How Knock Sensors Affect Timing Advance (Nerd Warning)

A Little Background

Regular followers of my YouTube channel know that I recently diagnosed and repaired a faulty knock sensor on my Porsche 986... The knock sensors retard (slow down) timing for low octane fuel. This prevents knocking from occurring in the Boxster's high compression (11:1) engine. I assumed the engine would default to a retarded timing setting if it knew one knock sensor was bad. With this in mind, I logged data from my car with a faulty knock sensor, then collected more data once the knock sensor worked. Since this was all done on the street, the two runs are not identical, but they show a clear advance in timing with a working knock sensor.

The Nerdy Charts

A little messy, because 4 charts are stacked, but the green arrow indicates the advanced timing with the working knock sensor.

Ignition Timing with the good sensor (measured in milliseconds) 
Ignition Timing with the bad knock sensor (measured in milliseconds)
For those of you who have access to Photoshop, I have these and many other graphs loaded as layers in this PSD. The file must be downloaded and opened in Photoshop to view all the layers of nerdy charts. 


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