Fuel Management 101

There are two ways to get more fuel into an engine, one is to increase fuel pressure, thus forcing more fuel through the same size hole (injector), and the other is to increase the size of the hole (injector) and keep the pressure constant. An FMU functions by raising  fuel pressure under boost, while oversized injectors function by simply letting more fuel though. However, you can't just plug in larger injectors and go - the stock fuel map tells the injectors to open for a certain amount of time based on the manifold pressure and rpm of the engine, but if the larger injectors are open that long, the air/fuel mix will be far too rich.

In a nut shell, the SAFC (or SMC, VAFC, Fields SFC, which all do the same thing without all the extra functions such as a screen, gauges and graphs) simply raise or lower the stock fuel map based on engine RPM. They do this by raising or lowering the voltage sent from the MAP sensor to the ECU, which largely determines how much fuel is injected into the cylinders. For a boosted application, you would install larger injectors (450cc and 550cc are common), and lower the signal from the MAP sensor accordingly (for example, by 40%). That way, at idle and in vacuum, the ECU thinks that you're in SERIOUS vacuum, and lowers the injector pulse length accordingly. Thus less of a signal for fuel combined with bigger injectors evens out to produce stock like driving. But once you go into boost, the ECU thinks that your engine is under the amount of load associated with a naturally aspirated situation (vacuum), and it sends naturally aspirated high load fuel signals, which coupled with larger injectors leads to a lot more fuel then stock, providing enough fuel to compensate for the increased air charge, thus maintaining a proper air fuel ratio. This also keeps your ECU from ever seeing boost, which is good, because if it does it assumes that something is wrong with the MAP sensor and throws a code.

The SAFC cannot, however provide for ANY ignition tuning, nor can you use it to tune your fueling based on any variable but RPM (of course, the stock ECU keeps everything in check to a point since it uses stock fuel programming which motors those other variables). This is generally fine for low/mid boost levels (under 12 or so psi), but it never a substitute for a proper engine management system, such as Hondata, the AEM EMS, the Power FC or FAST.