The speed density system is best described as a calculation procedure used by the electronic engine control(ECC) module that involves predetermined operating parameter values, volumetric efficiency tables, pre-measured airflow-through-the-engine values,and the known volume of fuel delivery required per combustion event for a single cylinder. Management of fuel and spark functions is based mainly on a predetermined range of pre-programmed data and also on real-time feedback data from an array of sensors.
The "speed" signal is based on the calculated volume of a single cylinder. The "density" signal is a function of temperature and pressure measurements. To determine an engine's fuel delivery requirements, the speed density system infers airflow from several monitored sources, including engine speed(in RPM), intake manifold absolute pressure(MAP, to determine load), manifold absolute temperature(MAT), throttle position, the oxygen content of the exhaust(via a heated lambda-sond sensor or sensors), engine coolant temperature and battery voltage.
There is no airflow sensor(airflow meter) in the speed density system. Signals form the manifold absolute pressure sensor relate operating conditions that are translated into relationships(engine-speed-to-load-to-throttle position, for example). The EEC then compares this data to ideal data curves based on the engine's volumetric efficiency. The speed density computer is pre-programmed for the desired fuel, ignition, and EGR characteristics, and it makes continuous(metering, timing, and cycling) adjustments based on those pre-mapped relationships.
Observations made by Ford Motor Company indicate that a 1987 speed density H.O. Mustang will out-accelerate an 1989-93 mass-airflow Mustang(with the slighlty revised 1989 camshaft) by .2 second in the quarter mile. A speed density car will also outrun comparable mass-airflow cars in top speed ability.
