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
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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Mass Airflow
The mass airflow system only partially relies on preprogrammed values. It also measures the incoming airflow directly as volumetric displacement. This measurement is calculated with a hot-wire sensor in the airflow path into the engine. This sensor type is based on the principle that if a mass is maintained at a constant temperature (above ambient air temp), the thermal loss is proportional to the flow of air over that mass. The sensor uses this principle by subjecting a hot wire (a thin wire element wound on a ceramic bobbin) to the incoming air stream. This wire is heated to 200 degrees Celsius over ambient (as measured by a second "cold" wire in the sensor). The mass airflow meter controls the temperature of the hot wire by constantly measuring the cold wire for any ambient temperature changes.

The voltage drop across the hot wire is sent to the EEC IV as a signal, which is proportional to the flow of air into the engine. This signal controls injector pulse width, air/fuel ratio, and spark advance. Controlling an engine with mass airflow management allows the computer to "see" the airflow into the engine, no matter the modifications to volumetric efficiency. The mass airflow system uses fewer calculations to do the same work as the speed-density EEC IV, and it does it much faster, making for better driveability and vehicle performance.

Now that you understand the differences, how do you go about converting your speed-density Mustang to that of mass airflow? It's easy, really. You decide on a stock or aftermarket air meter, locate a mass airflow EEC IV, and purchase a wiring kit. For one-stop shopping, we suggest you purchase these items in a mass airflow upgrade kit. We found just such a kit as an advertised special at Dave's Mustang Parts. While there are several options for air meters available from DMP, we opted for the stock parts in used condition and the aforementioned wiring kit. The company can also upgrade these kits with various aftermarket sensors, or it can hook you up with other induction goodies that are perfect for installing at the same time, such as throttle bodies and cold-air kits.

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