Man, this question is nearly impossible to answer correctly as the right response is "that depends". It's really up to the service manager who performs your work. Strictly speaking, Ford can void your warrenty if they really want no matter what.
For example, let's say you add a CAI. And you take your car in because you have trany problems. If Ford wants to be a prick, they could say that the trany failed because of the added hp from the CAI. Strictly speaking, if Ford claims that the mod resulted in the failure, you are not covered.
If I had just purchased a new '05 or '04 with a full blown factory warrenty, wanted to modify my car, BUT not risk voiding the warrenty, here is what I would do and not do as far as popular mods are concerned.
- Shifter (such as tri-ax) - Probably OK. But, if the part fails or it is installed incorrectly and you trash the trany, you are at fault. Fact is, though, an after-market shifter is safer than the stock one.
- Catback - Should not be a problem, besides it is such a common mod.
- X pipe - border line. If you remove the cats, then at least your entire emissions warrenty is void. You can always put the stock H-Pipe back on, but any prudent mechanic will be able to tell. If I was really concerned about warrenty, I would leave the stocker on.
- Under Drive Pullies - border line. Good thing is that if you go with Steeda, they look just like stock. Chances are they would not notice these. But, if you have alternator or belt problems and they catch this, they won't cover it. If I was REALLY concerned about warrenty, I would stay away.
- Gears - Definately a problem.
- Chip/Tuner - A chip will void just about your entire warrenty. They hate these. But a flasher such as a diablo or SCT you can get away with it. They will never know you used it. BUT, you MUST put your stock tune back on when going in for service. A prudent mechanic might be able to tell, if they look at your fuel trims closely. If it were me, I would get a flash tuner. If I needed warrenty work that required engine attention, I might be inclined to disconnect my battery to reset the fuel trims and drive the car for a while. If they say anything, I would just say I left my lights on and drained the battery.
- Suspensions mods: Depends. Subframes and strut tower supports should be no problem. Other things, could be a problem.
- Al Drive Shaft: Should not be a problem unless it breaks.
Well, those are my thoughts. Also, keep in mind, that Ford seems to be targeting the after-market a lot closer with the release of the '05s. One way around this might be to use FRPP parts and have them installed by a dealer. If a dealer were to install 4.10's for you, it might not void any warrenty. I would check with them first.
