say i built a 400+ hp motor...would it run without a new PROM? or will the PROM just tune it to max effeciency?

 

A stock computer will only go so far (and not very far) on a stock engine. Change one thing, like injectors (which you'll have to do to support 400 horsepower) and it'll run terribly. Put a cam in there (again to get 400 hp), and it gets worse, if it'll run at all. It's not exactly easy to get (learn to get) higher hp engines tuned, even with good equipment, but it's a fun challenge. I can't recommend using an aftermarket chip, as they generally are done poorly, and cannot cannot cannot get it right unless you give them your car to tune (and then it's still probably going to have slight/severe issues). Study up on prom tuning here and elsewhere, then, buy some DIY prom tuning stuff, and please please please get a wideband (too many things will require guessing without it, and guesses are usually wrong, and when wrong, people come here to have other people guess, and then an argument will break out in defense of opinions (j/k)). Last I checked, an LM1 from Innovate Motorsports was $350, and it's great for this application. My only advice here is just get it before you start, and don't look back.
And if you follow my advice: Welcome to the board! You'll be here a lot!

 

1. At what point do you need a new PROM?
Need is a relatively subjective word in this context. When would having a custom tuned chip tailored for your exact combination be of benefit? To be honest, at all points. That is even a stock thirdgen with no bolt on parts. Never mind one with lots of modifications, aftermarket cam, heads, exhaust, intake etc.

2. Would it run without a new PROM?
That depends. Theres info on this board about various tips to try in the tune to try and get a new combo to work. If you are doing upgrades in stages it is obviously best to tune in stages and progressively update.

3. Should you buy a wideband
In my opinion yes, but that's just my opinion, and lots of people have great success without them. Or by building their own DIY WB O2.

It's not exactly easy...... but after loads of reading and rereading, and unfortunately reading even further, you should get there eventually.

4. Will you be glad you got the stuff to do it yourself?
Absolutely! You'll learn a lot on the way and if you do it right you'll end up with a more powerful, efficient, 'smoother' engine.

 

Seems like lots of people on the boards are having some success with doing exhaust and/or heads. But it seems like anyone who does a cam change needs chip tuning work.

Just an observation. And one of personal experience.

 

The only thing is though, is that he's running SD rather than MAF so he MAY run into problems earlier.

 

AZ is running SD.

I'd have to concur on the mods mentioned above, you can get away with basic intake and exhuast mods, but once you start drastically changing the way the engine works with cams, and heads, then you HAVE to change the tune. I suggest you get into it right now, and learn with a stock motor, it's worth a tenth and a mpg at a bare minimum, and probably much more, then you'll be all set for the big nasty motor. You'll learn more about how an engine runs by DIY chip tuning than any other way. I used to think I pretty much had this internal combustion engine thing down pat, then I got into chip tuning and it blew my mind.

 

Ever since I've started doing the tune myself, I haven't been in a car I didn't want to make a change to! There's usually something that could use improvement.
Once you see what can be done and see some rewards from making that first small change to your tune, you'll be hooked like the rest of us.
You won't even think about looking into using a stock chip again.