Question About How Spark Timing Adjustments Work

[Salvin]

Still working on learning all of this and I want to clarify something that think I am beginning to understand. Please correct me if I am wrong.

The ECM uses the Knock sensor to adjust timing. Every time you flash the ECM it starts in the High Octane Table, as the ECM detects knock it retards the timing towards the Low Octane Table.

First Question: Does the ECM adjust the entire graph in a Linear manner or does it only adjust the timing in a section of the table that knock is detected in? For example, if knock is detected at High RPM does it retard the low RPM side of the table as well?

Second question: Let's say a given field has a timing advancement of 15 on the high octane table and 10 on the low octane table. My understanding is that if the ECM Detects knock at 15, it will then retard it to 14. If it does not detect knock at 14 it will eventually (based on recovery rates) increase it back to 15. If it detects knock at 14, then retards to 13, then retards eventually to 10 (low octane setting for the same field) if it keeps detecting knock. If it keeps detecting knock at 10 it is not able to retard any further correct?

My goal here is to tune my pickup to be able to safely tow a trailer on 87 octane if needed. But also be able to make efficient power on E85 for daily driving.

I have a couple different ideas to achieve this goal.

One idea is to tune the High Octane Table to not detect any knock on 87 octane then subtract a degree for safety. Then move the table to the Low Octane Table. I would then tune the High Octane Table for 93 octane.

My other idea is to simply leave the Low Octane Table stock and tune the high octane table for 93. I have read in other posts on here that it is generally not a good idea to attempt to tune based on KR for E85 so I would tune for 93 Octane and run E85 daily driving.

Thank you for any input and/or corrections to my thought process.

[KillboyPowerhead]

I believe this is how it works...

As knock is detected the knock learn factor will begin increasing from 0 toward 1. The knock learn factor is used to modify the spark advance by moving from the High Octane table to the Low table by taking the difference between the two tables for the cells in question (not the whole table) and multiplying by the knock learn factor, so if there is a difference of 10 between the two tables and the learn factor is 0.75, 2.5 degrees will be removed.

The knock learn factor will stick around for these cells and preemptively reduce timing as you hit these cells in the future, and as no knock is detected in these cells as time goes on the learn factor will begin decreasing back to 0. Flashing the ECU resets the learn factor to 0, so you'll start off in the High table as you mentioned.

The learn factor behavior (when it changes and by how much) can be manipulated with some tables like attack rate, recovery rate, enable tables, learn up, learn down, etc. There is also a Maxmimum Knock Retard table which I believe ultimately limits how much timing is reduced, so if the difference between the High and Low tables is 10, learn factor is 0.5, and Max Retard is 4 for the cells in question, timing will be reduced by a maximum of 4, not 5.

https://www.hptuners.eu/help/vcm_edi...rk_retard.htm#

Also search for other threads for Knock Learn Factor, High Low Octane, Maximum Knock Retard, etc.

[Salvin]

Thank you for the explanation. I understand it better now. I will research more with those key words.