Old vs New MAP Sensor

Concern about the accuracy of the MAP sensor on the car led me to removing the part and replacing it with a new MAP sensor that I had in my tool box.  I went and took a second set of logs with the new sensor and made a comparison.

Boost pressure reading from Old vs New MAP Sensor

Old vs New MAP Sensor

It looks as though the old sensor is still functioning correctly.

Pre-Turbo Pressure Drop

To determine if some of the RS4 intake components produced a measurable difference in pre-turbocharger pressure drop I installed a pressure sensor in the inlet pipe leading to the compressor housing.

Auber pressure sensor installed in intake pipe

Previously I had recorded the S4 intake components, and now I swapped in some RS4 parts to make a comparison.  The parts being swapped were the upper half of the airbox, the MAF housing, accordion hose, and Y-pipe.

Audi B5 RS4 Airbox Top, MAF housing, Accordion Hose, Y-Pipe

RS4 Pre-Turbo Intake Components

Three pulls were made with each configuration, two in third gear and one in second gear.  A comparison of the pressure drop in PSI for each setup is shown below:


There is clearly an improvement with the RS4 intake components versus the S4 counterparts.  Whether the difference is great enough to affect engine or turbocharger performance is a different matter.

This testing was done with K03 turbochargers and a moderate Stage 2 tune, tapering to 10-12 psi near red line.  A larger turbocharger such as the TiAL 770’s that are capable of maintaining over 35 psi at red line will likely yield a larger performance split.

MAP sensor check

Recently I’ve been logging pressure at the compressor housing and prior to the throttle body.  I’ve noticed that the stock sensor, before the throttle body, has been reading higher, which it should not.  I’ve switched to a Dwyer pressure sensor to try and eliminate quality issues with the sensor, but it was still reading a bit lower than the stock sensor.

I next thought that perhaps with the sensor plumbed in prior to the N75 valve that maybe some pressure was being bled off by the N75 valve.  To eliminate that possibility I ran one compressor line to the N75 and the other to the pressure sensor.  Still the pressure from the Dwyer sensor at the turbo was less that the pre-TB pressure.

That led me to question if the stock MAP sensor is reading correctly.  I’ve got two extra MAP sensors in the toolbox, one new and the other used.  I thought that if I hooked the sensors up to an air compressor and ran the pressure up to 20 psi I could log with VCDS the boost pressure being reported to the ECU.

I hooked up the contraption below to record these MAP sensor readings:


This is a pressure regulator normally used to perform leak down testing running into the MAP sensor which is electrically connected to the car.

As I turned the pressure up VCDS reported the increase in millbars until the reading reached 1300 mbar and then it stopped rising.  I thought perhaps without the engine running there wasn’t enough juice going to the sensor, so I started the car and tried again.  This time pressure stopped at 1390 mbar.

Apparently the car is limited on how much boost it will read if the boost is not being developed as expected.  So much for this idea, I’ll need to log the different MAP sensors on the road and see how they compare.

Intake and Exhaust Pressures

I previously installed a pressure sensor on the intake pipe to the passenger side turbocharger, and today I added a sensor to the driver side downpipe.

Exhaust pressure measuring rig attached to 034 motorsport downpipes

Then I wired up the pair of sensors to the Innovate LMA-3 Aux Box to record pressure drop prior to the K03 turbocharger compressor, and exhaust back pressure prior to the catalytic converter.

The exhaust system is the 034 Motorsport 3.5″ single with catalytic converters.  On the intake side I am using a paper filter in the stock airbox with some holes made in the lower half of the box, a stock S4 MAF housing, accordion hose, and y-pipe.  The turbo inlet pipes are TiAL 2.25″ stainless steel.

Testing was done over a full second gear and third gear pull with an additional third gear pull part way through the rpm range.

The results are shown below.  On the top half of the chart is the exhaust back pressure and the lower part of the chart is the pressure drop leading into the turbo.


While these results are with the BorgWarner K03 turbochargers, the S4’s stock components, I am still pleased at the minimal pressure drop through the exhaust system.

As a follow on test I may install the RS4 airbox top, RS4 MAF housing, RS4 accordion hose, and RS4 y-pipe to see if any different pressure drop results are seen with that different intake system.