I looked through the OBD Sys Op Summary for the 6.4 and didn't see any logic for checking a max EBP value that would set a trouble code. Just comparisons between EBP and manifold pressure ... deltas. Typically, I would expect the ECM to open the EGR or adjust the Turbo to adjust exhaust back pressure. Not much of a help though.

 

No, I do not. The EGR system is still on it, but it's suppose to be shut off with the H&S Tunes.

 

40 PSI EBP is nothing to worry about. The pcm reads the total atmospheric + Barometric pressure. Barometric pressure at sea level is 14.7 so you need to subtract that from the pcm reading. On a 6.4 you really do not want to see more then about 82-84.5 PSI EBP from the pcm which would be around 68-70 PSI actual exhaust back pressure, at 70 PSI actual there is enough pressure to potential start to push the valves open and cause piston to valve contact. This can be corrected with upgraded valve springs or installing a exhaust waste-gate to kept the EBP in check.

In a perfect world we want to see a 1 : 1 ratio on boost vs Exhaust back pressure. Un fortunately the 6.4 can produce as much as a 2 : 1 ratio resulting in high exhaust back pressure. Typically your gonna see the EBP about 1 : 1.5 ratio higher then your boost pressure.

The stock turbos are efficient at providing boost to about 40-42 PSI they will provide boost beyond that but the turbo will be operating outside its efficiency and as a result EGTS will go up and turbo life is reduced due to the turbo being over-speed.

Some people are stubborn and just think more boost = more power, I have worked on a 6.4 that was making 45 PSI boost and we put a wastegate on it and adjusted the boost to 39 and the truck made more power and ran about 150 degree cooler EGT's. More boost does make power as long as you have the right fuel and the right turbo set-up thats efficient for your HP goal.

So to directly answer your question anything less then 74.7 read by the PCM would be OK, that gives you a good comfort zone as your actual would be less then 60.