Tag Archives: IAT

Datalog analysis of Cadillac STS-V Ambient vs IAT1 vs IAT2

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I have been planning new data-logging experiments and modifications, so I have been reviewing the datalogs I did last August in the summer temperatures.  For my next datalog I will attempt to capture comparative data to this info:

This screen is an HP Tuners VCM Scanner guage snapshot of parameters after a 0-60 run culminating at 65 mph.

Baseline from August



MPH (Temps in Deg Fahrenheit)

Item 0 30 45 60
Ambient 90 90 90 90
IAT1 113 117 113 106
IAT2 118 120 127 135
MAT 120 121 136 169

What we can see from this table, is that at the beginning of the 0-60 run the Ambient temp is 90F, and stays at 90F through the run.

IAT1: The IAT1, which is incoming air temperature as read in the intake air tubing, starts at 113F.  It has rapidly gone up to this value because the STS-V is stopped.  However, as the V accelerates to 30 mph it rises to 117F.   By 45 mph it has returned to 113F, and by 60 mph due to the incoming 90F air cooling it has resolved to 106F.  Conclusion:  heating of the incoming air at the IAT1 sensor does in fact effect the air temperature, and thus performance.   Conventional wisdom is that the incoming air is not in the intake tube long enough to become heated. Net Rise: -7F

IAT2: The IAT2, which is air after it has been heated by the supercharger and cooled by the intercooler, starts at 118F, a 5F increment over IAT1.  By 30 mph it has risen to 120F.  At 45 mph it has risen further to 127F, and by 60 mph has peaked at 135F.  Net rise: +17F.  Keeping in mind the IAT1 went down 7F in this time.

MAT: The manifold air temperature is a calculated value the engine uses to predict the temperature of air in the manifold.  It tends to go up at a higher rate than IAT2.  The MAT starts the run at 120F, is about the same at 30 mph at 121F, but rises to 136F by 45 mph and on to 169F by 60 mph.  Improvements in the IAT2 should have even greater improvements in the MAT value.  Net rise: +49F.

As a result of the high MAT temp by 65 mph as shown in the image, IAT temps are causing 3.6 degrees of advance to be pulled.  This begins at 59 mph when the IAT2 hits 135F and the MAT is at 157F at -3.2 degrees.
This was a video capture of the dashboard of the 0-60 run from August:

Cadillac STS-V: MAT vs IAT2 vs IAT Advance

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I am continuing to study the effects and behavior trends for intake air temperatures (IAT) versus after the intercooler air temps (IAT2), versus Manifold Air Temps (MAT).

This graph is a tuner discussion taken from an .hpl download in this thread.  It is not my STS-V, just interesting to see the data from this run.  In the run the author was studying the relationship between MAT, IAT2, and the effect of IAT on timing advance (IAT Advance).

HPTuner Dashboard from a web example

What this dash shows is a moment in time as the STS-V is about to shift from 2nd to 3rd gear at 6188 RPM and 64 mph.  The IAT incoming air temperature is 79F, so a mild day.

The IAT2 is 117F, taken off the sensor.  The MAT is a predicted value for the actual manifold temperature as follows:

ok there is some hidden logic here you cannot see, the IAT2 PID displays the raw IAT2 sensor reading. The IAT Spark table uses a prediction from the IAT2 reading that is meant to compensate for the slow moving IAT2 sensor. To predict IAT2 temperatures faster than the IAT2 sensor can actually measure them. Now for the happy coincidence: The MAT value is calculated from this predicted IAT2 value (using the usual Bias and Filter tables) and for your application the bias tables are all 0. Meaning MAT = Predicted IAT2.

So in other words, the IAT2 is useful, but the MAT perhaps reflects the value that the IAT Advance will use.  What we see here is that the MAT temp of 109F the IAT Advance is 0 degrees.

Later in the run as the MAT goes up to 212F the IAT Advance falls as low as -3.2 degrees.

Hp Tuners test drive snapshot example2

Summary:

MAT is perhaps as important as IAT2 in understanding the LC3 Supercharged V8’s responses to temperature over time.  After adding MAT to the Table display in HP Tuners one can select F instead of the default of K degrees, and same for the gauges so that MAT shows apples to apples with the IATs.

Generally the MAT changes much more rapidly than the IAT2 value, which as described is the purpose of its use.  It swings higher than the IAT2 max, but also recovers faster to lower temps.

 

 

Adjustments for the Spectre Intake

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After some discussion I have made the following adjustments to the intake install:

After install at the dyno with change notes added

Re-route hoses under the corner of lines:

New MAF clocking closer to 11:00 when viewed from passenger side:

Next headed out to do new temperature comparison runs.

 

Update:

I reclocked the bottom of the intake (changed the relative orientation of the pieces) to ensure clearance away from the fan shroud.

It would be handy if the parts had indicators that showed the correct relationship.  I also had a great deal of trouble getting and keeping the circular gasket gasket in the hole.

HP Tuners Dashboard monitoring IAT1 during cruise

My initial results looks promising in that IAT1 during cruise now goes near ambient as expected.  After I reset the battery the Scanned Ambient went to 32F, slowing coming up to 52F by the end of test 2.  I assume it has a delaying function since generally ambient temperatures don’t change very quickly.