If your statement 12:1 is refering to compresson ratio, you are absolutely wrong. When forced induction is applied to (gas) internal combustion engines, you must lower the compression to accomodate the increase of air and fuel. He is right in that you must determine what your goals are, do you want low end torque or upper end power. This will determine how you set the turbo up in terms of exhaust turbine house size. Generally speaking a .63 A/R housing is best for low end torque and a .82 A/R is best for upper end torque.
Again, Good Luck
Originally Posted by Big Evil
T3/T4 dosen't mean a whole lot, what does the turbo map look like and where do you want it to "hit" in the power band? How much boost do you plan to run and what is your end game effective compression ratio? Do you plan on modifying the engine to handle more than 12:1? You WILL have to run 91 or better octane fuel. Go here http://www.turbobygarrett.com/
and under "Turbo Tech" start with turbo 101. You may be in for more than you're bargaining for, maybe not. But get the info from the guys who develop and make the stuff, and good luck.