Do anyone know how much rail pressure u can built in stock fuel rail, and how you can increase it Thanks for Your Help

wont a lift pump increase the pressure?

I believe you can build upwards of 24,000-25,000 before the pop off valve opens and releases it at which point it will rapidly drop. The only way to increase it is with either a fuel pressure box or tuning. As for the pop off valve, I'm not aware of one to raise the safety blow off.

I had my duramax up to 179 MPa (26,000 psi) for about 10 seconds before the relief valve opened. Now it opens within 2 seconds of reaching 179-180 MPa. I turned the rail pressures back to stock so I don't make it open any sooner than that!

The problem was, while commanding 174 MPa, it would spike to 180+ during defuel/shifting. I guess I should have run a bigger PW to drain the rail more ;)

I have never heard of anyone shimming a LB7 relief valve, but it might be worth a shot. Or see if a LBZ relief valve screws in? I'm kind of guessing here, but I also hope to fix this problem one day.

EDIT: Sorry, missed the part where you wanted to know how to increase it. It's pretty easy.

Increase desired rail pressure "{B1001} Engine Calibration, Fuel Pressure, Fuel Pressure Base"
You should probably only increase the areas where you are really getting on it, and leave the rest alone. i.e. >= 2000RPM and >= 70mm3.
Increase rail pressure limits "{B1011} Engine Calibration, Fuel Pressure, Rail Pressure Limits"
You should probably only increase the table below a fuel temp of 101 or so. That keeps the safety factor if your fuel gets REALLY hot.

That's it! Or at least that worked for me. Good luck!