Which computer chip mods work and which definately dont?

[Broken bones stink:)]

and what do they do to the engine to make it work better?

[Al Legend]

Chipping an "ecu" in my opinion is very much "Hype".
Yes it is scientific and exact in its "being" but for most auto enthusiasts there is a lot of "fluff" here.
It sounds, as they say in New England , "wicked awesome" to tell someone that you have a "chipped" ecu. But are they a "good" investment-questionable. And certainly they are only as good as the guy who "burned" the chip. And now it has to be installed ....
My point is - that for the money and reliability and flexibility - you should consider a stand alone "engine and fuel management" sytem.That if all you are going to do is change the intake and exhaust on your car - don't run the risk of chipping and the "downtime" of your car for a chip. If you are heavily modifying your vehicle go with a stand alone unit so you have total control over every aspect of fuel delivery and timing.
I will include a couple of websites that you might want to look at
moates.net
xenocron.com
Hope this helps!!!!!
Happy Motoring