I like the turbosmart line of wastegates. The 45mm hyper gate or 50mm pro gate would be good selections. If you're not going to be working the turbo hard id use the 50. If you are working it hard id use the 45mm. The XFI will be able to be used to control boost. You need to determine your strategy as there is a few different things you can do to control the gate. The ultimate would be using an electronic controller with a light spring and using an external source of pressure like pressurized CO2. This way you can have control to very low pressures with light springs and create nice ramp in profiles for street use and more aggressive for better prepped surfaces with total control.
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