[v8volvo]

Do not try to change the boost pressure setting without first installing a boost gauge inside the car. You need to be able to see the effects of the changes you are making. You can quickly destroy your engine if you mess with the turbo without a gauge to tell you what is going on under the hood.

The upper hose is the boost reference signal to the wastegate, to tell it when to open and limit charge air pressure. The lower hose is just an oil drain from the turbo inlet; fooling with it won't have any effect other than maybe making a mess. If you remove or crimp the upper (boost reference) hose, it will NOT just go to 12 psi like Cuaz said, so do NOT try that..... when I was experimenting with my 760, removing that line from the wastegate resulted in boost spikes of 20+ PSI in second gear, which is enough to do major damage. This is on a car with GTD spec Monark nozzles and an open exhaust which is part of why it went so high so fast. But you don't want to gamble like that. Make sure you have a gauge and a way to carefully control how much you restrict the boost signal to the wastegate. Do not just take the line off unless you want to blow up your motor quickly.

Stock boost level is ~10.5 psi. I am running 14 now and it made it a little faster, but to get real gains you need an intercooler. Just turning up the boost without other modifications (intercooler, airflow, exhaust, fuel) will not get you all that much more power, and will be hard on the engine. You don't really want to go much above the stock level without an intercooler, and an EGT gauge would be a good idea as well.