You can either turbo the 420a or use spray on the 420a. If your expecting to put a bolt on turbo kit and expect the same gain or more then your 91 TSI its not going to happen. The 420a engine internals are not forged so 5psi would be safe, 10psi varies. Anything higher then that your pushing it. If you want to run more boost safely you will need to rebuild the motor with forged internals and a few other fuel supporting mods (either Portfueler, Megasquirt, etc.). You will notice a difference with a turbo 420a. A few members on here that had/have turboed 420a's have beaten GST's. You won't be disappointed but don't expect so much so quickly because a 4g63t is already setup for turbo and you just keep upgrading rather then a 420a if you want to do it right you build it from the ground up.

You need to piece a nice kit together. Its cheaper then buying a kit. If your willing just to run 5-10psi to be safe on the stock motor then you can do that till it blows and build it to then run higher boost settings and have a better peace of mind.

420a's did not come with so much power out of the factory. So expect a hp range of 200-210 with just a bolt on turbo kit on the stock motor at 5psi(?). To read more about it visit... Hahn Racecraft - The Future in Performance...

Before you even plan on dishing out the money to turbo the 420a and buying a kit I would first have the compression checked to see if the engine compression is good and will withstand a bolt on turbo kit.