my buddys turboed gsr, having problems when boost is engaged. It backfires, flames out the muffler but no gain in speed, just keeps spitting. Can't seem to run it full throttle 8krpm in 5th either. Could it be the gas? he's runing 91 octane but the car is tuned for 93. Blown turbo? White smoke when car is started cold.

 

Get it tuned.