Falcon XR6T over boost with waste gate fully open!!!

Hi all, we have BA XR6T in our workshop with a strange issue,

It is standard apart from a big bore exhaust and has had an auto to manual conversion with a manual computer fitted.

When we run it on the dyno in 4th gear it over boosts and the injectors max out, we have disconnected the waste gate arm and wired it fully OPEN and it still makes around 14psi at 5000 rpm, we have tried two different big bore exhausts with no difference, we had the turbo sent out and a larger waste gate flap to be fitted but it still makes 12psi with the waste gate fully open, have checked ignition timing and “cam timing error” with a scan tool and all looks normal.

Can a big bore exhaust make that much difference on a standard engine? Or is there some other issue we are overlooking?

It is a new core in the turbo but have no idea how it ran with the old turbo as the customer purchased it with a blown turbo.

Thanks in advance for any help..

Bad port job. Was the port opened up and reshaped or just bored bigger and a bigger flap added?

What poundage actuator is fitted to it? Sounds like its an 11.9lb so you wont see anything lower than 12lb.