AM   #14
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No linear regression is required for this conversion... you just need to convert the deg C to deg F and truncate the voltage at the hundredths (or whatever accuracy your management allows). Honestly, going to the thousandths on voltage is overkill for such a slow-moving value (IAT), but since my equipment reads that precision, why not?

Since -40 deg C = -40 deg F, I think you may have a few slight calculation errors.
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Old 07-25-2016, 12:52 PM   #15
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Quote:
Originally Posted by mrsaturn7085 View Post
No linear regression is required for this conversion... you just need to convert the deg C to deg F and truncate the voltage at the hundredths (or whatever accuracy your management allows). Honestly, going to the thousandths on voltage is overkill for such a slow-moving value (IAT), but since my equipment reads that precision, why not?

Since -40 deg C = -40 deg F, I think you may have a few slight calculation errors.
My table was much larger in range than the range you posted, so I had to find some other values.

Yeah, -40 is -40, but with the regression run, I was much more interested in the middle range (where the engine will mostly operate).

It's also kinda slow, but much steadier than the MAF IAT.
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Old 09-27-2016, 02:55 PM   #16
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An example of a short data-log of IAT under normal (not angry) driving:



After this log, the car was parked for about 5 minutes. When started again, IAT had risen from 27.5 to 34.1 deg C, so without airflow, you'll get some pretty quick changes in the reading!

Even on a hotter (85-90 deg F) day, the IAT peaked around 45 deg C while in motion, under boost with the I/C spray used occasionally. Having a post-throttle IAT of 40 deg C is a great target for closed-loop I/C water cooling purposes (if you have the means).

Last edited by mrsaturn7085; 09-27-2016 at 03:02 PM.
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