You always hear about a "Stage 1 Turbo Kit" or more often a "Stage 2 or Stage 3 Clutch" Well what determines the stages?

I was told for clutches its the general WHP range, a Stage 3 would be good up to 300whp and under etc, etc, but as a sacrifice for a street car, the higher the Stage the more of a pain it is in traffic and parking lots etc.

But what about turbo kits, what determines a factory turbo kit, from a stage 1, to a stage 2 turbo kit?

reps for in depth correct answers