Mark Houlahan
Tech Editor, Mustang Monthly
November 1, 2002

Horse Sense:
There are several ways to convert your speed-density Fox Mustang to mass air. One involves the adapter harnesses and moving of wire terminals as shown in this article. Another way is to replace the complete EEC IV harness, which when swapped out makes for a plug-in conversion, though this is not an option for '86 models. Basically, you're upgrading your '87-'88 Mustang to '89 specs without having to get out the wire cutters or move wire terminals around in the PCM connector. The only downside to this plug-and-play alternative is finding good used EEC IV harnesses from '89 5.0s for the swap. The mass airflow system uses fewer calculations to do the same work as the speed-density EEC IV

In the late '80s and early '90s, Ford converted to mass airflow systems on many of its products, while GM was actually going in the opposite direction back to speed-density- based systems. GM's mass air meters were having an unprecedented failure rate in the industry, but that didn't scare Ford from converting to the hot-wire sensor system of mass air metering to more accurately maintain the engine's air-fuel ration by measuring the airflow coming into the engine.

So what is a mass airflow meter, how does it work, and why do you need it? Those are the most common questions we are asked when it comes to Ford EFI systems. Probably the easiest way to answer them is to define the speed-density system of the '86-'88 models, and then define the mass airflow system used in '88 (California only) and '89-and-later Mustangs. Then you will clearly see why your Mustang would benefit from the upgrade to mass airflow sensing.

Speed-density systems used on '86-'88 Mustangs are based on a set of known values. These values are inferred from various engine sensors that tell the EEC IV throttle angle, rpm, air-charge temperature, coolant temperature, manifold pressure, and so on. The values are cross-referenced into a set of fixed fuel and timing tables in the electronic engine control. The tables (think of the them as a series of multiplication tables) compare the sensor values until coming to the proper table for the operation of the engine at that given moment. This table is then executed, setting injector pulse width, spark advance, idle speed, Thermactor air operation, and much more.

The system operates within its parameters as long as you don't dramatically affect the volumetric efficiency of the engine. Remember, an engine is nothing more than a sophisticated air pump. If you pump too much air into it with high-flow cylinder heads, intake, cam, and so on, you'll throw the speed-density system loopy since it can't "see" these modifications. You've basically pushed your speed-density Mustang out of its realm to cope with the additional airflow. This is the main reason for upgrading to the mass airflow system.

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