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# Air Intake Temperature (IAT/AIT) Calibration and Response Time

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Good Afternoon,

I am in the process of setting up a map for a 2004 USDM Subaru STI that will be tuned using Cobb's Accesstuner Speed Density system. This is the first time I've needed to calibrate/setup an aftermarket Air Temp Sensor as the previous cars I've tuned kept their factory sensors. My issue is that that the Accesstuner software presents the Intake Temp Sensor Calibration as Volts vs Temp, where as AEM's product sheet presents it's table as Resistance vs Temp. How do I take the information AEM provided and apply that to calibrate the sensor in the Accesstuner software?

On a more product specific note. Does AEM's air temp sensor provide fast enough response time to properly operate with Cobb's speed density system? I've read in a few places that an intake temp sensor's response time to reading changing temperatures is very important. Link's Bosch Intake Temp sensor is advertised as having a very fast response, but I haven't seen any data to see how fast it really is or how it compares to the AEM. Any input would be appreciated.

First, you will need to know the pullup resistor value and the reference voltage. This may be in the OEM ECU, or may be an external precision resistor that you add. Now you can calculate a table (using a spreadsheet), the voltage for a given resistance will be Reference Voltage x Sensor Resistance / ( Sensor Resistance + Pullup Resistance).

So for example, using a 5V reference voltage, and a 2000 ohm resistor (2k ohm):

2000 ohms = 5 x 2000 / (2000 + 2000) = 2.5volts

1000 ohms = 5 x 1000 / (1000 + 2000) = 1.667volts

1437 ohms = 5 x 1437 / (1437 + 2000) = 2.09 volts

Response time is directly related to how fast your air temp is expected to change. On normally aspirated engines, it's not usually critical. On boosted apps that might change air temp very rapidly, more important.

Temp changes will need to be 20 degF to make more than a 1% change in density. So how much and how fast does you temperature change?

Thank you for getting back to me David,

I’m going to play with those formulas and do a little more research now that I have a base to start from.

With regards to how fast my temp is expected to change. I do not have a reference for the specific project I am working on as this is my first Subaru EJ25 project. However, from my past experience working with OEM IAT sensors on turbocharged Honda engines, the temp change during a drag race can be as fast as 1.5 Degrees F per second with a total temp range fluctuation of 10-15 degrees F. Now I guess these numbers could be skewed a bit depending on how fast the OEM sensors react. I’m not sure.

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