On a gas engine, if you are under boost your adding fuel because you need more fuel to maintain desired afr. Of course there is always a trade off between efficiency of the engine, clearly making more power under boost due to better VE. Everything is a little different with diesel. Diesels control engine speed and power output with fuel only, and as such lean is actually cooler so no potentially damaging heat related effects from lean.
Diesels with VGT, specifically a 6.0L in my case, has about 7lbs of boost driving down the road at 70mph or more depending on your VGT controls. How does boost effect fuel mileage driving down the road? On diesel, I would assume 30mg mass fuel at 7lbs of boost to maintain 70mph would be harder and less efficient than 30mg mass fuel at 3lbs of boost to maintain 70mph, just due to backpressure alone. I understand the advantage of more boost under cruise so that it will build boost quicker if you were to say go WOT, but would less boost and less backpressure be more beneficial for MPG? These numbers are just examples.
I know that EBP is directly effected by injection timing, and so is MPG. Most gas engines don't have VGT turbos. It would seem that opening the turbo some under cruise would reduce pumping losses. Since the diesel is naturally lean at cruise, the loss of VE from less boost should be neglible.
Hoping some advanced tuner could help bring this into perspective for me.