Dale Amy
October 1, 2003

Horse Sense:
Somewhat surprisingly, Pro-M says bad grounds are perhaps the most common culprit in diagnosing mass air meter problems. Not grounds within the meter itself, but poor vehicle grounds, such as failure to route a proper ground wire to the factory location when relocating a battery to the trunk. Residue buildup on the mass air meter’s thermistor and hot-wire element from oiled air filters is the number-two problem, causing lean conditions. Clean them carefully with a pipe cleaner or a Q-Tip.

To a great extent, we have the mass air meter to thank for the electronically fuel-injected Mustang's broad-minded adaptability when it comes to engine modifications. Were it not for Ford's 50-state adoption of mass air in 1989, the Mustang performance aftermarket-and indeed the readership of this magazine-might well be a lot smaller than the thriving and vibrant entity it is today. This is because, for high-performance street use, the mass air meter helps us enjoy a combination of power, economy, driveability, and emissions cleanliness that was once only a pipe dream.

The entire selling point of electronic fuel injection is its computer-controlled, unmatched ability to introduce the exact amount of fuel necessary for proper combustion, and to adjust this supply nearly instantaneously as driving conditions demand. Natu- rally, the amount of air headed into the engine at any given time is the main factor in determining how much fuel is required. Correct air/fuel ratios deliver combustion that is both powerful and clean, making everybody-from gearheads to Greenpeace-happy. The point is, an EFI system relies on having an accurate and real-time means of figuring out how much air is inbound to the combustion chambers.

It's sometimes easy to forget that fuel-injected Mustangs haven't always used a mass air meter to provide the EEC processor information on induc-tion airflow. From 1984 through 1988, that information was instead provided by speed-density calculations, a process that, although extremely accurate in stock appli-cations, did not have sufficient flexibility to adapt to the heavy-breathing modifications that have since become commonplace.

What's the Difference?
Before turning our complete attention to mass air meter systems, let's first be clear on how they differ from a speed-density configuration. With mass air, a metering device is placed smack in the induction inlet path, upstream of the throttle body. This meter measures airflow, while a speed-density system has no such meter and instead merely estimates airflow by monitoring various engine sensor inputs-including manifold absolute-pressure and temperature sensors, engine rpm, throttle-position and oxygen sensors, and coolant temperature-and comparing them to preprogrammed tables within the EEC processor.

Among other factors, the speed-density EEC's programmed fuel (and spark, and EGR) responses to these inputs were predicated on the known volumetric efficiency of a purely stock engine. Change that volumetric efficiency through significant modifications and the programmed assumptions would no longer be correct. It's this lack of latitude in that programming/sensor relationship that limits a factory speed-density system's ability to respond to major modifications such as free-breathing heads, a cam, or supercharging. Let's be clear, though, that these comments relate to a stock speed-density system, not the aftermarket digital fuel-injection systems specifically developed for extreme performance applications. But that's a discussion for another issue.

Photo Gallery

View Photo Gallery

Getting Wired
Unless your Mustang has a 2.3 turbo engine, its mass air meter uses a heated sensing element to measure airflow, it has no moving parts, and it is called a hot-wire mass air meter (Ford's turbo fours, as did the Mustang SVO's, instead used a mechanical-vane airflow sensor). Though we often use the terms interchangeably, the mass air sensor is this sensing element, and it is positioned in a sampling tube somewhere within the mass air meter assembly. Of the two "wires" in a hot-wire mass air meter, one is a thermistor that simply measures incoming ambient air temperature, while the second is heated by an electronic control circuit to 200 degrees Celsius above ambient. The amount of current required to maintain that temperature differential above ambient will increase with the mass of incoming air-the more air that passes, the more cooling effect, and the more current drawn to compensate. The degree of cooling will also vary according to the passing air's density-determined by temperature, humidity, and barometric pressure-meaning the sensor is indeed measuring air mass rather than volume.

The mass air meter's electronic control circuit monitors this current draw and simultaneously outputs a directly proportional voltage signal to the car's EEC processor. The sensor is calibrated so that a given output voltage-within a working range of between about 0.5 and 4.95 volts-results from a given mass of inducted air. This relationship between incoming airflow mass and the resultant output voltage is electronically mapped by the mass air meter's transfer function-an exponential flow-versus-voltage curve that can be manipulated by the process of meter calibration,and can be visualized as a line-graph plotting airflow along the x-axis against voltage on the y-axis.

As it monitors the mass air meter's voltage signal, the EEC processor also considers other real-time sensor data-including engine rpm and throttle position-to calculate engine load. As dictated by its programmed tables, the processor orders not only the appropriate shot of fuel, but also the correct spark advance and EGR response. This means that an improperly calibrated mass air meter can throw both air/fuel mixtures and timing out of whack, costing horsepower, driveability, and maybe even engine longevity.

A Meter's Got to Know Its Limitations
Factory meter calibration for a stock engine is, as you might expect, fairly good (though, thanks to production tolerances, it can be as much as 4 percent rich or lean), and a stock meter will respond well to some modifications that would have thrown the old speed-density system completely for a loop. But as modifications become more serious, the stock meter can begin to show its limitations, both physically and electronically.

You've heard it before, and you'll hear it again: An engine is nothing more than an air pump-the more air it can inhale, the more fuel that can be added, and the more power it will subsequently produce. In the process of installing better heads, cams, exhaust, and forced induction to increase this breathing potential, it's likely you'll exceed the physical flow limits of your Mustang's original mass air meter. It was factory-sized for stock airflow requirements, but it can become a real restriction after modification-similar to a marathon runner trying to breathe through a drinking straw.

As a result, aftermarket mass air meters having a larger cross-sectional area for unrestricted breathing have increased in popularity, such as the well-known units from Pro-M, now available in sizes said to be capable of supporting up to 2,000 hp. But a bigger hole is just half the battle. Equally important is that the overall design of the meter housing and sample tube must permit accurate air metering all the way from the trickle at idle to the torrent at full throttle. This is complicated stuff and, in this regard, Pro-M holds several patents on its meter designs. Still, even the best meter housing is nothing without the calibration side of the equation.

Calibration Electronics and Mathematics in Action
Meter calibration is an exceptionally precise operation, requiring flow stands that can measure airflow with almost unbelievable accuracy and consistency. To our knowledge, Pro-M is the only aftermarket source of electronic meter calibration. The company can recalibrate your Ford meter, and every one of its own in-house-manufactured meters is specifically calibrated to the customer's application.

Before trying to make sense of calibration, it's important to understand certain basics of the EFI fuel system. Fuel injectors have a maximum flow rating expressed in pounds of fuel per hour, calculated at a specific fuel pressure (normally around 38 to 42 psi). Injec-tors are binary devices having only two operating states-wide open or completely closed off-and the EEC processor controls how much fuel they inject for combustion by dictating the length of time they remain open, something known as injector duty cycle, or pulse width. The EEC's fuel tables rely greatly on the mass air meter's output voltage level to determine the required pulse width at any given time. When it comes to airflow, all the EEC sees is this voltage, so it therefore takes the mass air meter's word on how much air is inbound. Calibration basically entails establishing a particular mathematical relationship between this voltage level and the mass of incoming airflow-a relationship called the transfer function-by adjustment to the meter's electronic circuitry.

One essential job of mass air meter calibration-whether factory or aftermarket-is to make sure the full extent of the subject engine's airflow capacity is encompassed and mapped within a limited voltage output range topping out at about 4.95V DC. This is a voltage limit on the EEC's part, not the meter's. EEC fuel tables increase injector pulse width in proportion to increasing mass air meter voltage, but any signal higher than about 4.95 volts exceeds the tables' programmed upper limits, above which the processor will order no further increases in fuel regardless of how much air is actually inbound. Can you say "lean"?

In a factory meter's original calibration, then, 4.95 volts would be equated to the greatest mass of air the particular stock engine could ever conceivably ingest, with some fudge factor thrown in for basic modifications. That way, a 4.95V mass air meter output would theoretically never be exceeded, and sufficient fuel would always be assured based on the corresponding injector pulse widths programmed into the EEC.

Photo Gallery

View Photo Gallery

Major engine modifications that open the door to a lot more air usually dictate larger fuel injectors, along with a larger mass air meter specifically calibrated for the new combination. This calibration must not only compress the modified engine's newfound giant airflow potential down into a 4.95V range, but also compensate in overall scale for whatever new injectors are on board. Why the compensation? Because the EEC's factory programmed pulse widths are based on the flow rate of 19-lb/hr injectors. If you now have faster-flowing 30-pound injectors on board, those factory pulse widths would create grossly rich conditions at nearly anything but wide-open throttle and high rpm (your 400hp pony may need the bountiful flow of those bigger injectors at full throttle, but it sure doesn't need it at idle). The meter's calibration must therefore be altered so that it takes more airflow to generate any given voltage any time you change injectors.

Calibration must also take into consideration the specific details of the air inlet tract ahead of the meter. Any changes made in front of the meter-such as eliminating the factory airbox or even swapping to a high-flow filter-can significantly affect meter calibration, creating A/F ratio inaccuracy. What this means is that, when shipping you a new meter or recalibrating your old one, pro-M will need to know the details of your car's inlet tract, and will duplicate that hardware on the flow stand when it performs the calibration. Elbows directly ahead of the meter, such as found on some cold-air and supercharger kits, can have a particularly perverse impact on accuracy, leading Pro-M to develop a specific configuration of meter, known as the Univer or new Univer Plus, for such elbowed inlets.

Calibration is accomplished by altering resistance within the circuitry of the meter's electronic element, with the meter installed on a flow stand and monitored by a computer. This resistance matching of airflow to voltage is done only at the top and bottom of the particular application's airflow limits, with proprietary curve-fitting software calculating all the points in between. We don't need to know the compli-cated mathematics behind these well-researched transfer function curves, but suffice it to say that curve shapes are roughly similar for any naturally aspirated application but differ for forced induction, due to the extra enrichment requirements of boost.

Admittedly, we've really only touched here on the science of mass air meters, but hopefully you've at least gained some insight into the delicate relationship between your mass air meter and a good-running engine.

Photo Gallery

View Photo Gallery

How Do You Know When You Need a Meter?
The folks at Pro-M produced the following chart showing maximum recommended brake horsepower levels for a given meter.

METER MAXIMUM BHP
OEM 55mm 320
OEM 70mm 380
OEM 80mm 485
75mm Bullet 600
77mm Pro-M 700
80mm Pro-M 800
83mm Pro-M 900
92mm Pro-M 1,000
117mm Pro-M 1,500
209mm Pro-M 2,000

Care for a Chip?
Pro-M's Scott Beer tells us that calibrating a meter for anything beyond two injector sizes up from stock should be accompanied by a custom chip in the EEC. This is because the mass air meter signal is used by the EEC to infer barometric pressure for load calculations (there is no longer a separate barometric sensor on current Mustangs), and recalibration for large injectors can throw off this "inferred baro," resulting in timing-advance errors.

By the time you read this, Pro-M will have its own line of custom EPROM chips.