is there any point in turning up a 3406B..........

If your making 27-28 psi it's already turned up. Stock there 19-21 psi cat spec. If your timing is good and everything else is sound, I would throw more air at it. Get your self a borg Warner 177148 turbo and enjoy.

 

Altitude versus oxygen. If your over fueling add more air via single turbo or compound turbos. Get a B or early C dialed in right and they will live and be pretty impressive.

 

Do remember gentlemen, that the b-model throttle controlled rpm, not fuel ( at least not directly.) It's why the throttle lock on a Cat could be used like a cruise.

 

Rank do you have actual service manual?

 

A 550 C15 is set to only run 26-28psi. It seems like cummins liked to run their engines at a little higher boost than other manufacturers, most of the 435-460 N14s run well over 30 wether they need to or not.

 

Sounds like you need to experiment to find a nice balance between power/smoke and economy. Do you get into Hamilton at all?