engineermike
Well-Known Member
The way the Ford system works is the GDI pump has an output pressure setting that varies with speed and load; stock is up to 2900 psi. The pump is variable capacity and adjusts to achieve the pressure setpoint. If you hit 100%, then the pressure will start dropping. When the pressure drops, the PCM automatically extends injector open time to compensate for lower pressure.
If the system calculated that there isn't enough time to inject desired fuel through the direct injectors, it will automatically reduce the GDI blend and supplement with the port injectors.
As for timing, you have to think about GDI injector timing more like a diesel injector or even valve events.
The start-of-injection (SOI) is typically commanded during the first half of the intake stroke, not long after the exhaust valve closes (EVC). At WOT, I believe the stock Mustang GT commands this at 30 ATDC. To increase the available time, you might command this earlier but I there are some hard limits in there that won't allow it less than about 28 deg ATDC.
The end-of-injection (EOI) is calculated by the PCM determining how much fuel is needed and how long it takes to get that amount of fuel injected. It could be just a few degrees at low load, low speed, or low DI blend. Or, it could be over 200 deg at high speed, high load, or high DI blend. There are a couple of parameters in the PCM that limit EOI but so far I've been able to push mine back to about 75 deg BTDC before combustion occurs. If it hits any of the EOI limits, again, it automatically supplements with PFI.
I actually have a real-time graph set up in VCMScanner where at a glance you can see the timing of all the events from TDC (overlap) to TDC (combustion). From top to bottom, you can see how EVC, SOI, IVC, EOI, and Spark all lay out. All are variable.
GDI gives you knock suppression, simple as that. The stock GDI system is good for ~500 hp. When power exceeds this, the PCM (or tuner) starts reducing GDI blend and sacrificing knock suppression. This isn't a big deal on E85 where knock isn't a limit. But on pump gas at 12/1 compression and boost, you need all the knock suppression you can get, because knock limits your power before anything else (the reason why E85 gains so much on these cars). The first limit is the stock GDI pump. XDI makes two pump upgrades, the bigger one outputting 45% over stock. It takes some tuning tricks to take advantage of this on the stock injectors, but basically the result is having them stay open way longer to take advantage of the extra pump capacity. The next limit is the injectors because you will eventually run out of time to inject fuel between EVC and Spark. Larger injectors reduce the time they need to be open. The next limit is the pump again. A couple of cars out there got a special exhaust cam with a larger lobe to increase stroke of the GDI pump, so there's that option too.
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