here's what I have personally found on mine. 4 works best all around except when you try to launch the car above 2500rpm. I found that with 3 washers I can hold the rpm to 2800 or so and launch the car. with 4 washers anything beyond 2500-2600 rpm and (this part is a guess as I am not familiar with turbocharged cars) I believe the wastegate opens and dumps the exhaust and lose immediate boost. With 3 washers it never happens to me, but with 4 it does it . Anyone else experienced this? (might be that 4 washers holds perhaps a hair more of boost and activates the wastegate whereas a stock or a 3 washer setup wouldnt. Just a guess.