Had lunch today with a friend that consults to GM and EDS on some of the firmware. It appears that automobile manufactures, being the conservitive creatures that they are have some interesting limitations. Does 8 bit counters mean anything? 8 bits is 0xf in hex and is basicly 16. The problem with this is that it starts at 0 so the max decimal value is 15. One of those small counter locations stores and set the boost variable for the fuel and timing state machines. While the engine is still in vacuum my uderstanding is that it is set to 0x0 and has a inactive bit set. There is a seperate code path when the engine is running in vacuum. When the active bit is set high it uses another path ( another logic route through the state machine ) I also took away from the lunch conversation that there is not a lot of out of range checking on some of the state machines. Thier opinion was that if something broke and sends stuff out of range other conditions would catch them. In short, it might get a little hairy pushing the boost over 15psi... Just a guess but as my friend stated, the limit has to be somewhere...On a side note.... The tables can read 3 digits past the decimal point the state machine only uses the integer.
Mark
Oh yea... The boost/vacuum, timing, fuel pulse/rate, air flow, etc., are used all over the place. As stated, you would not believe which modules has use for some of the powertrain info.