[Mitch P]

For DI control, you must at least send the DI box 4 separate injector signals plus another signal for the spill control valve in order to control the amount of rail pressure. You can use injector, coil or other crank angle timed outputs properly setup in software,with the correct hardware interface, to control these types components.

The amount of fuel injected is a combination of the injector pulse time and the fuel pressure, just like a regular port injection system. With direct injection you only have a limited crank angle window to inject fuel (you don't typically want to inject fuel with the exhaust valve open as it just goes out the pipe, but sometimes you might) so the crank angle through which the direct injector fires is important. To get more range in available fuel flow for given crank angle, you also need to adjust the fuel pressure.

With a port fuel injection system, if you raise your fuel pressure from 36psi to 81psi, you get approximately 50% more fuel. Physics tell us that a fuel pressure change will effect fuel delivery by the ratio of the square roots of the pressure change. In our example: sqrt(81)/sqrt(36) = 1.5, or in other words, a 50% increase. Now, a DI system runs at a much higher pressure range. According to the FR-S/BRZ manual, this system runs from 2.4 MPa to 20MPa (~350 to ~2900 psi) so from lowest to highest pressure, you get an approximate fuel flow change of sqrt(2900)/sqrt(350) = 2.88 or a 188% increase in base fuel delivery. (min to max rail pressure) Now, in reality, this might not hold exactly true due to a number of factors (injector non-linearity, cylinder pressure, etc...) but the idea is generally true.

One additional facet of fuel pressure control is that the spill control valve output must also be timed specifically to a certain crank angle window for proper operation of the high pressure fuel pump. The pump is mechanically run off a triangular-ish lobe on the camshaft. (Kinda looks like a 13B rotor) The ECU control signal must be timed to each of the three lobes for proper pump operation.

So in the end, you needed send the DI box 5 signals with correct pulse widths and engine position timing to get it to work properly. Calculating or mapping those value is a matter of software setup and some math. The DI box also may need additional error checking signals to ensure that it operates as though the stock ECU is still controlling it. We have not tested that yet as we need to get the DBW throttle going to characterize what the DI box is looking to see without the stock ECU closing the throttle on us.