Is there really 2 knock sensors?
 

I just noticed that PP lists these ( 944-606-145-00-M14) as 2 being required per car. I only ever noticed one on my 89 turbo - under the intake, near the ISV. Where is the other one located?

There is only one knock sensor on the 951, regardless of year.

I believe the second knock sensor was used on 3L cars (but don't quote me).

Pelican Parts - Product Information: 944-606-145-00-M14

They seem pretty adamant about the turbo having 2 of them That's why I asked. But it now appears obvious that it IS a mistake.

there is a spot next to the single factory knock sensor to accommodate a second knock sensor, but it's taken up by the factory ISV bracket (i think, will have to double check). i'm actually planning to relocate the ISV bracket, and install a second knock sensor just as a failsafe. :)

That doesn't sound like a bad idea. Is there a connector for the 2nd one?

Yes.

Not on the factory harness.

If commencing such a mod, would you just wire it in parallel with the other one? Wouldn't that throw the sensitivity off? Or does it it need a separate line to the DME? If so, will the DME be able to read it without modification of the firmware?

The DME does not monitor knock, rather the KLR does.

Adding a second knock sensor is not an option for the DME/KLR... nize is running a stand-alone engine management setup, so what he does is not a point of reference for anyone running the DME/KLR.

How does the S2/968 handle the 2nd one then?

Completely different DME.

I c.

Furthermore, on an inline-4 cylinder I don't see much of a point to adding a second knock sensor... There are only two ignition events per revolution, with good separation, so one knock sensor, placed reasonably, should be quite sufficient at picking up knock.

Adding a second knock sensor, especially so close to the first is only good for a redundancy-type of setup - in which one knock sensor fails (highly uncommon).

i'm installing the second one for redundancy, in case the first sensor fails.

Since I am using stock engine management hardware , I have no choice but to stick with a single one. That's fine though.