Sub-Ambient IAT

Here’s something to keep in mind when looking at intake air temperature data reported by the Cobb Accessport (AP) that seems too good to be true.

The AP will report the real time IAT, as well as the maximum and minimum readings during the time it is on.

Cobb Accessport Display Screen
Cobb Accessport Display Screen

The minimum IAT value might be less than the outside air temperature under the right conditions. One of these situations is shown in the chart below:

The red line is the IAT reading during a pull and the pink line is the outside air temperature. Of interest is what occurs in the time right after lifting off the throttle, indicated with the black vertical line. The IAT reading soon drops from 68F, four degrees above the 64F ambient temperature, to 61F, three degrees below ambient.

If only the Accessport minimum data from the display screen (picture above) was evaluated at the conclusion of this pull it would seem that the intercooler cooled the charge air to three degrees below ambient.

More likely what is taking place is an example of the Ideal Gas Law (PV=mRT) in action. There is a sudden rapid drop in pressure (P) in the intake as boost pressure goes from ~25 psi to half that amount in a fraction of a second, and this is followed by a temperature (T) drop.

The intercooler setup in this example performs very well, but sub-ambient cooling from an air-to-air intercooler, while operating in boost, is not consistent with how this type of heat exchanger functions.

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