Im currently running an evo IX turbo with FP 84mm comp cover and getting ready to switch to SD and running 93 and E85, but unsure if I should run a 3", 3.5" or 4" intake setup.
im also running hks280's if that makes a difference and might possibly be getting an fp red or black BB later in the future

 

I would go with a 3.5" or 4" pipe since 84mm=3.3"

Most people go with a 4" pipe.

 

Ideally, you want to lower the pre turbo restriction as much as possible, which helps your turbo efficiency. Its like creating a bigger head volume or pressure, im no physicist but you get it... more atmosphere to find its way to your compressor wheel!

 

Thanks 4" it is then