high/low octane spark tables.

[s10mods]

how does the PCM switch between the two. I know if there is too much knock it defaults to the low table, but how does it go back to the high table? does it know by when you re-fuel? I am wondering because I have a 6.0 in a 93 Yukon and I dont have the fuel gauge running to the pcm so it has no idea when I re fuel.

[BlackGMC]

It was explained to me that the computer always tries to run the High Octane table and if it senses knock then it goes to the low octane table.

[gmh308]

how does the PCM switch between the two. I know if there is too much knock it defaults to the low table, but how does it go back to the high table? does it know by when you re-fuel? I am wondering because I have a 6.0 in a 93 Yukon and I dont have the fuel gauge running to the pcm so it has no idea when I re fuel.

I recall this is explained very well in one of the EFILive manuals. From memory a GM LSX PCM will adaptively operate between or at the lo or hi octane tables. If it detects knock it pulls timing out, and then reapplies it until it detects knock again or gets to the hi limit (occurs in minutes?). If the tank of gas is lo octane, the PCM will tend to maintain spark at optimum for the gas situation by adapatively advancing spark while there are no knock events and backing off again when it gets one or more. Not sure if the re-try timing broadens over time so it stops trying to reapply advance so often.

Quite smart.

Cheers and have a nice xmas.

[Biggsy]

It scales between the two tables with a % number
ie 100% scaler will make it use the high octane table only
0% will make it use low octane table only
50% will use a value halfway between the high and low table figure
and so on...

When the PCM is reset, it will start at 100%, if it detects knocking, it will start lowering that number. If no knocking is detected for a while, it will start raising that number.

[s10mods]

Cool, I guess I am ok then, I wasnt sure if it used the fuel level to reset or not.