Catless downpipes remove back pressure and allow the turbo to spin more freely, which generates more heat. More heat requires MORE lubrication to cool it down. More specifically, FRESH lubrication. If the hot oil isn’t evacuating the turbo properly via the drain line, it stays in the journal and gets even hotter and loses its viscosity, burns up and cokes up the turbo. Also, because the oil isn’t getting out, it pushes past the seals and burns off in the exhaust, causing the “smoky turbo” problem. Adding restrictor bolts to this problem may solve the symptom by reducing or even stopping the smoke because less oil is pushing past the seals, but it makes the problem of heat and poor lubrication coking up the turbo even worse, as you now have too little fresh oil entering, which will just cause it to fail even faster.
