what would happen if i replaced the turbo on a stock s4 turbo with a larger one and fabricated a manifold for it and did not change anything else??

 

Probably lean out and blow up when you hit boost.

 

why would you bother?

 

The engine would blow.

 

More Air + More Fuel + Timing = More HP

That is the equation your are aiming at yes? But you are missing two VERY important variables: FUEL AND TIMING!

You can't just crank up the boost and expect everything to be ok. Modifying a car safely and correctly is expensive and takes time. You need bigger injectors to keep the air/fuel ratio within safe limits to prevent knocking. You need a computer that can deal with the extra fuel correctly and properly advance the timing beyond 8psi. You need to do more research.

 

^^ This is true. OR you can retard the ignition all the way back 3 degrees manually, and 1 degree for every 2 psi. (After 8 psi, that is) If you don't have the money to spend on a ems. Also you do need injectors regardless.

But in short, boom boom.