[Gregski]

Timing

First the High Octane & Low Octane table names to me are a bit of a misnomer, I look at them as Aggressive Timing table and Moderate Timing table. The PCM does not sense octane levels in our fuel, it uses something called the Knock Learn Factor to determine which timing table to depend on or a combination of the two. We will cover Knock Learn Factor later on.

GM considers 93 Octane to be High Octane. Well I live in Northern California and I can't seem to find 93 Octane at the pump, so what table am I using?

The PCM wants to run off of the High Octane table however it is capable of averaging out between the two tables when it senses knock so your timing may not precisely match the High Octane table nor the Low Octane table at any given time, and that's ok. Also if you put in cheap gas you may find yourself running off of the Low Octane table exclusively and even then may still have some knock.

We will address all that, but first we take a peak at these two tables. Please note the differences at the higher load (larger Cylinder Airmass numbers) and RPM levels aka bottom right hand corners of the tables, ha ha.