I thought this maybe useful...
Speed density uses the manifold pressure sensor (MAP) to measure the intake manifold pressure, then uses volumetric efficiency lookup tables indexed by engine speed to find the mass of air entering the engine. Other parameters such as coolant temperature, battery voltage and intake air temperature are used to compensate the table lookup values for the engine.
Formula: Pure SD (where injector pulse width = an injector constant * VE(kpa,rpm) * MAP)
When speed-density use RPM , temperature and TPS only, it is called "Alpha-N". Alpha-N uses the only throttle position and RPM to calculate the amount of fuel to inject as opposed to using the manifold absolute pressure and RPM to calculate the amount of fuel to inject. Using the speed-density algorithm, MAP is the main variable and VE is a 'tweak'. On alpha-N the VE table is the main variable, as TPS is used as a lookup into this table. Actually it is a fuel map rather than a VE table.
Alpha-N is useful for cars using ITB & long duration cams where the resolution of manifold air pressure (map) would be small. It is also useful to get smother idle on engines that have erratic map values. These vehicles rarely see idle as they run WOT 90% of the time.
Formula: Pure alpha N (where PW = constant * VE(tps,rpm) * TPS)
VE Calculation Table
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