After conducting an additional flow test with the stock S4 MAF and a different power supply, to confirm the results obtained, I swapped to an 85mm EPL MAF for the S4.

To my surprise the resulting airflow for each voltage point that I measured corresponded closely with the readings obtained with the stock diameter MAF housing.

Added a comparison with the MAF attached to the stock modified airbox.

After conducting the flow test of the stock BMW turbo inlet pipes I was contacted by MM Performance Engineering with interest in seeing how their aftermarket inlet pipes compared with the stock pipes.

Shortly thereafter a set of the MMPE inlet pipes arrived and I tested them out on the flow bench.  Results below show the MMPE pipes compared to the BMW stock inlet pipes.

The aftermarket pipes show a substantial improvement in airflow at the test points when compared with the stock inlet pipes.

This is my third attempt to get an airflow reading from the MAF sensor on the flowbench that will match relatively close to what the stock MLHFM curve predicts airflow to be.

The latest results are the 5.1x values.

I’m thinking that it is worth trying to adjust the output from the 5v power supply to pull the green line down and get it to match the stock curve.  Possibly the 5v supplied to the MAF sensor should in fact be slightly more than 5v.