Hunter of Trolls
Join Date: Nov 2012
Location: east coast
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there are several obvious factors- forward speed vs. engine speed ect.. there are some tucked away..
I know for a fact that the Fuel Rail Pressure Sensor (FRP) plays a heavy role.. I lost mine some years back while traveling a long run, and clocked 40+MPG in the next 300 miles or so.. and it was actually about 19ish iirc..
the FRP is singular with no back-up or redundancy capability.. the pressure in the rails determines how much fuel slips past the piezometric injectors and a known duty cycle/ pulse width, otherwise known as 'how long the injector gate is open (piezometric injectors shrink a crystal under power to allow fuel past as opposed to HEUI injectors used on the 7.3 and 6.0 which generate their own pressure via a tiny piston)... if you lose the FRP there is no other way for the computer to know how much fuel is being used with a certainty... if you lose the FRP the PCM dictates a weird unannounced 'limp' mode that disallows hard accelerations, and cruise control is gone.
THE way to make more power is to add fuel... the means to 'tune' a rig for more power is to use it's available functions to send more fuel... one of the ways that is accomplished is by altering the fuel pressure to engine speed ratio, ALONG WITH altering the pulse width of the injectors to allow more fuel past... when you tell a computer to forget its previous translation of (out of left field not accurate numbers, but for discussion) 24kpsi for 2400RPM and (x) throttle position and tell it to provide 27kpsi to that same 2400rpm and throttle position, whallah- more fuel.. more power.. but the files/tables on the computer concerning economy are still equating 24kpsi to 2400rpm and throttle position haven't been altered- and there is the disparity you see on your dash...
it also happens with rigs that aren't tuned.. those can be attributed to two things in my opinion.. one: unhealthy FRP operating at the top or bottom of it's parameter instead of in the middle (where they are calculated to work), and also due to regeneration function- the regen burns fuel unaccounted for in engine cycles and 'regular' circumstances, and it is a lot.
the FRP I KNOW beyond doubt plays a huge role.. the rest is conjecture, but the only way it makes sense to me.