Sorry to confuse you.
Its the point I have been trying to make from the get go, that PCM makes injection timing whatever it wants regardless of base timing. But then someone again comes in saying Im completely wrong, I dont understand, and then ends up saying it himself.
The PCM sets injection timing to desired programmed timing with the stepper motor regardless of base timing. So to get this extra performance at the top end you need to see actual 8.5* when the desired is 0* in time set.
The parallel with the vacuum advance unplugged is what I was paralleling to stepper motor being disabled, to where it can't change the timing. Its not entirely accurate, I apologize for saying it that way.
You had stated you can get -1.94 TDCO when at 3.8* base timing by revving engine a little and thats what you would do, but that is not the same as -1.94 TDCO at 8.5* base timing. So I wanted someone to explain the difference, which is the max advance ceiling.
So I asked the questions to have it come from gmctd instead of me saying it and someone poking holes in the wording.
This is where confusion is coming from to me it looks like you don't have it quite right here or a least it reads that way to me that you don't have it quite right, base timing of 3.5 does have an impact along the entire operating range.
I'll try to splain it as I see it, JD correct where I get too far off base.
Let go back to my orignal gasser analogy, piston engine basics, step 1 cam and crank have to be positioned via chain/gears so that they match at TDC, distributor rotor in a gasser has to be positioned so that at TDC rotor is pointing at plug number one, if too far BTDC or ATDC vehicle fire will not happen corrctly.
Same thing with the 6.5 EFI Diesel IP has to be positioned in a way so that the injection for injector 1 is at TDC or pretty close to it, in time set you disable the timing control feature if the PCM so it does not try to correct a too far off TDC timing event, and you are only reading the position feedback of the IP,
If too far retarded in time set the engine will stumble and die, if too far advanced truck may also die but usually runs smoky & rough. Base time has to be established 1st so that when in PCM control the full timing window of operation is there.
So for the IP in normal control may be electronically commanding a actual/desired of 8.5, physically IP timing will be offset by mis-adjustement by mechanical position from TDC, PCM has no idea of mechanically where the IP is sitting on the engine in relation to cam/crank, that has to be established by setting that relationship by moving the IP.
Think of a optic bump moved too far away inside the IP, IP cam ring vs optic sensor out of whak and truck timing gets jacked up.
So similar again to the distributored gasser, where you unplug the PCM spark advance, or vac advance so there is no spark advance that will add extra spark to try keep the engine running if the engine tries to cut out. The 6.5 EFIs PCM will also try to adjust timing to keep a laboring engine running smoothly.
When setting base timing you don't want that input, you want just base timing so that distributor or IP are calibrated to basic running at TDC squirting or firing the #1 cylinder, once that mechanical position is established/satisfied you can come out of time set for the 6.5 or enable vac advance/PCM spark advance in the gasser, then let the computer take over timing.
With the 6.5 control you are regulating a pump (IP) that sucks, squeezes, holds. releases as a "timed" event, if mechanical base timing is wrong performance suffers, or emissions suffers so TDC without PCM bias has to be correct 1st, PCM can not correct for far out of adjustment, yes engine can run, but further up in rpm one goes further out of whack it goes with timing vs fuel vs rpm schedule, resulting in stumble-or overfuel general or general driveability suffers.
