Pete,
My experience with other L-Jet cars indicates that you are on the right track with your questions/comments. The typical range of adjustment for the O2 sensor is pretty narrow, and I suspect that it is similar (or even identical) across the range of L-Jet installations.
As I have stated here several times before, tweaking the AFM simply adds fuel relative to the position of the flap in the AFM, but it does nothing to address the mixture at WOT. So from that perspective I agree with Gregory that altering the AFM in an attempt to compensate for engine mods is a half measure at best.
I agree with you that the easiest way to substantially change the mixture on the stock FI system is through the use of an adjustable FPR. This is pretty commonly done on a variety of cars, even today.
Gregory, I don't share your concerns over the use of higher fuel pressure to compensate for minor to moderate engine mods. The system is speced to operate at and beyond the pressure that the pump is capable of delivering. FPRs fail on occasion, delivering full pressure to the injectors-- the engine runs like carp but it doesn't exactly hemorrhage fuel. Again, higher fuel pressure is commonly used to compensate for other mods and it works just fine. I do share your concerns about going to larger injectors (unless truly warranted), which is why I recommend higher fuel pressure.
I also do not understand your logic regarding the use of an O2 sensor in conjunction with injection mods like an adjustable FPR. I think we all agree that the range of adjustment offered by the O2 sensor is small, so it obviously cannot compensate for a large deviation in mixture (assuming the ECU is even operating in closed loop mode). But that doesn't mean the O2 sensor is not still useful...
Let's use the mild interim engine I'm building for my '85 as an example-- I'm raising the compression, polishing the intake and ports (the *inside* of the intake and ports, Bob), installing 37.5 mm valves, changing cams, and installing a header and free-flow exhaust system. The engine will flow more air, and will therefore need more fuel. I intend to use an adjustable FPR, as I think it'll be sufficient to compensate for these mods until I can upgrade the EFI itself.
Now, let's say I raise the fuel pressure enough to provide a slightly richer mixture than stoichiometric. So maybe the engine will be closer to the 12:1 A/F ratio that's desirable for max power. But in cruise situations when the ECU is in closed loop mode it'll lean the mixture out a bit, getting it back closer to stoichiometric. Will the L-Jet ECU take a 12:1 ratio all the way to 14.7:1 based on the O2 sensor data? I doubt it'd get anywhere close, but in that case every little bit helps...
Note that my engine will not have a catalytic converter, but it will have an adjustable FPR and an O2 sensor. Fuel mixture would be much more critical if a catalyst was planned, in order to avoid overheating the converter with an overly rich mixture. But that doesn't mean the O2 sensor data won't be useful, even if it won't fully compensate for an major variation in mixture.
Think about it this way-- let's say your X overheats in stop and go traffic unless it has two electric fans fitted. Does that mean that a single fan is useless? Heck no-- it may not be moving sufficient air but it's keeping the engine from running even hotter than it would with no fan at all. Same thing with an O2 sensor-- the ECU may not use its data to compensate for a major mixture anomaly, but it's better than nuthin'.
Cheers,
///Mike