....from Mitch @ The Corral Techboard

First of all....everything that I will state here IS my opinion,
but it is based on FACT. Furthermore, it also reflects the tuning
philosophy of all the chipmakers who use the M. Wesley hipmaster.

You're the kind of guy who likes to experiment, fool around and
learn things on his own. That's all well and good, but there are
some things that will work and other things that will not work
when it comes to tuning an EEC computer. Lets start with the mass
air meter (MAF). The EEC uses the MAF to accurately determine the
quantity (mass) of air entering the engine at any given instant.
The MAFs duty is to do the measuring, that's all! Within the EECs
programming logic, there is no place for altering the MAF
calibration as a tuning aid. Period. Ford flow tested all of
thier MAFs on a flow bench and with the data they obtained, a
plot of air flow in Kg/hr vs voltage output is constructed. This
is called the mass air transfer function and it is incorporated
into the EEC as part of it's software. While in operation, the
EEC looks at the MAFs output voltage, goes to the MAF transfer
function and converts this output voltage into an actual air flow
(mass). It then ratios this measured airflow over what the engine
can hold at 100% volumetric efficiency. So if the engine is
injesting air at 50 Kg/hr, but could injest 100 Kg/hr for
complete cylinder filling at that given instanteneous RPM, the
ratio is 50/100 or 0.50. This is equal to the LOAD. Once the EEC
has a load number, it references a fuel load table where a given
A/F ratio is selected as a function of load vs RPM. A similiar
calculation is used to determine the spark timing from a spark
load table where timing is given as a function of load vs RPM. In
the case of the fuel load table, once an A/F ratio is determined,
the EEC knows the correct pulsewidth to use because THE INJECTOR
SIZE IS PROGRAMMED INTO THE EEC SOFTWARE. So you can see that it
is not the MAF that is calibrated for a particular injector size,
THE EEC IS. The proper way to tune for an injector change is to
reprogram the EEC for the injectors. This way it will continue to
correctly calculate the load and thus, properly read the fuel
load table. In this case, the proper A/F ratio will be selected
and with the proper injector size now known, the EEC will give
the corect pulsewidth. Now if you alter the MAF by "recalibrating"
it, the transfer function can no longer be used to accurately
determine what the MAF is measuring. Therefore, the EEC will
incorrectly calculate the LOAD. This error is then carried over
to the spark and fuel load tables where incorrect fuel and spark
timing are thus selected. In the case where you use a MAF
recalibrated for, say 30# injectors, the MAFs output voltage is
lowered by 19/30 or almost 37%. With this alteration, the EEC may
never read the high load areas of the spark and fuel load tables.
And this is assuming that the meter's output follows the curve
for the transfer function. It is possible to modify a meter
whereby it's behavior does not even follow a linear relationship
to the transfer function, but instead deviates differing amounts
depending on where you are on the curve. Now consider the FMU.
The FMU is actually a second controller overlaid on the EEC. Here
you are taking the EECs calculated fuel delivery and modifying it
with a mechanical boost sensative device. The FMU will not
necessarily give you the best fuel curve becuase, at best, it is
limited by a linear relationship of boost VS fuel psi. Who says
that this will necessarily produce the best fuel curve? Ideally,
the fuel curve should be based upon the LOAD. The EEC already
performs this load calculation and here, the best way to go is to
use a reprogramming chip. With regard to Pro-Ms calibrations, if
there is actually a difference, why couldn't they tell you?
Remember, any change in the MAFs calibration not only effects
fuel delivery, but spark timing as well. I don't dispute that
perhaps you can recalibrate the MAF to give decent results at
WOT, but not over the entire performance envelope which is where
you drive 95% of the time. All of these tricks, ie: recal. MAFs,
boost timing retards and FMUs do not work because they try to
defeat the programming logic of the EEC. Any tuning modifications
or updates should instead be incorporated into the EEC. This will
give you the best possible results.