919 FI System Overview
For best combustion, you want all the fuel droplets at or near the same size, and a precise amount of fuel delivered (injected). The way to do this is to match the injector sprayer design to the fuel rail pressure to the kind of spray pattern you want, then control the duration that the injector is open to control the amount of fuel delivered. So, the sprayers shape and direct the fuel droplets, and the injector opening duration controls the amount of fuel delivered. When you adjust your PCIII or similar, all you are doing is modifying the injector open duration that the ECU says is needed. The key here is that fuel metering control is best derived by injector open duration (time) control. The on/off action is so precise, that precise time control results, and this is what gives you accurate fuel metering control.
Here's an overall of your 919 FI system. Keep in mind that Air Fuel ratios are on the basis of mass (moles on a Chemistry level), NOT volume ! The Honda engineers have a plan for what kind of A/F on a mass basis they want for the entire engine cycle. All revs, all throttle openings. They construct a co-ordinate map for fuel delivery. The co-ordinates are Throttle Position x RPM. For every co-ordinate on that map, they design the ECU to tell the injectors how long to be open in order to meter the amount of fuel they intended in order to get a certain A/F ratio. The baseline assumption is that the air temperature and pressure will be some constant. The density and temperature of the air affects the actual mass of air flowing past the intake valves into the combustion chamber. Hotter air is less dense, and higher pressure air is more dense. So they add an IAT Sensor and MAP Sensor. (Intake Air Temp & Manifold Absolute Pressure). The signals from those two sensors go to the ECU and the ECU uses that information to calculate the actual air mass flow rate into your engine, and adjusts the baseline map accordingly by changing the pulse width signal sent to the injectors in order to keep getting the target A/F ratio. Honda also uses a ECT Sensor (Engine Coolant Temperature). This feeds into the ECU and even further adjusts the baseline map in the ECU. Cold coolant = cold engine = richer needed = longer duration pulse width signal to the injectors. At normal coolant temps, there is little adjustment resulting from this sensor. But when the cooling system is running higher temps, the ECT signal to the ECU results in significant richening in an effort to cool combustion and reduce heat load into the cooling system. There are actually 7 distinct richness adjustment steps for the ECT sensed cooling temp range of 168 - 218 F. (this is why keeping the coolant in the normal zone is so important when doing dyno runs). Now we add in the PCIIIusb. Let's now look at an instant in time.The ECU has just done to accounting for Throttle Position, RPM, air mass flow corrections based on the AIT and MAP, and ECT info, and come up with a injector time open requirement that is derived by a pulse width signal that sent to the injectors. All the PCIIIusb does is adjust the duration of that ECU output pulse width signal by the amount of time change needed of injector opening to get the change in A/F ratio that the modified map you installed into the PCIIIusb calls for. Further, the PCIIIusb is a piggyback system. It piggbacks the ECU. It just adjusts what the ECU outputs. The IAT and MAP are still being used and are still working and sending their adjustment signals that the ECU still gets and uses.
Last but not least.
Remember, they construct a co-ordinate map for fuel delivery. Again, the co-ordinates are Throttle Position x RPM. Again, for every co-ordinate on that map, they design the ECU to tell the injectors how long to be open in order to meter the amount of fuel they intended in order to get a certain A/F ratio AT THAT PRECISE MAP CO-ORDINATE OF THROTTLE POSITION x RPM. This is why it is so crucial to synchronize your PCIIIusb to the Throttle Position at both Closed and Full Open position. The Closed position is more important, but always do both. Why you ask ? If you don't, your fuel injection map will be out of phase with the engine ! If your engine is at 5 % Throttle opening and 1500 RPM, but the PCIIIusb is reading 8 %, you will get a 8 % x 1500 RPM dose of fuel injected per the map, instead of the 5 % x 1500 RPM dose that the engine actually needs. To give you an idea how much this really means in real life, my mapped and synch'd 919 can walk along in first gear as low as 800 RPM, with a passenger, on a slight uphill grade, and have perfect throttle control that needs no clutching to control it.
I hope you find all of this useful and informative. Don't hesitate to ask more questions. I'll answer if I can. If I don't know I will say so.