Induction Component Flowbench Test
I had the opportunity to flowbench the HFM meter, stock air box, and my self-made K&N cone filter setup from my car (a 1995 M3) on a SuperFlow 300 flowbench. The HFM meter used on my car is the same that is used on all BMW's equipped with the M50TU engine (this includes 325's, 525's, and '95 M3's). The air box from my car is the same as used on all E36 M50/M52 powered cars (325's, 328's, and M3's).
All measurements were made at 6" of water. I have also converted
the results to 20.4" (where carburetors are typically measured) and 28" (a
common standard) of water. The temperature as indicated on the flowbench was 78 degrees
fahrenheit for all measurements. The mounting fixture used was a piece of 3" exhaust
tubing bolted to a piece of masonite. The mating point between the pipe and masonite was
tapered and smoothened out with some clay.
The stock air box draws air through a snorkel mounted on the front of it. This snorkel is then mounted next to the radiator, drawing cool air from the front of the car. As the air passes through the snorkel, it enters the air box through an internal baffle. The air box that I used had this internal baffle removed. The air then passes through the panel filter. The filter I used was a stock replacement from K&N. After the filter, the air goes through a tube that is slightly flared on its end, and then into the HFM meter.
My cone filter setup consists of a K&N Cone Filter (part # RE-0920),
a 3" exhaust pipe about 9 1/2" long and mandrel bent at 45 degrees, and a
3" rubber coupler to mount the pipe to the HFM meter. (And yes, I have installed a
heat shield next to the filter in the engine compartment :)
The HFM meter has plastic screens installed at its entry
and exit points. I figure BMW put them there to ensure that the air passing through
the meter is straight and uniform so the meter gets an accurate reading. Out of
curiosity, I removed both screens and redid all of the flow tests.
It seems as if the K&N cone filter setup is the hot ticket to better intake airflow. It's apparent that removing the HFM meter screens also leads to more airflow. But removing the screens might also lead to poor driveability due to inaccurate meter readings. Personally, I've had the screens in my meter removed for several weeks and haven't noticed any change in driveability. I also haven't noticed any increase in power with their removal either. Maybe the increase in airflow isn't required by my engine just yet. That is something I'd like to check on the dyno some day.
I would also like to flow test a 540i/Euro M3 meter someday, as well as the meter from an M52 engine (328 and '96-newer M3). I have a 540i meter to test, but I don't have the proper adapter to mate it to a 3" pipe. I basically need the machined aluminum adapter that's sold with this particular meter when it's used on a '95 M3. I could measure the meter without the adapter, but the results wouldn't be comparable to the stock meter readings. It would be more fair to measure both meters using the 3" fixture, as both meters attach to the factory rubber elbow connecting to the throttle body (this elbow happens to measure about 3"). So if you're in the South Florida area and would like to donate your intake components one Saturday afternoon for some testing, drop me an email.