Mach 1, Open loop just means that it isn't making adjustments based on feedback from O2 and other sensors. If you reset your computer and never run in closed loop mode, yes, it will run with preprogrammed settings. While it's in closed loop operation, however, the computer is determining the length of the injector pulses that go with the sensor signal feedback. It uses this information to build adjustment tables. These adjustment tables are then used at WOT to figure out how long to open the injectors to provide a fairly rich air/fuel ratio.
If your engine never gets warm enough, it will stay in open loop mode. That's why you should never get a thermostat below 180. Maybe that's why you see a difference with your pressure turned down. I find that hard to believe, though, since the leaner you run, the hotter the engine. Do you have a really good cooling setup? Aluminum radiator, water pump, etc.?
As far as running richer for better performance, the SVO extender will go to a maximum air/fuel ratio of 14:1. That's still on the rich side. I read an article in one of my magazines that referred to that as a piston melting ratio. A some of the sample cars in that article were running best with 10:1 ratios which is very rich.
Poopstang, this is where the benefit of adjustable FPRs is gained. If the amount of fuel being delivered by the computer is too much at WOT, then you drop the pressure so that the computer cannot deliver what it wants. Or, if your injectors can't deliver the amount of fuel that your engine needs, you can increase their overall capacity by bumping up the fuel pressure. I don't believe that you are in either situation so saving the money to buy something else is more beneficial.
I hope you at least have a 77mm MAF sensor to go with that 70mm TB.
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351W 89 Mustang GT Convertible