Originally Posted by Hugh Keir
So on 4.5 and modified turbos you are boost / torque limited in the mid range compared to stage 5.
Stage 5 with upgraded turbos can go for a bit more boost to provide an additional 70/80 ftlbs mid range torque over 4.5.
BHP is +50 higher up the revs on Stage 5 when torque may not be quite such a limiting factor.
Stage 5 is just a higher bhp turbo and an intercooler. It does not include internals according to standard nomenclature. In short, you can't run any more torque with stage 5 than stage 4.5.
4.5 is a cheaper turbo halfway house. Beyond 5 is where rods and pistons change.