Buying the triple clutch stall yank converter that warwagon is currently selling (or a new one) is a good possibility to help then. This is because the 16psi with the volume the ATT is pushing is a descent amount of air.
(If you are doing that on TTY head bolts and or high mileage stock head gaskets- that would scare me.)
So you are pushing enough air to until the performance tune which is then pushing more fuel than the turbo, heads, valves, exhaust system can flow. Having a higher stall forces the rpm up into the power band of the turbo much sooner. While this system doesn’t work for everyone like myself (wanting descent power and movement of the rig before 1100 rpm for off roading) if the bottom end rpm can be given up- it will help you here. The higher stall basically is a stop gap for a wastegate.
In the end, if the performance tune can push more fuel out the ds4 than the ATT turbo can supply, I would really begin to suspect head flow is also a limiting factor.
The tune can ease the fuel at bottom end and put a wonderful curve to it’s increase opposed to a db pump which is more a dump the bucket method. But when you outrun capacity on a ds4 pump- you left a lot on the table for a hot rod.
The egt is simply because you cant burn it fast enough in the cylinder and it is getting burned on its way out, or it is being pushed out the valve in such volume it hasn’t had time to dump its heat into the piston, head, and precup.
I remember talking to Heath years ago about 1100f egt and asking how far he felt it was ok for a stock built 6.5 to drive at that temp. His response was “To the moon.” I never panicked on it but was searching others opinions at the time. True the cooler the egt and combustion chamber, the easier it is on everything. But if you are playing with these boost numbers and that much fuel- you already know you aren’t being easy on anything- haha.
Seriously 1100,1200 going across the valve edge means you’ll need a valve job 10% sooner if you do it all the time. That increase in heat also beats up the edge of the turbo blades to cause sooner wear. The other part of concern is the crack spot between the valves.
All this is quite acceptable if you only do it once in a while. If you’re doing it all the time, properly built heads becomes much more important.
