>>The Mass Air Sensed System was still being used for the '89 production year. The only physical difference between the Mass Air Sensed system and the Speed/Density system is whether a Mass Air Flow (MAF) sensor or a Manifold Absolute Pressure (MAP) sensor is installed.<<
All the port-injected engines have MAPs whether they have MAFs, or not.
Engine controls that use mass-air flow to set the fuel schedule use a MAF to measure the mass of the air going into the engine. A mass-airflow system tends to be a bit more accurate in its metering of fuel, however, this accuracy doesn't really affect performance but it does affect driveability and emissions to a small extent.
Speed density systems meter fuel by computing the air going into the engine by measuring it's density and the engine speed. "Speed-dense" systems are less expensive to manufacture but require faster and more accurate processing.
When wide-spread use of electronic port injection began, many engines used MAFs because the computers weren't robust enough to make the calculations for speed-dense to work well. In the late 80s, GM switched over to speed-density because the technolgy advanced enough such that computers capible of making the fast computations were available. In the mid-late-90s MAF systems became popular again because of the improvments this type of system makes in exhaust emissions.