Boost on stock top end?
 

I have a 2003 GT, Auto swapped 4r70w, mmr built bottom end, and on3 turbo kit. Wondering how much boost I can run safely on top end. Or hp numbers. I want to run 18-20psi at some point this year. I know the trans, rear end, bottoms end, and fuel system will handle it, just not sure on a stock top end. I have replaced the intake with a track heat intake from trick flow. Heads are 100% stock. Of course I'm gonna need cams just not sure if I'll need the heads worked on or have to replace them.