 Money/horsepower/NA
Blue has it right. The Mustangs run motors too small to win the NA wars with the GM boys. That's why power adders are so popular with ALL performance guys who run cars with small motors.
The other problem is the budget. Building a na motor that can deliver more power costs just as much (if not more) than achieving a lot MORE power with power-adders. Its just he way the physics and technology work together.
OK, the usual routine to add some guts to the 4.6 works for the Mach 1 just like the GT...
1. Intake. Not a good idea with the Mach One - for obvious shaker/oriented reasons. There are few good choices open to the 4.6 stangs anyway! Some folks are port-matching their intakes - and my comment would be that for a NA motor, every little bit helps.
2. Heads. Porting and polishing the heads always yields some power, though your DOHC design is already breathing pretty easy. You can expect maybe 30 hp from reworked heads. Expect to spend about $1700, plus exchange, plus head gaskets, plus shipping, and another $1100 for labor if you can't do it yourself.
3. Cams. Lots of good upgrades available for cams, but if you want to keep it idling smooth and streetable (or just streetable), there's a limit to what you can do. If you go much beyond mild to medium head specs, you also need to swap in some stronger springs and retainers. Figure $1300, maybe more, and just a little labor since you ARE doing this when you have the heads already out for porting, right?
4. Headers. The pick of the litter are the long tube headers, which will net you 10-20 more hp. Get the ceramic coated versions, and keep in mind that installing them is a cast iron witch (the 2005's and the Fox bodies have LOADS more space for headers than we do). Oh, and that mid-pipe you have is almost certainly the wrong size for the new headers. Figure $900 installed, plus $500 for the new X-Pipe w cats (deduct $200 to run naked of cats).
5. Underdrive pulleys. Sort of a given for the NA set. You can squeeze out as much as 10-12 hp from this. Not an expensive mod - call it $170 if you do the install yourself.
Now come the expensive parts...
6. Bottom end. There's no replacement for displacement, particularly with an N/A philosophy. With that new low mileage motor, I doubt you want to consider a stroker or over-bored and sleeved increase in displacement - particularly with the price being pretty steep...
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