I was doing some datalogging up at Flagstaff this weekend and I specifically wanted to track down a possible source of why some folks report bad MPG where they live up high. And I think i found something.
It is very pronounced. I will look more closely at the logs later but for now,
The rail pressure drops significantly at cruise settings. Why, from what I can tell, the PCM is commanding it per a barometric schedule. What does this mean? When rail pressure is commanded lower, this act increases pulse width, or the time duration that the injector remains open. This is very important because injection timing (which does NOT change) is suppose to align with pulse width. So if timing was optimum, and then a change occurs to increase pulse width with lower pressure, then timing is now non-optimum, in fact too retarded.
With EFI, you can eliminate the baro effect on rail pressure, but unfortunately I did not have the time, my days were full with retreat activities. But if you have EFI and would like to eliminate this, it is easy to do, and maybe you can report back the results on economy.
Timing is important, and there is a lot of room to advance it at cruise, particularly at altitude. So hopefully this nute is at least partially cracked.
Vehicle Systems Engineer/Professional Tuner
04.5 GMC 2500HD SRW LLY
Learn about INDUCTION OVERHEATING
ECM's FOR SALE