The point of tuning the a/f ratio to specific point, say 13:1, is so the computer always strives to met that point.
I don't understand why you needed to retune the car after the bolt on plenum. I think that given enough drive cycles over time, the car would have eventually worked the a/f ratio back up to 13:1. That's the point of burning a chip and setting a specific a/f ratio. It's not like you added a blower.
Sure, it ran rich at first because the plenum was allowing more air into the engine and the computer compensated and dumped more fuel than it needed (rich) in the mixture. I believe this is a safety reaction of the computer so the car doesn't run too lean. Once it has time to slowly fine tune the a/f mixture, it eventually backs off the fuel to get back to the programmed 13:1 ratio.
Does this sound logical?
You may ask "why does anyone tune the car at all then?". Well, the stock a/f ratio is more than likely set up very conservative (i.e. rich) so Ford doesn't have to worry about owners complaining about pinging engines. Think about it, it's easier for Ford to just have our cars run rich than to set them up lean from the factory. These cars are made to drive in any conditions, anywhere in the world. A factory lean set-up wouldn't work too well in some areas in some conditions. The safe route is a rich setting from the factory.
We tune our cars because we know that leaner = power. We fine tune them according to our mods. on a dyno so we can squeeze out the most possible power with the a/f ratio.
