I agree about the weather conditions, dyno calibrations, gearing and even the dyno operator causing variations. But that is not the total picture.
This stuff here, is absolute fact.
Not every engine built by the same manufacturer are 100% identical, not even on the same line, not even on the same day, and not even built during the same hour.
This is also true.
It is farily common knowedge that all manufacturers do, in fact, underrate their motors so that the absolutely WORST examples produced will still make the bare minimum rating if they were hooked to an engine dyno.
Now this...this is pure speculation. There is absolutely no evidence that ALL manufacturers under rate their motors. That's a completely false statement and I have three pieces of information to refute that.
A) The Mazda RX-8 was underrated by the factory
B) I actually did the factory tour at Porsche and when we got to the engine assembly room, they mentioned that every motor (at least for the 911) is dynoed after assembly to make sure the output is within a certain percentage of error--and I want to say that it was 1% error.
C) This one is going way back, but I even seem to recall some controversy with some older Mustangs, maybe a SVT version, where ford advertised a certain hp and supposedly it was underrated. I believe ford made updates the following year, IIRC, to the heads to make up for the power.
So no, there is absolutely no evidence that over rating engine output is a standard practice. The track times pretty much confirm this fact with regard to the current mustang. No one is legitimately making 430+ hp in a stock mustang GT. There isn't enough variability in the production process to add an additional 30 hp. The ECU also plays a role in keeping things within spec.
Will many hit 420 to 430? Yes, absolutely. But don't be pissed if yours doesn't, as long as it makes the minimum 412.
You're talking out your rear, sir. You have absolutely no legitmate evidence of this. Find me an engine dyno of a stock 5.0 making more than 420 hp and I will absolutely change my opinion on the topic. But, some dude posting up chassis dyno results isn't legitimate evidence and anyone who knows anything about dyno tuning will tell you this. There are significant differences even between dynos. DynoJet and Mustang dynos are notorious for being 10+ hp off from each other. There are simply way too many variables associated with a chassis dyno to come to any legit conclusions about what Ford is doing with their motors. I mean I could potentially even see your side of things if every chassis dyno was even a little bit above the rated hp, but even that's not true. People dyno all over the place in reality. Even the calculation of whp to crank hp is whacky. We see it here with one guy claiming 15% DT loss and someone else saying it's like 8%. That's a HUGE difference and that difference is merely an estimate, not a well defined calculation. So I'll say it again--either you provide an engine dyno and put the argument to rest, or you stick with the chassis dyno data and not prove or disprove anything.