what im trying to say is,if the turbo housing is 3 inch and the flange is of the 3inch down pipe,then there was no other way of making the adapter,the adapter has a shoulder of around 12mm at 3 inch i/d,then it goes to 3 1/2.forming a taper would make no differance to the flow at all,if the turbo had a bigger housing then i would of made it 3 1/2 inch from the turbo