The different stages of turbo depends on what the track is like: If it's a high speed track like High Speed Ring, Tokyo etc then you want Stage 3 as that gives you the high end boost whereas a technical track such as Deep Forest, London etc, you will want Stage 1 for the initial acceleration out of corners. Stage 2 is almost a compromise between the 2 as it will give a boost to cars that accelerate quickly through the low revs but then lose that acceleration through the mid range so will give them the boost there