Im running 15lbs on an all stock motor aside from a Starion intercooler and a suppliment injector that comes on at around 7psi for added fuel. How about u Z Ferrari? ...you running stock zxt injectors with 14psi?
There is a limit, but as said above, as long as you don't detonate...
'THe question reaally isn't "how much" but "how long with how much"?
Stockers will handle enough HP for scary rides, but if you try it for an extended ammount of time, anyhting is going to fail. Your limitation will be more poinst on the license or running out of road, than how much boost you can pump into the mill. Again, as long as you don't detonate...
I've seen turbo motors go south running 10# of boost, spectacularly melt-down the pistons altogether! Really more a detonation problem than a mechanical stress proposition if everything is running correctly.
I know folks running 25 psi on stock motor. You can run what you like as long as you cool the charge and are conservative with the timing. Detonation is what kills pistons, not power level. All the forgies will do for you is allow you to detonate for 2 seconds instead of 1/2 second.
My stock pistons see 18psi regularly. Last time at the track I was having boost control problems and the motor saw at least 25psi before detonation set in, but wow what a ride! I'm going to keep the boost below 20psi because I've talked to a couple of people who say that 20+ psi will break the skirts due to the stress. My current motor is all stock except for the HKS headgasket. I also cool the charge with a very effeicent Spearco intercooler and methanol injection. My advise is to start with stock pistons and learn from there. Good luck,