what is a bad knock level?

[qvester]

when i run and it is warm my safc reads sometimes over 200 on the knock sensor level what is a bad knock number is it all just relevant i have never actually heard it knock just read what the safc reads out and sometimes it hits over 200 is that bad or what is bad? thanx

[qvester]

anyone know and tips or reasoning for this ?

[theicecreamdan]

when I had mine hooked up on my s13 fastback I dont think i ever went higher than 30 or 50 actually... but I dont really understand exactly what the numbers mean on them so I don't know

[qvester]

does anyone know i tried searching couldn't find anything on what is a bad knock level or anything to such topics

[kaje36]

I am wondering the same thing. my engine tens to climb to 230 raw data over time.. it didnt used to.. but what does it all mean ? spikeing I know is normaly knock/detination..... ya I am just confused

[rabid_monkey]

i also asked this from a few builders of lots of SR cars after this last drift event. it was really humid and i was concerned after like 8 straight runs on the course. My Power FC was reading peak at 30 on the knock.

I was told that anything above 100 u should start getting concerned about your engine. basically i think the knock sensor is measuring internal vibration noise that things like ur pistons or valves give off. so im assuming if this vibration is big enough, obviously that is meaning bad things are happening... not quite sure tho, maybe a more technical person could enlighten.

but yea, over 100, be concerned about how ur engine is running, air temps, air/fuel mixture, etc...

[msaskin]

First thing to realize is that knock as measured by the Power FC and knock as measured by the S-AFC are two totally separate things.

A knock sensor is a microphone...nothing more. Knock is created by opposing flame fronts colliding in cylinder itself. The microphone picks up this noise, then reports it back to the ECU as a 0-5v signal. This microphone also picks up other engine noise and reports it back as the same 0-5v signal.

For the S-AFC, you're intercepting this 0-5v signal before it hits the ECU and then it displays some number, based on no real logic. For the stock ECU (and probably the PFC) there is some logic on the board to "clean" the 0-5v signal and produce a bit more meaningful data, but it's still not too helpful.

Basically, knock as measured by either is kind of flaky. The true way to measure knock would be with an ECU such as the AEM EMS, or an aftermarket knock control device like the J&S safeguard. Basically with both of these you calibrate them by doing a full pull through the entire RPM range while ensuring that there is no knock (i.e.; run real low boost, with race gas, and less timing). They then create a table of RPM vs. 0-5v signal, which basically is used to represent engine noise. From this point on at a given RPM and 0-5v signal that's higher than the table is used to be "real knock".

Basically, knock is bad. Audible knock is even worse. Beyond that, it's not a great way to tune.

~matt

[89s132b]

If you are trying to interpret "raw" knock reading from an uncalibrated SAFCII, you are just playing a guessing game. Break out your SAFCII manual and calibrate it on a nice cool day when you are sure you don't have knock (or an exhaust leak, or any other funky engine noise). After you calibrate it, the background noise will be subtracted out from your SAFC display. From there the scale is 0-200. Mine nver really gets above 30 or 40, even on hot days when I push it pretty hard.

Even a calibrated safcII knock reading is a rough guestimate. IMHO, the should have more that 2 low rpm calibration points.

[msaskin]

Can you give a rough overview of how you calibrate the SAFC2 for knock? I knew it was possible, but was never too sure on the procedure (especially since I don't run an AFC myself)

~matt

[qvester]

ya i guess i should calibrate it and see what it is from there but i was just wondering cause i am running good a/f levels and plugs always look good no signs of detenation so i wasn't sure what was going on or what the numbers were based on thanx.

[89s132b]

Originally Posted by msaskin

Can you give a rough overview of how you calibrate the SAFC2 for knock? I knew it was possible, but was never too sure on the procedure (especially since I don't run an AFC myself)

~matt

It's pretty easy, you just go to the right menu and hold rpm at the level it tells you too and then press the button. This basically zeros out the noise reading. You do the same thin for a different rpm. I think the rpm levels are about 1500 and 3000, iirc.