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What causes the pcm to revert to the lower octane table?
Obviously if you run lower octane it should revert to the low table but how does the vehicle tell what octane you are running
What conditions are a deciding factor when it comes to choosing high/low tables?
i think it has to do with amount of KR being seen it averages the tables to some extent and if u have a lot of KR the car runs in low octane
Here is an explanation of the high and low spark tables , it is from the efi live site and has been around for a while . It was about the best explanation i could find but if there are any objections to it please let me know and i will remove it . I did remove a couple of lines but they weren’t about the spark tables.
theres a little more info the link belowQuote:
Posted by GMPX
Quote:
Hi all,
There appears to be alot of myths and confusion about the High and Low Octane tables in the LS1 PCM.
The PCM will ALWAYS try to run with the High Octane Table values, however if there is a MAF or Knock DTC set, it defaults to the Low Octane Table only.
But with a vehicle that has neither of those faults set it actually does a 'blend' of the two tables, it does not totally switch between one or the other.
Say for example the High Octane table has a value of 20 degrees, the same load point on the Low octane table has 10 degrees, and your octane multiplier is set at 50%, the final timing value would be 15degrees, if the octane multiplier is set to 75% it would result in 17.5 degrees, etc.
So what changes the Octane Multiplier value?, I have personally logged this on my car so I have seen it first hand. Basically any sustained amount of knock above about 2 or 3 degrees causes the Octane Multiplier to head towards 0%. It is a constantly changing value, it does NOT get reset to 100% when you fill up with fuel, the only thing that will bring it back to 100% and keep it there is minimal knock activity.
This has been tested on my own 2000 Commodore and emarkay's Camaro.
http://www.ls1tuning.com/iboard/index.php?showtopic=431
I watched a 2004 truck traverse most of the range the other night,
it appears to be driven by the degree and duration of knock retard.
High retard levels drive you downward faster than more modest
KR.