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.

For instance, 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 be absolutely known 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 unpredictable results.

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”.