It's a relatively easy thing to do, of course actual firing order is determined by the plug wires, but the shutter wheel in the distributor also times the injectors at low speed. The "easy" way could be to rearrange the injector wiring harness for low speed, and adjust injector size to use the stock Ford programming, once you get to higher RPM injector firing order it doesn't matter as much
However, I did not do it that way, I run a rider chip made by Moates, it allows me to use a program called Binary Editor on a laptop to change anything I want inside the A9L. I can adjust any field Ford does. Quarterhorse is the access chip, but the right way to do it is build your program using a laptop and the Quarterhorse, then burn an F3 chip and put it in the A9L, then you don't have to worry about the battery in the Quarterhorse. (Of course, mine lasted 12 years, so it's not like that's a big danger, but I did recently swap the QH for a J3 chip which is a normal chip like we bought from Superchips, Hypertech etc in the 90s.
I changed firing order, injector timing, ignition timing, you name it, as well as change the values of some of the bolt on parts Some examples:modify injector slope and pulse duration, then injector timing to match cam events and firing order, timing initial, rate and total as well as modifiers for temp, vacuum etc, added a 102mm LS Mass Airflow Sensor and programmed accordingly, and shut off most the the fields that the A9L uses for emissions (decel fuel shut off, etc)
There is an incredible amount of control once you get inside the computer Additionally, I added a WB O2 sensor, although you cannot make the A9L use one, you can log up to 2 hours of driving through the chip, and then a second program, EEC Editor analyzes it with all the logged data and gives you recommendations to tweak it with the additional input of the WB O2.
Neat thing, you drive an hour, log it, analyze it in a parking lot, try the new load, if you don't like it, pull over and load the old one and voila, back to normal
The toughest thing is you can't get the program off the A9L, you have to get someone to build you your initial file, the F3 or QH bypasses the original chip and you need to load it.
I haven't done it, but I also believe you can access and control the Ford speed density computers and do the same, less a MAF of course, this would give you control without having to run a MAF sensor.
