firstly, see here for my latest conversion As you can see you need no engine mods and it all holds together, done 1000 miles now, no problems http://www.twin-turb...IN_20_HOURS.pdf
This used a T2 from an R5 GT turbo, this a great all round turbo for the 998, it won't boost efficiently over about 12psi, and will make a maximum of around 140bhp.
Other turbos I have used include the T3 of a metro and a GT17. The GT17 will make bags of power (160 or more I suspect) I have 155bhp and 1/4 mile times to prove it!. The T3 is a pile of junk, old technology, high inertia and enormous. The T3 will work and make 140 bhp easy, if not more but at the expense of driveability, you will not get boost below 4k, ok for track use, but little else.
Edit: I put 100 miles when it should have been 1000.
Edited by Wil_h, 15 September 2007 - 04:42 PM.