I've seen turbo Honda Civics making power from 3" exhaust post-turbo, so if a 250-300HP 1.6L engine can make power over a 2.5" setup, than a heavy truck with 250-300HP stock could definitely use 3.5" or 4" exhaust.

The 3" downpipe should be fine though.

 

So, if someone has a 3.5" DP and you would recommend a 4" exhaust off of that? Not 4" DP, and 3.5" exhaust?

 

you never want to run smaller pipe than what the outlet of the turbine is. so if its a 3" outlet run a 3"+ DP. It always helps to have the most free flowing exhaust you can after the turbo.

 

without the boost controller and the vac reference hooked up to the bottom port on the wastegate, it makes a 5psi in 1st and 6-7 through 3rd gear

 

Is that in the dp or atmosphere?

 

atmosphere. last time i tried it, it made about the same in the dp. i am only having issues over 10psi. anything under 10psi, and its perfect. im stumped.

 

Did you try 12+ psi with no controller?
Just swapping the spring out?

 

no i havent. i dont have any other springs for it. just whats in it now.

 

I think something weird is going on here. Someone already said it but it should be more consistent w higher boost setting.

 

Ok. How old is this kit?
My kits use precision gates. And they come with 5 springs.

After seeing what happening. I think you have a controller glitch here.