"Jetting" the maf...

[Daniel Ciobota]

 As I've decided to abandon the low temp thermostat thread, since didn't
want to get into a pissing contest, I started looking at other
interesting threads... the "maf flow" thread caught my eye, and I
noticed the same old problem of pegging the gm maf rearing its ugly head
yet again.  I believe the original poster mentioning an 89 vette with a
355 and vortech, among other things.  As everyone knows, that's the old
maf system, using a plastic housing with an annular ring sensor bonded
to it.

 In the past, I mentioned that ford aftermarket mafs are easily modified
for different fuel pressures or injector flows simply by using a
different size sample tube for the sensor.  Alas, that can't be easily
done with the 89 vette maf, even though later model ones can apparently
be modified.  I was considering building a bypass to lean out the maf,
but that would look like a rigged job no matter how much I'd pretty it
up.

Sooo, that brings me to the topic of the post, an inexpensive way to
calibrate (read, lean out) the sensor reading on those old annular
mafs.  Measure the inner diameter of the annular ring, then calculate
the area.  Since flow is _approximately_ proportional to the area,
calculate the new area you need to sample based on the ratio of old
injector flow over new injector flow, or the square root of the ratio
between old pressure and new pressure.  For example:

old injector = 22 lb/hr
new injector = 24lb/hr
old sample tube area = 20 sq mm
new area = 20 * (22 / 24) = ~18.3 sq mm

 This is a fairly rough aproximation, but it will get you close.  Next,
find a hardware store and find a piece of tubing that would snugly fit
inside the annulus, and has the right inner diameter to give you the new
area you calculated.  Cut the tube long enough to fit inside the annulus
but not touch the sensor, and make a lip at the front end of the tube so
it won't have a chance to slip through the annulus and hit more
important parts.  Voila, you've "jetted" the maf similar to the way the
old carb folk used to change venturis on the holleys.  You can buy
different thickness tubing and experiment with leaning out the maf.

Remember that although you now can run higher airflows through the maf
without pegging it, the dynamic range of the sensor stays the same.  In
the ecm, you will still get only 255 discrete values, and now they will
be mapped to higher airflow numbers due to the rejetting.  This can
cause a problem if the shifting is too severe, because the low airflow
numbers will suffer in resolution, maybe causing idle problems, or the
all too familiar "hunting" most radical efi motors experience.  Because
of the limitations of the ecm, nothing can be done about that unless
someone builds a non-linear transducer to skew the sensor dynamic range
towards the airflows that need the most resolution.

 All in all, it's a very cheap, "dirty", way to quickly lean out the maf
calibration, especially when doing last minute tuning at the track.

Have fun with it,

Daniel