Before you start tuining your PCM

[351CJ]

There have been several threads showing dyno pulls posted here.
One of the things several people have noted was how rich the A/F ratio on the GT engine is.
Here is something I found on streetrodstuff.com. It was writting by someone who worked on the GM Northstar engine, but what he says applies to any modern PCM controlled engine:

Consider the Northstar engine. If you do a full throttle 0-60 blast, the engine will likely run up to 6000 RPM at a 11.5:1 or 12:1 air fuel ratio. But under sustained load, at about 20 seconds, that air fuel ratio is richened up by the PCM to about 10:1. That is done to keep the spark plugs cool, as well as the piston crowns cool. That richness is necessary if you are running under continuous WOT load. A slight penalty in horsepower and fuel economy is the result. To get the maximum acceleration out of the engine, you can actually lean it out, but under full load, it has to go back to rich. Higher specific output engines are much more sensitive to pre-ignition damage because they are turning more RPM, they are generating a lot more heat and they are burning more fuel. Plugs have a tendency to get hot at that high specific output and reaction time to damage is minimal.

Sounds like the PCM on the 05 Mustang does the exact same thing under WOT (runs the A/F mixture richer).

It also confirms what some people who have tuned their PCM have found, that is you can get more HP by leaning out the WOT A/F mixture, but you are taking a risk by doing so.

It also implies that you have to take into consideration the time span of the dyno pull. That is does the Ford PCM intially give an 11.5 - 12 A/F mixure but after a certain time period richen up the A/F mixure to protect the engine?

This opens up a can of worms. If you are doing a >20 sec dyno pull where the PCM richens up the A/F and you lean it out, does it also lean out the intial WOT A/F mixure too much? Are the HP gains you see on the dyno from leaning out the A/F mixure a FALSE improvement because they only kick in after 15 or 20 seconds, so when you run a 1/4 mile you never will see this PCM A/F enrichment?