RS4 MAF Flow Test

The Question:

I’ve wondered about some of the different intake configurations and how they affect pre-compressor pressure losses.  To get some idea of the impact a few different options have on airflow rates I collected some parts that I have used on my car for a comparison flow test.

The Parts:

For this evaluation I obtained a stock S4 MAF housing with Hitachi MAF sensor, an RS4 MAF housing with the Bosch MAF sensor, and an EPL MAF housing using the Hitachi MAF sensor.

MAF Housing Lineup
MAF Housing Lineup (L-R) Stock, RS4, EPL

The stock S4 MAF and EPL MAF work with the stock S4 airbox, which I had available for use.  The RS4 MAF affixes to the RS4 airbox top, which I also have, and is able to attach to the stock S4 airbox bottom half.

My S4 airbox bottom half has been modified using the popular ‘Darintake’, which is simply cutting out portions of the airbox bottom to allow more air passage.  There are a variety of ways to do this modification, images of how I modified mine are contained on the page where I tested the airbox flow with different areas removed.

Attached to the airbox is a standard S4 intake snorkel.

The Test Setup:

For this test I put each MAF housing onto a complete airbox with intake snorkel and attached the assembly to the flow bench.  The airbox had a filter in place with approximately 1,500 miles on it.  The flow bench airflow was increased until a depression reading of 10″ of H2O was reached and then the airflow measurement was recorded.

epl maf housing
EPL MAF on the test bench
RS4 MAF Housing
RS4 MAF housing on the test bench

The Results:

The airflow reading for each of the MAF setups is displayed below for comparison.

MAF airflow test results

The raw numbers are:

  • Stock S4 MAF – 354 CFM
  • RS4 MAF – 438 CFM
  • EPL MAF – 445 CFM


I was a little surprised with the RS4 MAF / RS4 Airbox combination flowing slightly less than the EPL MAF and Stock S4 airbox.  The RS4 MAF has an 83mm inlet and the outlet from the RS4 airbox is 93mm.  The RS4 airbox does have what looks like a sleeve inside the outlet area, which makes for a smooth transition from the airbox to the MAF housing, but therefore makes the airbox outlet equal to the MAF housing 83mm diameter.

By comparison, the EPL MAF has a 79mm inlet while the stock S4 airbox has a 78mm outlet.  The EPL MAF does have a taper, with the outlet side opening to an 86mm diameter.  If the pressure losses were only dependent upon the cross sectional area of the piping I would have expected the RS4 MAF to show the highest flow rate, though only by a small amount.

The most likely reason for the EPL MAF having the greatest air flow is presence of the screen on the RS4 MAF causing restriction that the EPL MAF does not have.

MAF housing cross section picture
MAF housing inlet cross section
Audi B5 S4 airbox
Airbox outlet cross section

A follow-on test with a MAF housing that has a screen, and one with the screen removed, would likely prove if the screen on the RS4 MAF housing is the significant factor impacting airflow rates versus the EPL MAF housing.

The most dramatic change comes simply by swapping out the stock S4 MAF housing for the EPL housing.  The result is a 25% gain in airflow.


I was loaned an Autospeed 90mm MAF housing to test that fits onto the RS4 airbox.  Below is a picture of the Autospeed MAF housing on the bench with RS4 airbox attached, and the updated results which include this latest MAF housing.

autospeed maf housing on flow bench
Autospeed 90mm MAF housing and RS4 Airbox on the Flowbench

autospeed maf airflow results


Audi B5 S4 Information and Testing