Off the line, I can hit almost 40psi with just my EDGE and when im rolling around 70 or so and I punch it, the most i see is about 25psi. Why is that?

 

Don't know for certain, but I'm pretty sure your aftercooler is more efficient with a 70 mph wind blowing through it than with not even a breeze.....

 

does it have anything to do with de-fueling?

 

I don't know if I can explain this or not

Your engine is the resistance to the airflow from the turbo, low RPM = most resistance so the turbo builds its max pressure at WOT and lower rpm. At higher RPM more air is released through the engine so the turbo cannot build its max pressure as the resistance is lower. If you blow through a tube with a pinhole outlet(more resistance) you would build pressure, if the outlet is larger you don't get as much pressure as the air is released too fast.

Like I said, I don't know if that explains it or not

Cya

 

what can I do to fix it or is everything normal, so what would be a mod to make me faster off a roll at HWY speeds, bigger turbo, stack chips?

 

There are 2 major factors that go hand in hand when it comes to turbo boost, heat (EGT) and engine speed (RPM). If one of them is limited, you cannot hit max boost pressure. Volume would also be a factor if you start talking about different turbos, but not a factor in your question.

It's because your torque converter is not locked from a stop. You can get your rpm's up to redline immediately. A quick RPM and EGT increase will allow you to hit 40psi.

At 70mph your TC is locked. When you hit the pedal hard at 70mph, it is the EGT increase that primarily creates more turbo pressure, because the rpm's of the engine do not spike like they would with the TC unlocked. That is what is limiting your boost pressure at rolling speeds above 50mph.

It is normal. I'm sure there are different turbo's that can affect the actual PSI, but as long as the TC is locked, heat is the major factor in turbo boost pressure.

 

Not so much defueling as just the fuel map. Highest boost occurs at max rpm's. Thats how the fuel maps work. TC locked and you won't get enough rpm to call for enough fuel to build max boost.

 

Not to mention the truck is already rolling so not as much resistance.

 

Yeah, I'd say in this scenario the main difference is the load on the drivetrain.