"The question = Why can't Ford make more power with the 4.6 when the 3.9 has 280?"
Because it would cost more money. Also, as a Muscle car, Ford is concentrating on torque, not HP.
"In no way, shape, or form did I ever suggest, theorize, hint at, or demand that the 3.9 should go into a Mustang. I actually went back and re-read my own post just to make sure that I didn't do such a thing, and I can see nowhere that anything I said could have been taken in that context."
I didn't take it in that context bro.
"I guess I am trying be nice when what I really mean is ... please do not quote me if you do not understand what it is I am saying."
I am sorry about that. However, I didn't know I misunderstood you when I quoted you! I also didn't think you meant that the 3.9 should go into the Mustang. What I meant was, Mustang owners don't care about HP/liter--unless they are comparing their car to a Camaro or something which makes more HP and gets better fuel economy
. Muscle car owners, if anything, would rather prefer the higher displacement, lower HP/liter car because that car is easier to modify.
However, rereading my intial post, my tone was rude, and that was uncalled for. So I do admit my error in that regard.
To answer your question regarding mileage, the Mustang will probably get better mileage in 2007 when the engine gets upgraded. However, now it has been tuned very conservatively (and thus runs a little rich) to be on the safe side. Basically, the engine is a new design for 2005 and Ford wanted to play it safe. Because of that, they let it run rich but in a few years, when they are better able to gauge the durability of this new design, MPG should improve a bit.
Of course, there are other issues such as the gearing and the low-grade fuel; these issues will still contribute to poor gas mileage in 2007. However, there are things that Ford will improve upon with time.
I didn't think you suggested that the 3.9L engine should go into the Mustang. I just meant to say that HP/liter is pretty meaningless. I mean, look at the 2005 Corvette. It has 400 HP and 6.0L, but I have read a few reviews that suggest it is a mid 20s car when driven mostly on the highway. Now the Mazda RX-8 is 1.3L but it is an 18 MPG car like the Mustang. The S2000 is a light car with a 2.2L engine but it sucks gas like a 400 HP Corvette. The basic point is that when you get that much HP out of so few liters, the gas mileage isn't going to be great anyway. Even though 1.3 or 2.2 liters don't suck gas, 9,000 revs do. (Especially compared to the Mustang's 6,000 revs.)
If the Mustang had different gearing, it would be a better fuel economy. However, Ford must have been hellbent on getting a 5 second 0-60. Because of that they tuned it way for the low end; as such, both fuel economy and top end acceleration suffer.