two big $EE questions

[steveo]

it's nearly 20 years since EE was released, and i've taken too long of a break from disassembly.

there are two questions i really want to answer this year as i start tuning yet another LT1

1. where does the spark variance in power enrichment mode come from, and can it be tuned. i have (and other people have) observed at least a few degrees of additional timing that can't be explained by other tables.

2. how exactly is the VE table used when calculating airflow in MAF mode. the table lookup isn't skipped when using a maf. setting the VE table to rediculous values such as *1.9 or *0.2 causes a giant swing in AFR, so it's definitely used somehow.

[RobertISaar]

the EE speed-density vs MAF thing has always been something I've been curious about but never investigated..... assuming I have a day off soon, i'll probably dig around and see what comes up.

as for PE advance, that should be fairly easy to find, assuming other spark modifiers have been found/added to the XDF.

[RobertISaar]

looking through the code for VE vs MAF now.

at 6196, the code tests for the "speed-density" bit, if it is set, the MAF calcs appear to be skipped. so, speed-density appears to be in effect at all times.

there's actually so much important stuff missing from the XDF.... and it's ridiculous to accurately identify what a bunch of 2D and a few 3D tables are doing that aren't defined. I'm trying to find what I assume to be a MAF vs speed-density blend factor, but I haven't come across it yet.

[steveo]

that ku4ro guy that was kicking around before was making progress towards understanding fueling calcs but im not sure how far he got.

the thing is the factor which VE affects fueling in MAF mode is kinda the unknown variable. a blend table would make sense.

there are some really odd looking tables that kinda scale like fuel would, but i've never been able to track anything backwards to anything i understand as part of fueling calculations.

[steveo]

... but on the upside, i finally have the stuff to build a bench testing setup, a spare ECM, and i've mastered a lot of memory i/o stuff. i have sockets on order so i can protect against flash failures. so i can do a lot more discovery from the aldl side really soon..

mode 4 in this ecm is great. you can offset timing on the fly. it's nearly as good as an emulator for definining timing tables while driving. you can also do straight fuel offsetting in closed loop by forcing trims to stick in the current blm cell, then adding or subtracting.

[RobertISaar]

a lot of the later PCMs with mode 4 are similar in capabilities. dig far enough and you'll likely find some bits that will allow you to shut off individual or combinations of injectors. interesting for diagnostic purposes, along with seeing what happens when you shut off a bank or alternately shut off cylinders in the normal firing order. I found out a 3100 will apparently idle somewhat happily on 3 cylinders.

we need to get a good collaboration thread going and demystify what is probably the second most complex OBD1 controller GM released.... the 93-95 northstar PCM still feels like it has even more craziness going on.

[steveo]

dig far enough and you'll likely find some bits that will allow you to shut off individual or combinations of injectors

yep; and wrote a routine to use it to do automated cylinder drop test and report a snapshot of each cylinder's map sensor reading to find non-firing or weak cylinders easily. definitely nice to have.

we need to get a good collaboration thread going and demystify what is probably the second most complex OBD1 controller GM released

i wish i could be a bit more helpful with the disassembly. i can write complex branching routines in my own programs but when it comes to reading existing assembly, i still struggle to understand more complex routines. the amount of work that goes into writing a full 'hack' of an ecm is staggering.