mark944turbo

... but only 1 is used. Does anyone have definite info on why the second one exists?

Factory experimentation? Did the location change for certain years? Which is recommended?

hosrom_951

From what i understand, firstly the knowck sensor is not on the cylinder head, it's on the block. Secondly, where did you find the second knock sensor? did you see one in the car ? there is one that i know of

mark944turbo

My fault, I thought it was on the head because you have to remove it to remove the head. I was mixed up.

I do not mean that there are 2 knock sensors, just two possible holes with the right thread next to each other, that the sensor could be mounted in. I am wondering if this was the factory figuring out which hole was best and never bothering to delete the second hole for production blocks. I think that is too bad if they only tried 2 locations.

Tom Carson

the 16v engines have 2 knock sensors and a 'hall effect' sender on the cam gear letting the DME pinpoint which cylinder(s) need spark retarded

cruise98

The second one is the mounting hole for the idle stabilizer.

mark944turbo

Oh, cool

Bengt Sweden

Any hole in the block would probably do since it's the frequency that count's and the amplitude is probably sufficient anywhere.
Bengt

mark944turbo

Except all good manufacturers try many locations to find the ideal one for minumum noise max signal. The valvetrain making the same frequencies as knock sometimes. The 951 has individual cylinder tuning capability when it comes to knock. So location is pretty critical for this I would imagine.

Bengt Sweden

I belive finding the individual cylinder is done by timing as Tom says. Yes noise is probably a consideration and therefore a position on the head is not ideal but I would be surprised if you were able to find very different signals in different parts of a homogenuos block.
Just speculation though.
Bengt