Ford used speed density on the 86,87 & 88's. Mass Air is much better about accepting engine modifications that change it's airflow characteristics because it actually measures the volume of air going thru the engine. Speed density has problems with this because it can't measure airflow so it has to keep referring back to preprogramed maps in it's memory and when the info from the sensors doesn't match the maps in it's memory it goes a little nuts trying to set things right & that usually causes drivability problems. Some basic info sometimes helps:
Once upon a time an engine needed three things to run: fuel, air, and fire. That's what carbs, coils, and distributors are for. Modern EFI engines still need these three elements, but they use different hardware to provide them, and a computer to run the whole process.
Today's electronic engine management systems can process millions of instructions per second to continuously adjust spark and fuel for optimum performance. The computer regulates the electronic fuel-injector pulse width (the time that the fuel injector is open) and ignition lead with input from various sensors. One of the key things the computer needs to know is how much air the engine consumes under a given set of conditions. Three different measurement strategies have evolved to supply the computer with this basic information; in order of sophistication they are: N Alpha, Speed Density, and Mass Flow metering.
N Alpha
A relatively simple design, N Alpha systems use only engine speed and throttle angle to calculate the amount of fuel needed by the engine. This system doesn't measure airflow directly; instead, engine load is assumed based on throttle-angle versus engine rpm. The various load-rpm points make up the computer's lookup table, with the amount of fuel needed at each point manually programmed by the tuner. N Alpha systems work well on engines that operate primarily at wide-open throttle such as race cars but are much less accurate at part-throttle than more sophisticated systems because of their relatively simple fuel map. They generally do not have a closed-loop mode for air/fuel correction, resulting in part-throttle calibration that is crude at best when compared to other systems. This also makes them incompatible with modern catalytic converters. Any significant engine change requires remapping.
Speed Density
Speed Density systems accept input from sensors that measure engine speed (in rpm) and load (manifold vacuum in kPa), then the computer calculates airflow requirements by referring to a much larger (in comparison to an N Alpha system) preprogrammed lookup table, a map of thousands of values that equates to the engine's volumetric efficiency (VE) under varying conditions of throttle position and engine speed. Engine rpm is provided via a tach signal, while vacuum is transmitted via an intake manifold-mounted Manifold Air Pressure (MAP) sensor. Since air density changes with air temperature, an intake manifold-mounted sensor is also used.
Production-based Speed Density computers also utilize an oxygen (O2) sensor mounted in the exhaust tract. The computer looks at the air/fuel ratio from the O2 sensor and corrects the fuel delivery for any errors. This helps compensate for wear and tear and production variables. Other sensors on a typical Speed Density system usually include an idle-air control motor to help regulate idle speed, a throttle-position sensor that transmits the percentage of throttle opening, a coolant-temperature sensor, and a knock sensor as a final fail-safe that hears detonation so the computer can ****** timing as needed
All 86-’87 and 88 non-California Ford 5.0L-** engines used Speed Density metering.
Because a Speed Density system still has no sensors that directly measure engine airflow, all the fuel mapping points must be preprogrammed, so any significant change to the engine that alters its VE requires reprogramming the computer.
Mass Flow
By contrast, Mass Air Flow (MAF) systems use a sensor mounted in front of the throttle body that directly measures the amount of air inducted into the engine. The most common type of mass-flow sensor is the hot wire design: Air flows past a heated wire that's part of a circuit that measures electrical current. Current flowing through the wire heats it to a temperature that is always held above the inlet air temperature by a fixed amount. Air flowing across the wire draws away some of the heat, so an increase in current flow is required to maintain its fixed temperature. The amount of current needed to heat the wire is proportional to the mass of air flowing across the wire. The mass-air meter also includes a temperature sensor that provides a correction for intake air temperature so the output signal is not affected by it.
The MAF sensor's circuitry converts the current reading into a voltage signal for the computer, which in turn equates the voltage value to mass flow. Typical MAF systems also use additional sensors similar to those found in Speed Density systems. Once the electronic control module (ECM) knows the amount of air entering the engine, it looks at these other sensors to determine the engine's current state of operation (idle, acceleration, cruise, deceleration, operating temperature, and so on), then refers to an electronic map to find the appropriate air/fuel ratio and select the fuel-injector pulse width required to match the input signals.
Ford has used MAF metering on 88 California 5.0L engines and all 89-and-later V-8 engines.
MAF systems are much more flexible in their ability to compensate for engine changes since they actually measure airflow instead of computing it based on preprogrammed assumptions. They are self-compensating for most reasonable upgrades, as well as extremely accurate under low-speed, part-throttle operation. On the other hand, the MAF meter, mounted as it is ahead of the throttle-body, can become an airflow restriction on high-horsepower engines. On nonstock engine retrofits or EFI conversions on engines never produced with fuel injection, it may be hard to package an MAF meter within the confines of the engine bay and available intake manifolding.
Which Is Best?
In a perfect world, virtually all street-performance engines would use Mass Air, due to its superior accuracy and greater tolerance for engine changes.
One more thing, adding the truck info to your signature is helpful because it shows up automatically in every post.
/emoticons/
