from what I remember, the highest safe numbers a stock bottom end rb25det would be 450-500, anything after that is a gamble because of the ring-landings. Max boost on a stock rb25 turbo is 14psi, but even at that the turbo's been known to blow because of the ceramic wheel's inability to withstand higher boost. keep a stock turbo at 12 to be safe. As for the aftermarket turbo... make sure you have all the proper supporting mods like injectors, engine management, etc. and have replaced the basic parts that came on the motor (i.e. water pump, belts, oil pump, etc.) before you upgrade the turbo.