My father has a '95 540i... we got talking about airboxes and one thing let to another and now it too is Fogged!
Unfortunatly, since his is OBDI (OBDTOOL is for OBDII) so I have no airflow charts to back up this
up and my G-Tech is out on loan. However, consider that the Z3 1.9l M44 stock breathes through a
1.75" hole and it is rather restrictive. (See article below for much more info) Then you pull the airbox out of a 540 (4 liter 8 cylinder 110% more displacement) and see that it's airbox breathes through a 2.25" hole (only 60% more area) you start to wonder what might happen when that 2.25" hole goes away. I know I did!
On the 540i airbox it is incredibly easy to get rid of it and it doesn't cost a dime! The hole is part of a
venturi muffler for intake noise, just like on the M44 airbox. The muffler is located almost totally inside
the airbox itself. That is in the direction of airflow it is an expanding funnel. For the direction of intake
noise it is a decreasing funnel which blocks out lower frequency noise. It is also rather restrictive.
The air intake on the 540i is basically in the bumper. That venturi muffler actually just pops into the
airbox itself. There are two clips on the inside of the box that hold the muffler in place. You need to
take the lower airbox out of the car to remove the muffler. Spin the muffler until you can feel the first
clip. If you can push it in to release it but you might need to break it off. Then spin the muffler around till you feel the clip on the other side and either push it in or break it off and the whole muffler will slide out the front of the airbox.
Now instead of a 2.25" hole your car needs to breathe through it is just under 4" (sound familiar ;).
There is a small lip just inside the 4" hole that is what the clips pushed against. I used a dremel with a
small sanding wheel to flatten that out flush with the rest of the airbox. Then sanded it smooth. I washed the box out and reinstalled it.
It actually is only slightly louder with the muffler gone but according to my father felt like it was
pulling harder at high RPM. And that is with two people in the car... normally it is just him.
Just thought you might want to know,