Definitely come here for questions concerning turboing the 420a. Most 4g63 guys haven't even seen our engines, and a lot of them have a chip on their shoulder (which some people on this board can easily take away). This is what I was told when I asked the club DSM which I am a member of, on their opinions between Star and HRC:

"Just sell the GS and buy a GS-T or GSX!
hahahahahaha."

So this is the place to come :)

You need to get a boost controller to keep the boost down, but I think the lowest you can go is 5psi. That is what I was told by a friend who has a T-25 on his TSi AWD. If I remember correctly, the T-25 has an internal wastegate, so that shouldn't be a problem. THE biggest concern you should have is your fuel system. Upgraded fuel pump, FPR, and make sure all your gauges are installed before testing the enging to make sure it responds correctly. If you have everything needed, I think 5psi is perfect. If the engine can take 5psi non-intercooled from a HRC super 16G, it should have no problem with a T-25.

Jeff

http://gs.dsmpower.com