I am trying to understand how the computer comes up with the SA value and why its not what I expect it to be by adding up all the spark adders .

If I pick a point on my main spark table then find the same spot on the CTS spark table why then does my displayed spark advance not equal the sum of those two?

Lets say, just cause its what my log happens to be paused at while I type- I'm at 2000rpm, 50kpa, my main table has 25.3 in it. No AE or PE, up to temperature, CTS should be adding 10* but my displayed SA is 26.7. Why is the CTS spark missing 9*? What it subtracting the 9*?

I've got 2 theories, maybe their both wrong though.
1, the displayed value is 10* lower than actual advance because 10* is subtracted from the # for the main spark bias but even though its reading 26 its really at 36.
-or-
2, the PCM only needs to add 26* because I have my disty set (and the bin to match) at 10*.

The more I think about it and read the more confused I am.

Tuning Guide Book sticky, Chapter 5, '8746/'8063/'7747 section, SA Logic. Explains it all.

RBob.

You've got a winner with theory # 2!