Dave, Thanks for that explanation. In light of other things I've learned in the past few days, it does make sense... and for the same reasons, I now know that any time I've tried that in the past, I've done it wrong.
Unfortunately for me, there is no BSFC for my engine... it is not stock, nor are the programs I run on it.
For example, my main "eco" program takes me down to about 140 HP / 250 ft-lbs (wheel), and my main "not eco" program takes me up to about 500 HP / 1000 ft-lbs (wheel). Everything on the intake/exhaust sides of the engine is not stock, and with extra air comes a need for extra discipline (or, the "nanny mode").
If I were to P&G on my "fun" program using 6th gear and the 75-95+% throttle position figures I've seen around here, it'd turn the skies black behind me... not something I like to do, because even though diesel smoke is black, we know it's really green... wasted money, that is. The "fun" program takes a different methodology for clean burning power than burying the skinny pedal, one must ease into it to "light" the turbo, then you can crank up the load. Obviously, this program isn't where I'm getting my higher number tanks.
On the eco program, though, there is very little rhyme or reason to its behavior. Keep in mind, this program wasn't designed to be eco, I just use it as my nanny to keep me within more efficient ranges of operation. The manual says "Use this program when you absolutely, positively cannot make any smoke." And it won't, no matter what I do. But, because they didn't tune the program to be for economy, per se, or for anything really other than "no smoke", its drive-ability suffers greatly. In order to rev out enough (500 rpm) so that I hit my target RPM on my next shift point, I have to progressively increase throttle position gradually from maybe 10% to 60% from 1500-2000. Timing is incredibly advanced, ensuring that all the fuel in the cylinder burns completely before exiting. Because the tuner made stupid decisions on the timing map, running *any* amount of load below 1500 causes the truck to complain, partly from timing knock and partly from gear rattle, both a result of too much advance at too low an RPM without enough throttle to warrant it.
My UG will be here sometime next week. It's my hope that in using it, I can better determine where the peak efficiency is either on the stock program or this "half power" nanny-mode, and perhaps then I can start to P&G my truck effectively. I would drop the nanny-mode entirely in favor of stock, but even then I manage to slip up and shoot 200-300F past my EGT targets, or 5-10 psi over my boost targets.
My long-term goal is to switch over to EFI Live so I gain total control over the engine's behavior, and the ability to tune the maps exactly where I need them to be for best eco running. With this tool, one has total access to the entire chain of Throttle Position commands X Fuel, X fuel @ Y RPM commands Z rail pressure, Z pressure @ X fuel commands A duration, and Y RPM @ A duration commands B advance. With creative planning, one can build the tables to not over-advance or over-pressurize at low loads and throttle positions and low rpm, enhancing drivability and smoothing out the overall engine performance and efficiency.
Don't get me wrong - the stock program does quite nicely by itself, but my engine will tend to fall a bit outside the BSFC given by cummins due to the hardware modifications, so now it's time to build the programs that will adapt engine running to those mods and find the efficiency from them.