Originally Posted by TheShodan

The fact that you're at 7.49:1 hurts you a bit (it does a lot of vitara guys at least from my experience w/ the D-series turbo ninja) no matter what housing you put on there. It doesn't effect "spool times" as much [I can't stand that term, but I get ya.] But this means that you will require over 2.5PR (a bit over 20psi) to get to about 400whp regardless.

Remember, whether you're 7.5:1 or 10.0:1, the total amount of power of a given turbocharger is the same; the difference is where a given amount of boost is most effective to make that power. Say for instance one has a GT35R, and it takes 15psi to make 450whp at 9.0:1. On the lower compression car (say 8.5:1), it takes over 23psi to make the same 450whp. The turbo can do the power level, regardless, but with the lower compression, it takes much more boost to get there. If you have something that is a bit more effective in terms of WHEEL CHOICE that can work a bit better with your compression, you may help with response, but a turbo can only do so much, the rest is on your engine and its ability to combust that air /fuel. [good cylinder head, and possibly a "turbo" cam comes to mind).

Perfect real world example.... Question: why is it that DSM guys need lots of boost to do minimum power that Honda guys need. Well, the reason why DSM guys w/ 7.8:1 compression (1st gen) use over 20-25psi to get to 350whp on smaller turbos is because 1) they can take a lot of back pressure, so the higher amount of boost doesn't effect them in the way of heat and backpressure as much) and 2) they choose smaller turbochargers who compressor maps and characteristics are most effective in high pressure (over 20psi) situations unlike hondas. They also use camshaft profiles specifically to enhance the abilities of a small turbocharger. This is why you don't see too many B-series 9.0:1 + 16G turbo around, though the Greddy kits use them at low boost pressures.

On a side note, and I reiterate as Tony1 has once stated, that "turbo cams" are made for the purpose of cars that use smaller turbochargers that are most effective in low end torque and midrange power. NA cams have been shown time and time again to help with top end power on BIG turbochargers (over 70lbs/min) whose purpose are mainly are for maximum airflow input.

In your case, going with a GT3251b or similar size in a .63/Ar is best because you need to eliminate backpressure as much as possible with amount of boost needed to make that power goal and be efficient. Changing backhousings later is both a waste of time and money. After that, you have to change downpipes anyway, unless that e-bay one is 3"..