This morning I was able to log a 3rd gear pull that was not quite to redline, but far enough to show how important intercoolers are on this car.  The compressor outlet temperature (COT) really climbs during a full load pull but the intake temperatures (IAT) remain close to the starting values.  Temperature is on the second y-axis (right-hand side of chart).

Also, shown again is the pressure drop across the intake piping and intercooler by the difference in compressor absolute pressure (CAP) and manifold absolute pressure (MAP).  The left-hand y-axis shows pressure in psi.

In response to a question about the compressor outlet pressure exceeding the manifold pressure (from about 2700-3500 rpm) I started investigating potential causes.  A couple candidates that came to mind were conversion errors when moving from millbar absolute pressure to gauge pressure psi and differences in sensor calibration and response time.

Another possible cause was brought to my attention when someone asked about the placement of the MAP sensor, which on the S4 is typically located at the throttle body boot, or the juncture of the two up-pipes on a one piece bipipe equipped car such as mine. In both cases the sensor is located prior to the throttle plate.  It occurred to me that when the throttle plate closes, restricting airflow into the engine, this would probably create a high pressure region on the side of the throttle plate that the MAP sensor is located on.  Additionally, when the ECU works to reduce air into the engine, lowering the turbocharger wastegate duty cycle (wgdc) would cause the compressor outlet pressure to decrease.  I thought about where the anomaly was occurring, just as the boost was peaking on initial ramp up, and it seemed plausible that at that point the ECU would be closing the throttle plate slightly, as well as dropping wgdc, in order to transition from boost rise, to level boost pressure.  That would bring into play both potential sources of the pressure disparity.

I added Throttle Plate Angle and N75 Duty Cycle to the chart with the MAP and CAP pressures to see if this theory held up.  Each has a value measured as a percentage, which is shown on the scale on the right side of the chart.

My thought about the throttle plate closing is not validated by the data, the throttle plate never closes at all during the period of interest.

The wastegate duty cycle does come down, but that occurs prior to the boost differential reverse readings.  Interestingly, later during this pull, around 5000 rpm, the wgdc dips, and approximately 200 rpm later the pressure values dip as well.  The delay from the time the signal is sent to the time the outcome is achieved may be part of the cause, possibly as the wgdc drops rapidly during the transition from boost rise to level boost the impact is not seen right away.

At this point I cannot provide any explanation with confidence, I’ll explore some of the other potential causes as well as review future logs for recurrences of this anomaly.

Finally after corresponding with Doug at FrankenTurbo and the support at Auber Instruments for a few days I was able to get the Auber Air Temperature sensor properly wired up to the LMA-3 so that I could log air temperatures coming out of the the turbocharger compressor.  The sensor probe is located in the metal hard pipe that joins the turbo compressor outlet and the intercooler inlet – so these readings are approximately what the compressor is putting out without the benefit of passing through the heat exchanger.

It was a bit late in the day when I finally got it hooked up and the S4 out onto the road for a test drive to see how the logging worked, so there’s only a very small boost event that is in 2nd gear.  Nevertheless the results below are interesting to me.

The chart above shows on the left hand vertical axis boost pressure, both at the compressor (CAP), and at the Intake Manifold (MAP).  The AFR is also logged on this axis.  Even at these low boost pressures there is some noticeable drop in boost pressure from the compressor to the intake manifold.

Along the second vertical axis is the temperature scale for the Outside air temperature (OAT), the intake air temperature (IAT) as measured by the vehicles intake air temperature sensor, as well as the new addition, the Auber recorded Compressor Outlet Temperature (COT).  Even the performance of the S4 Stock Intercoolers looks impressive when the flat line of the IAT is compared to the compressor outlet temperature.  On the other hand at four to five psi the turbo’s are pushing the air temps up approximately 30 degF, I’ll be curious to see what happens with these temperatures with higher boost levels.