Dale Amy
April 16, 2003

Like it or not, we have the often mysterious world of electrons to thank for the modern Mustang's jaw-dropping performance potential. Were it not for the unparalleled speed and precision of electronic engine control processors when it comes to spark and air/fuel mixture, we couldn't begin to approach the blend of power, economy, and prep-school manners we've come to take for granted these days.

But a computer is only as good as its software, and, as good as they are, factory tuning calibrations aren't always perfect--even for a stock vehicle, since they're generalized and even compromised to allow for production tolerances and other variables. That gap from tuning perfection will widen to a chasm after hop-up modifications are made, because the original calibrations were based on assumptions that will no longer be valid.

Instead of using wrenches and screwdrivers to close that gap, today we must rely on "tuning" the EEC computer itself, since these days an engine does exactly what its electronic brain tells it to. Most of us are familiar with tuning via EEPROM chips--such as those from Autologic, DiabloSport, and Superchips--that go in and substitute revised data in the EEC's programming tables. Well, when it comes to manipulating air/fuel ratios, there's now another way to trick the EEC into doing our bidding: We can lie to it about how much air is inbound to the combustion chambers. Enter Pro-M's MAFtuner, which can be thought of as an air/fuel ratio tuning tool disguised as a little black box. Used properly, it can root out the last bits of power, driveability and mileage hiding in your Ford's current combination, whether heavily modified or factory stock.

The MAFtuner functions by intercepting--and modifying--the electronic signal between your car's mass airflow meter (MAF) and computer (EEC IV or EEC V). Remember that a mass air meter's one and only job is to tell the EEC processor as accurately as possible how much air is being ingested at any given time, so that the EEC can then order the appropriate amount of fuel injected for proper combustion. Maintaining an ideal ratio between air and fuel--an ideal that varies somewhat depending on a number of factors--is oh-so vital for exhaust cleanliness, idle quality, driveability, power production, and ultimately the engine's survival. An A/F ratio that is at any time either too rich or too lean can adversely affect any or all of these conditions.

In Ford applications, the mass air meter talks to the EEC by sending it a signal that increases in voltage with the mass of incoming air. More air equals more voltage (within a range of roughly 0-5 volts DC). In the EEC's software cerebrum, any given voltage input is equated to a certain mass of air, to which the program assigns a corresponding amount of fuel to be injected. In other words, the amount of fuel injected is directly related to the voltage received by the EEC from the mass air. Raise that voltage and the computer would call for more gas, and vice versa.

The MAFtuner does its trickery by intercepting the mass air meter voltage signal and selectively raising or lowering it, according to the user's preference, before sending the modified voltage on to the EEC. This makes the computer believe that either more or less air is being ingested than is actually the case, fooling it into generating a slightly leaner or richer A/F mixture. The voltage changes applied by the user need not be universal; they can vary from point to point in the airflow curve. In other words, you can leave the mixture as is at some points, richen it in some areas, and lean it in others, depending on what your engine's combination responds to best.

These voltage alterations can be ordered up either by using the combination of four potentiometers and eight LEDs arrayed on the face of the MAFtuner, or by attaching a laptop. Since it's best to have a clear A/F ratio feedback, detailed tuning will be best done on a dyno equipped with a wideband oxygen sensor that can show the ratio real-time. But Pro-M can provide general suggestions for what has worked on particular vehicles and combinations.

The MAFtuner can also save you from needing to have your meter recalibrated after changing injector sizes or adding a cold-air kit. For instance, let's say you have a 75mm Bullet mass air calibrated for stock 19-lb/hr injectors and you then upgrade to a set of 24-lb/hr squirters. Rather than removing and shipping your meter back to the manufacturer for recalibration, you can simply hook up your MAFtuner to your laptop with the supplied cable, and use the included software to reprogram it using specific voltage alteration tables available from Pro-M.

Will the MAFtuner completely eliminate the need for chip tuning or an EEC reflash after radical engine mods such as ported heads and a blower? No, it won't, because it can address only the fuel side of the tuning equation and not the spark. But it does add real-time A/F tuning to your bag of tricks. Will the mere act of bolting on the MAFtuner instantly gain you horsepower? Nope, it's a tuning tool, not a power adder. But it's a flexible way to gain complete control over all-important air/fuel mixtures, and getting those right can maximize power, driveability, and efficiency, making the MAFtuner a valuable addition to your tuning arsenal.

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The modern equivalent of fiddling with idle mixture screws and carb jets, Pro-M's MAFtuner can help optimize air/fuel ratios from idle to full throttle. Best of all, you can fiddle all you want and still not smell like a leaking gasoline tanker.
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The $299 Pro-M kit nets you the MAFtuner module itself, a serial/USB cable to link it to a laptop, a CD containing the MAFtuner software, a couple squares of Velcro for mounting, and a harness to hard-wire the MAFtuner module into your car's EEC harness. Not shown is the kit's instruction booklet, included both in hard copy and on the CD.
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Should you not cherish the thought of cutting into your car's main wiring harness, for an extra $99 a "plug-and-play" cable is available that will eliminate any such electronic surgery. In the following photos, we'll show you the basics of installing the MAFtuner either way.
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Hard-wiring the MAFtuner requires unbolting the main wiring harness female connector from the EEC processor with a 10mm socket or wrench. But before doing anything, disconnect the negative battery cable. Because it's nearly impossible to photograph down in the passenger kick panel where the EEC normally resides, we're using a cannibalized computer/harness setup on the workbench for clarity.
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To separate the black plastic harness shield from the connector, use a small flat-blade screwdriver and gently lift and push out the four retaining tabs (two top, two bottom).
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Now pull the shield out of the way and, to gain access to the individual wires, use a utility knife to make a few-inches-long slit in the tape wrapping the wire bundle. We hope we don't need to emphasize the importance of using extreme care during this cutting step--visualize yourself as a surgeon using a scalpel really close to the spinal cord of a particularly litigious patient.
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You'll be working with three wires in the EEC harness. The first two are simply tapped into for ground and power. For ground (shown), Pro-M's Scott Beer connects the kit's black wire to a black-with-white-stripe wire in the car harness (pin No. 77 on the EEC connector). For key-on power, he then connects the kit's red wire to a solid red wire (pin No. 71 in our case, though having a factory wiring schematic is never a bad idea). As you can see, Scott prefers heat-shrinkable butt connectors over wire taps.
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The third wire Scott cuts into is pale blue with a red stripe (pin No. 88 in our case). This is the MAF signal wire, and to the MAF side of the cut, Scott connects the kit's yellow wire. He then connects the kit's orange wire to the EEC side, thereby placing the MAFtuner electronically in line between the MAF and the EEC. By the way, there's also a fifth wire on the MAFtuner harness--a blue one--that is for GM applications only and is not used. About now you're probably beginning to understand why the plug-and-play harness is a popular option.
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With the connections complete, the harness can be taped back up and reconnected to the EEC processor. Be careful when reinserting the female connector not to bend the pins in the EEC. Proper torque for the harness bolt is 22 in/lb. The kit's wiring harness then simply plugs into a connector on the back of the MAFtuner.
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Installation is quicker and less electrically nerve-wracking with the optional plug-and-play harness--a method that would also make it easier to move the MAFtuner to another vehicle. As always, begin by disconnecting the battery ground cable. Then unplug the factory MAF electrical harness from the MAF. The Pro-M plug-and-play harness has the appropriate weatherproof connectors to simply plug in-line in between the MAF and its factory harness (so as to intercept the MAF signal and forward it to the MAFtuner).
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The only downside of the plug-and-play option is having to drill a hole through the firewall big enough to route this end of the harness through to the MAFtuner module, which will be mounted inside the car. If you have the appropriate tool, you can remove the pins from the back side of the plastic connector, drill a smaller hole, and reinsert them after routing the cable through the firewall.
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Plug the connector into the back of the MAFtuner, and you're done. In our case, Scott simply Velcro'd the MAFtuner module to the floor of the GT's glovebox, making it accessible but out of the way. For real-time tuning using the potentiometers on the MAFtuner, place it in a location where you can easily twiddle the knobs and see the LED array.
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The supplied interface cable can be used to connect the USB port on the back of the MAFtuner to a laptop's serial port. A laptop is not absolutely necessary to tune with the MAFtuner, but it does open the door to some additional tuning options, such as using preprogrammed calibrations.

Horse Sense: While in closed-loop (part-throttle) operation, the EEC uses feedback from the car's oxygen sensors to monitor and maintain a "stoichiometric" air/fuel ratio. This ratio of around 14.7 parts air to 1 part fuel is that which produces the most thorough combustion. At wide-open throttle, or in open-loop, feedback from the O2 sensors is ignored and A/F is dictated strictly by the EEC.