I can hear knock but the detector...

[KEVINS]

I can hear knock but the detector isn't picking it up. Why does this happen?

The last few test runs I went on to trouble shoot low fuel psi had some low O2's around 770-740 and I can hear the knock before the audible detector goes off and it never showed up on PL. I KNOW it was knocking, no question in my mind and of course I let up as soon as I heard it. I upped the fueling and got the O2's back to where they should be and the knock went away. It seems that during sever knock the sensor won't hear it but minor knock it will.

Anyone else experience this?

I'm running 35psi boost with alchy.

KS

 

[Chuck Leeper]

Kev, is the knock sensor properly torqued? AIRC, it should be 14 #/ft.

 

[KEVINS]

Kev, is the knock sensor properly torqued? AIRC, it should be 14 #/ft.

It's still all the original factory long block. I've never touched the sensor and it does work. I went on three test runs last night and during the first two runs the O2's were showing very lean and I can hear it knocking pretty bad so naturally I let off right away. The PL never registered it and the audible knock detector never went off.

I changed the fueling before the third run and the O2's were fine at the start of the run but I started running out of gas in the tank and the sensor picked up on the car running a little lean during this moment and the detector was sounding off. You can see this happening in PL as the KR was between 0 and 6.

Forcedfed3.8 apparently had the same issue from his comment in one of my posts too:

I had a car running at 16.7:1 AFR on a wideband and the narrowband showed .810!!!!!!! Get a wideband. You are 10psi lower than it should be, you are lean for sure.... The car had alky and showed no knock!!!!!!! 3 months later it had 2 blown headgaskets, all main bearings were toast, the crank was junk, and a thin spot in # 5 cylinder was cracking.

If I remember correctly these sensors are listening for a certain frequency and I'm betting the frequency of severe knock is different than what the sensor is listening for so it never detected it. I'm glad my car is real quiet otherwise I may not have heard it.

KS

 

[jasjamz]

You better back that boost back 10 psi and re tune. Audible knock at 35 psi means its chipping away at the headgaskets LOL. What plugs, you should step it down a notch colder.

 

[Jerryl]

Kevin, I have similar experiences during earlier years of my GN career. Lol

I remember having some serious audible kr in first gear, stomping it from a standstill. DS did not record the events. Heck, Caspers knock gage wouldn't even register anything. Lol

DS/gage would however record kr under similar test conditions when there was no audible kr.
Chip did not have kr ignore feature.

 

[KEVINS]

You better back that boost back 10 psi and re tune. Audible knock at 35 psi means its chipping away at the headgaskets LOL. What plugs, you should step it down a notch colder.

Kevin, I have similar experiences during earlier years of my GN career. Lol

I remember having some serious audible kr in first gear, stomping it from a standstill. DS did not record the events. Heck, Caspers knock gage wouldn't even register anything. Lol

DS/gage would however record kr under similar test conditions when there was no audible kr.
Chip did not have kr ignore feature.

I tuned it at 30psi and the tune was good with no KR and O2's were where they should be. When I raised the boost I was trying for 32-33psi but it went to 34-35 and that's when I heard the knock due to O2's being low. Once I added more fuel to get the O2's were they should be the KR went away.

I'm very happy with where it's at right now so as long as things don't change I'll leave it alone. I'm just pissed that the knock sensor didn't here the knock and wondered if anyone else had this issue and if they knew why. It sounds like others have experienced this same issue so I'll definitely keep an open ear tuned to listen for knock not just the audible detector.

ks

 

[RmvBfrFlght]

The knock system is going deaf, likely from too much back ground noise. Kinda' like being at a rock concert and can't hear/talk to the person next to you {note that I edited this to be a more precise analogy}.

The knock filter acclimates to back ground noise that is picked up by the sensor. If the level of noise is too high the filter saturates and goes deaf.

The filter responds to two scenarios, one being a high amplitude from the sensor within a particular frequency band. The second is a rapidly changing frequency.

RemoveBeforeFlight

 

[Jerryl]

The knock system is going deaf, likely from too much back ground noise.

The knock filter acclimates to back ground noise that is picked up by the sensor. If the level of noise is too high the filter saturates and goes deaf. . . . .

RemoveBeforeFlight

Wow. Is acclimation temporary or permanent?

 

[KEVINS]

The knock system is going deaf, likely from too much back ground noise. Kinda' like after being at a rock concert and can't hear very well afterward.

The knock filter acclimates to back ground noise that is picked up by the sensor. If the level of noise is too high the filter saturates and goes deaf.

The filter responds to two scenarios, one being a high amplitude from the sensor within a particular frequency band. The second is a rapidly changing frequency.

RemoveBeforeFlight

Good info! Does it reset when the key is turned off and "relearn" during each run or is it going bad and should be replaced because it's hearing device has been listening to too much rock music?
I'm nervous about replacing the factory sensor b/c I'm worried that the new one won't be installed as accurately and may not hear things properly.

ks

 

[RmvBfrFlght]

The acclimation to background noise is a continuous process. I never measured the time base of this, but it is fairly rapid. Some time ago I was playing with knock sensors & filters on the bench. When I got tired of whacking sensors with a bolt, switched to using a frequency generator in place of the sensor.

With this I could find the frequency band that the filter was most sensitive too. And found out that they can go deaf. This is also how I discovered that a rapid change in frequency also triggered knock. I didn't take any real notes when I did this, basically I was just goofing around a bit. Just to get a better idea of what triggers them to report knock.

If the input frequency to the filter changes over a span of say 2 seconds, no knock is reported. Change that to say a 1/4 second and sure enough knock is reported. Even though the '86 & '87 TB's use different knock filters they have identical specs. Center frequency of 5.9 KHz, with the '86 version being a 4-pole low pass filter and the '87 version being a 2-pole low pass filter (cost reduction).

RemoveBeforeFlight