BMW E30 airbox & filter flowbench tests
The Rig
Out of curiosity, I wanted to find
which air filter designed for the BMW E30 stock airbox flows the most air under
ideal conditions. Filter performance after 10,000 or 50,000 miles is not
the point of this exercise. To that end, I built my own flowbench assembly
using a Black & Decker leaf blower. I connected the leaf blower intake
to the airbox assembly with a short length of 3" ABS pipe. A 3/8" hose
adapter is plumbed into this length of pipe and connected to a water manometer
which can measure pressure differences in units of inches of water. (hereafter
referred to as just "inches") This is not a calibrated flowbench - it
can't measure airflow in CFM, but it can reliably show pressure differences
caused by flow restriction. That's all I really care about for this
test.
After assembly, I goofed around with it to see
what it could do. First thing I did was put a good pair of sound barriers
on my head. It's just an electric blower, but it's LOUD. Without
anything attached to the adapter pipe, I measured 2.5" of vacuum.
Obstructing the intake caused the vacuum reading to decrease all the way
down to 11" (.4 PSI) when fully plugged up. With the airbox assembly
attached to the unit, the vacuum reading is in the 5" range.
The AFM
The AFM itself is responsible for most of the flow
restriction - without the box in place, the pressure drop across the AFM &
intake horn is 4.2"
How much air is involved?
On the test bench, the AFM flap is pulled open nearly all the way.
This corresponds to the 3-4000 RPM range on the engine.
The Intake Horn Thingy
I attached the AFM & airbox
assembly to the rig and began taking readings. The first thing I tried was
to remove that intake horn / velocity stack thingy inside the airbox that looks
like it might restrict airflow. If you've removed yours, you should
probably find it and stick it back in there. The flowbench shows a
pressure difference of .1" for horn vs. no horn configuration. Air flow is
measurably better in the stock configuration with the horn in place.
The Filter tests
Next, I obtained five different air
filters for testing. Since I'm only interested in air flow through a clean
or new filter, I won't discuss filter performance after dirt begins to load the
filter. This may be important stuff to know if you live in a dusty
environment, so don't rely on me to tell you which filter is best in that case.
I tested each filter several times, swapping filters between runs, and
obtained very repeatable results, which was very encouraging. Accuracy
Note: The estimated accuracy of the numbers in the following results is .02
based on my squinting really hard at the scale whilst taking measurements.
The reported numbers are averages of 3 runs.
Mahle paper filter, OEM part
This filter consists of 74 pleats, each 1.8" deep.
If you unfolded this filter, it would be 22.2 feet long! The surface
area is 1438 square inches, or pretty much exactly 10 square feet. The
pleats have small ribs molded into them to keep them a uniform distance apart.
In addition, no wire mesh is required to keep the filter in shape.
As a result, this filter is the #1 performer in this test, adding only
.11" of restriction to the airbox assembly.
K&N oiled gauze
This filter is the oiled cotton gauze type that every
performance enthusiast is familiar with. K&N is successfully marketed,
but isn't renowned for the highest filtration standards. This filter is #2
in this test, adding .13" of restriction to the airbox assembly. This is
the filter I expected to come out on top, but no matter how many times I reran
the test, I could not get this filter to top the Mahle. K&N boasts on
their website that their filter media flows 50% more air than an equivalent area
of paper. With 18 pleats each .7" deep, the area of this filter is 202
square inches. That's less than one seventh of the Mahle filter area.
I'd say the cotton filter media flows a lot better than they claim.
I'd feel better about it if I couldn't see so much daylight through this
filter though.
Fram paper filter, el cheapo auto parts store special
I included this filter because Fram is generally
considered to be the cheapest & worst. It's similar to the Mahle
filter, with 76 pleats of 1.8" depth, for 1477 square inches of surface area
(10.25 square feet). Like the Mahle, the pleats have ribs to keep them
apart, but they're not so well defined, and you can't see as much space between
them. This filter also has a wire mesh on top for added structural
integrity, which may restrict flow a tiny bit. This filter is #3 in this
test, adding .23" of restriction to the airbox assembly.
ITG oiled foam
The ITG filter looks like a quality assembly. It's a
2-stage type foam filter. The outer foam is a coarse type, roughly an inch
thick. The inner foam is finer to catch the smaller particulates.
The oil is a tacky, stringy, viscous goop that is probably excellent at
trapping dirt. Amsoil foam filters use the exact same type of oil.
This filter is #4 in this test, adding .24" of restriction to the airbox
assembly.
Amsoil oiled foam
This is the filter I've been using for years under the
assumption that oiled foam flows better than paper! The Amsoil unit uses
just one type of finer sized foam throughout with a metal screen inside to lend
structural rigidity. The screen itself is of a rather dense construction,
which is good for rigidity, but probably not ideal for air flow. This
filter placed #5 in this test, adding .37" of restriction to the airbox
assembly. I'm still a fan of oiled foam filters in many cases where OEM
filters aren't so well built, but in this case it's probably just as well that
Amsoil doesn't sell this filter anymore except by special (large) order.
How much restriction is this, really?
To give a better idea how
much restriction an air filter makes, I set up the airbox with no filter
installed and fired up the blower. I then partially blocked the opening to
the airbox with a piece of cardboard until the manometer gave me the same
readings as I got with the air filter installed. This is the air box
opening - with the plastic endpiece installed that lives just behind the left
side headlight area.
This is the
blockage necessary to emulate the installation of a stock paper filter (the best
case) with .1" of restriction. (The translucent cardboard trick is merely
a photo edit enhancement for illustration)
This
is the blockage necessary to emulate the installation of the Amsoil foam filter
(the worst case)
Conclusion
If you want the max airflow through this
particular unit, you can't go wrong with the Mahle OEM filter. Paper
filters clog up faster than other types however, so you should be prepared to
replace the filter at regular intervals. Since foam filters hold such a
large quantity of dirt before choking up, you'd be well served to consider foam
if you live in an extremely dusty area or if you just don't want to fuss with
filters for long periods.
If some bright engineer type
wants to chime in with some flow calculations that can better explain the
significance or lack thereof of this little study, I'll put them up
here.
You can reach me using my bolhuijo.com email address. I'm
bolhuijo.
What's Next
(October 2002
Now that I've got this spiffy
setup for measuring flow restrictions, I plan to keep using my new Mahle paper
filter and take periodic readings to see how often I need to replace my filter
to maintain minimum air flow restriction. Info to be posted here as
mileage accumulates. Don't hold your breath, my car sits in the garage
most days.
Links
Larger
photographs of this experiment can be found in my photo gallery
ITG air filters
Amsoil air filters
K&N air filters
Schulba BMW, Belmont CA Helpful
local shop
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