Talk about modifying cars and eventually someone will mention a high flow air filter of some sort or another.  I purchased a K&N air filter since I figured my S4 could use all the fresh air the turbos could inhale.  But, I wondered if a filter actually made any difference.  Ahhh, one of my favorite things: more testing!

I had, in my possession, a fairly accurate airflow measuring device--namely--the mass airflow sensor that the engine computer uses to measure incoming air.  And, the means to read and capture said information via VWTool. While I was at it I stuck my GTech on the windshield to capture HP numbers. The numbers were meaningless since I was doing rolling starts.  Oh well.  I fired up my laptop and 1.5 hours later I had plenty of data to sift through.

For my testing I did 5 - 2nd gear runs with each filter.  For each run, I steadied the car around 1200 rpm in 2nd gear then applied full throttle until around 7000 rpm.  I chose this method to eliminate shifting and to keep the runs as consistent as possible.  All runs were eastbound with the return westbound trip to let the intercooler cool off.  The stock filter has 6000 miles of service under it's belt, the K&N was fresh out of the box.

I was surprised.  The mass airflow sensor didn't measure one bit of difference between the factory filter and the K&N filter.  None, zero, zip, goose-egg.  I didn't really expect huge gains but I did expect some difference!  The airflow numbers from both filters were, for practical purposes, nearly identical.  There were small variances here and there but the same type of variances existed between each group of runs on the same filter.  As far as the airflow sensor was concerned nothing had changed.  (see the large, slow loading graph below) My apologies for the different RPM scales on the graphs.  Nothing I could do about it.

The graphs

Ohhh, I can hear it already... "you didn't measure any difference because you have that evil, restrictive factory airbox.  Yup, that's your problem... you need one of those big cone filters so those turbos can really breathe deeply."  Uhhh, I'm afraid not... you won't ever find one of those on my car.  With the massive amount of underhood heat the S4 produces why would I want my engine ingesting power-robbing, super-heated air when nearly ambient temperature air is picked up by the snorkel behind the grill?

So what does all this tell me?  At this point, unless someone can tell me why these results are invalid "me thinks" that popping in a high performance air filter in my S4 yields zero horsepower, zero improvement.  Some would argue throttle response is more crisp.  I don't feel it.  If no extra air is making it's way into the engine it CAN'T be making any more power.  No extra air,  no extra fuel. No extra fuel, no extra power.  All the high performance filter manufactures have graphs showing how their filter outperforms all the others.. under different conditions they probably do but my S4 does not have air filter induced asthma.  I'm not suggesting that the K&N does not, under different conditions, flow more air just that in this particular case the air filter is not a restriction.  I wish I had a stock filter with 20,000 miles on it to see how dirt effects its performance. 

And finally, the filter did not change the sound of the turbo or intake noise