A SAFC intercepts and "corrects" the voltage signal from your AFM in order to fool the stock ECU into running normally with larger fuel injectors. AFC’s are not recommended in my opinion because they feed the ECU false airflow data to “correct” the fueling; this is not a good way to “tune” because it causes the ECU to use its mapped ignition timing based on less airflow. Some units (such as the GReddy eManage) are able to correct ignition timing, but not generally with the precision of the stock ECU. Some units also have the ability to compensate for a maxed-out AFM by using another form of metering (such as a MAP sensor) or replacement of the stock MAF altogether. By the time you've wasted your money on one of those units, you could have already purchased a ROMulator and 88-89 ECU.
That having been said, the SAFC (made by Apex'i, inc) or units that are similar can be purchased online for under US$300. Most units can run +/- 50% of your stock injector size. This means the Z's stock 260CC/min injectors can be replaced with up to 390CC/min injectors.
A few things need to be done to install a SAFC:
1. Find higher flow direct "drop-in" injectors.
2. Install the TPS from either a late model 240SX or a (so it has been reported) Z31 with an automatic transmission. The TPS needs to have a variance through the throttle movement range (known as a potentiometer). A stock Z31 with a manual transmission only has a single set of contacts on the TPS for idle. Don't limit yourself though; many TPS's can be made to fit your Z's throttle body.
3. Wire in the SAFC to the new TPS and the ECCS wiring. You can find wiring diagrams in both your Haynes manual and the Factory Service Manual. Below is a chart for installing a SAFC with an 84 Turbo ECCS:
4. Begin to alter the airflow readings fed to the ECU and tune to the desired AFR (using the instructions for the particular unit you purchased). As you timing will be severely advanced, back out base timing using the distributor so you don't get detonation under boost. Keep in mind that this shifts your ENTIRE timing map, so while the car may run great under boost you will kill your low end and engine response while in vac.