Timing is not the only thing that is adjusted. The MAF sensor sees increased flow when you go into boost. While it is not a MAP sensor you need more air to produce boost and the MAF can sense that increase. I agree that final adjustments need to be done but it is better to tune for 12 psi and ultimately decide to go back to 9psi than it is to tune for the opposite. You can safely drive on a 12psi tune running 9psi but not necessarily the other way around.

Regards

Jean Marc Chartier

 

Concur with SAFE, but not OPTIMAL