Category Archives: Flow Tests

More MAF Flow Testing

Today I conducted a flow test comparison of a pair of RS4 MAF’s and an EPL MAF housing.

For some time I’ve wondered how much, if any, restriction the screen on the RS4 MAF housing causes.  A parts car that came through the garage was equipped with a Bosch MAF housing that is the same size as the Bosch RS4 MAF but had a set of screens that are removable.

MAF housing lineup

I decided I would pull the RS4 MAF from my car along with the RS4 airbox top and stick them on the flowbench with the rest of the stock intake to see how the screen affected the airflow through the intake.  I also decided I would retest the EPL MAF housing attached to the stock airbox top as that combination had out-flowed the RS4 MAF and RS4 airbox combination previously.

The results of the flow test are shown below:

Interestingly the removal of the screen from the RS4 MAF provided only a little benefit.  The EPL MAF and stock airbox still provided a little more airflow than the screen-less RS4 MAF and RS4 Airbox top.  All tests were conducted with a stock air filter, modified lower airbox (Darintake modification), and the snorkel attached.

I also have a large K&N cone filter available, so I attached the RS4 MAF to the K&N filter and retested using the RS4 MAF housings.

The results of this test are shown below:

The removal of the screen from the RS4 MAF housing produces a larger increase in airflow with the K&N filter versus the RS4 MAF with the screen in place.

More dramatically, the K&N filter allows for much higher airflow at the test depression of 10″ of H2O.

A couple of things should be noted, first is that a test depression of 10″ of H2O is not very much pressure.  In these tests there is a substantial amount of airflow passing through the intake with minimal pressure drop.  It’s common to test at 28″ of H2O, nearly three times the level this test was conducted at.  This test was performed at 10″ due to a bench limitation of around 600 CFM.

From this test it would seem that the K&N cone filter would be a desired substitute for the stock intake.  What this test does not account for is the potential disruption of airflow over the MAF sensor caused by the cone filter, which could lead to tune problems.

Nor does it show how engine compartment temperatures may alter intake air temperatures with the cone filter.


Turbo Concepts Stage 1 Bench Test

Prior to installing a turbocharger on my S4 I make an effort to flow test the turbine side to see how it compares with the other turbochargers that I have tested.


For this test I run a 100 CFM calibration plate since it is the closest to where these turbo products fall out, generally around 45-55 CFM at 28″ of H2O.

Turbo Concepts Stage 1 Turbocharger on Flowbench
Turbo Concepts Stage 1 Turbocharger on Flowbench

After performing the bench calibration I place the turbocharger onto an adapter for transitioning to the flowbench.  The compressor wheel is fixed in place, I hold it still with a finger, and then I slowly run the flow bench up to 28″ recording several data points along the way.


The chart above is a consolidation of the various turbochargers that I have tested thus far.  Of note, the BorgWarner RS4 K04 has been the highest flowing turbocharger that I have tested.

The Turbo Concepts Stage 1 performed similar to the FrankenTurbo and Turbo Engineers products, as well as BorgWarner K03’s, still below where the K04’s placed.

While this is a test that I don’t put a lot of stock in to give insight into how the product will perform on the car, it is notable that the exhaust backpressure measured on the vehicle has been lowest with the BW K04’s, supporting the flow bench measurement.

BMW MAF Part 2

Follow up to the post about the BMW MAF with the K&N Cone Air Filter.

BMW MAF with K&N Air Filter
BMW MAF with K&N Air Filter

This time when I put the BMW MAF on the flow bench I tested the part in several configurations.

  • MAF with both grills in place
  • MAF with large space grill removed
  • MAF with small space grill removed
  • MAF with both grills removed
  • MAF with both grills removed and K&N filter attached
BMW MAF Housing
BMW MAF Housing
BMW MAF Housing
BMW MAF Housing

The results are shown below:

BMW MAF and K&N Air Filter
BMW MAF and K&N Air Filter

For the most part the results were as expected.  As the grills were removed from the MAF housing the airflow increased.  The removal of the small grill led to a greater increase in airflow than the removal of the large space grill.

What was a surprise was the increase in airflow that occurred when the grill-less MAF housing had the K&N cone air filter attached.  This is most likely attributable to the filter neck smoothing the airflow as it enters the MAF housing.  I’m impressed by the fact that adding the air filter increased airflow, that was not the case with the stock airbox when I tested it using the EPL MAF housing.