Up to 86.5, there is only room for one knock sensor, at the front. You'll need a used S4 plug to mount it. (
AFAIK, noone has found a part number for it new.)
I used a KnockLite for the display. I also made a automatic retard using the KnockLite's 5v low output.
After all this, I find that S3s are highly resistant to knocking.
