The Mass Airflow Sensor (MAF) is a precision
instrument. It provides a voltage output based on a 5-volt source
that responds directly on a roughly proportional curve to the mass
of the air that passes by it. For the PCM to compute the actual mass
airflow value, it must take into account a known, fixed value for
volume and that value is correlated directly to the inside diameter
of the tube in which the MAF sensor is enclosed. If one were to
change that diameter, the calculation will be skewed accordingly.
the RX-8 original equipment MAF tube has an inside
diameter of 3.375 inches (86mm). This gives the MAF a calibrated
limit of roughly 370 grams a second of mass airflow, which is about
49 pounds of air a minute, at 4.7 volts, which is about the point
where the factory calibration ends its scale.
Based on the PCM’s MAF scaling and calibration, an
output of 1.25 volts would correspond to 6 g/sec of air going by it
at idle. Increasing the MAF tube inside diameter by a mere .067
inches would change the PCM calculation to 5.88 g/sec at the same
MAF output. This tiny difference (about 2%) is enough to change the
target fueling air/fuel ratio value by up to a half point (for
instance, from 14.7:1 to almost 15:1)! This is not insignificant.
So, the inside diameter of the MAF tube is a measurement that must
for accurate fuel delivery and load calculation.
This is also the area where some of the aftermarket
intake manufacturers have worked their “magic”. By slightly
increasing the inside diameter of their MAF mounting tube, they have
effectively lowered the airflow calculation at all points on the
internal MAF calibration chart, which has the effect of making the
target air/fuel values leaner and, in open-loop operation where the
effect of stoichiometric-seeking fuel trims is negated, producing
more power in most instances. This slight increase may also increase
the actual volume of air the intake can pass to the throttle body.
It also has the effect of convincing the PCM that the load on the
motor is ever so slightly lower, which leads to slightly more
advanced ignition timing.
Just as important to the accuracy of the MAF as
inside diameter is consistency and total length of the MAF tube. The
MAF needs the airflow that passes it to be as smooth as possible. To
that end, it is important that the tube in which it is suspended to
be of a constant, uniform inside diameter and direction for a length
that is roughly equivalent to twice its inside diameter both before
and after the MAF itself. Anything shorter runs the risk of
producing eddies and swirls in the airflow that can cause
The original equipment tube contains a mesh screen
several inches before the MAF. It is helpful, if not imperative that
this design feature be emulated in any aftermarket intake. This
screen stabilizes the air passing through it and causes the air to
flow toward the MAF with less error-inducing “swirl”.