Green89IROC305

Hey All,

I'm pretty sure I am experiencing false knock here but thought I'd ask either way. Most of the time the knock shows up is during downshifts at WOT, however this last run it was showing a lot of knocking in the 1800 rpm range while cruising. I have altered the SA tables to be inline with what my engine wants.

I have the EBL Flash II, adapted my 87 MAF system to it. Car runs great!.

However I get some pretty high knock counts.

I recently had a new 383 installed. The installer installed an SD knock sensor (3.9k ohm one), where I need a 100k Ohm MAF style, which is also the style the EBL Flash II needs according to a chat RBob.

The installer is installing the proper knock sensor this Monday for me free of charge.

My question is this: Is the 87 MAF style knock sensor more or less sensitive than the SD style?

I am essentially trying to confirm my false knock suspicions.

Thanks

GeneralDisorder

iTrader: (1)

The knock control module is external so I'm not clear on why the knock sensor would be changed for the EBL computer? I must be missing something.

GD

Green89IROC305

On the 87 MAF systems knock signal runs through the ESC module. I'm suspecting that because the knock sensor is 3.9 k ohms instead of 100 k ohms the ESC signal for the knock filter is producing false knock readings.

The main question though is what factor does resistance play in knock sensor sensitivity.

It seems to me that a higher resistance would require more voltage to trigger a signal than a lower resistance, which would make in this scenario the higher-resistant sensor less sensitive?...

a secondary question would be what result would be produced by running a lower resistance knock sensor through the ESC module's knock filter.

I mean after all the stock knock sensor is 100K ohms for a reason.

GeneralDisorder

iTrader: (1)

It's not immediately apparent to me what the correlation is exactly between the resistance of the sensor and the sensitivity of the sensor. The sensor is essentially a piezoelectric crystal that emits electricity in response to vibrations that cause physical stress in the crystal. I don't know enough about the underlying science or the specifics of the crystals used in GM knock sensors to comment on which sensors are used for which frequencies..... I have read enough to know that the sensor used is generally chosen based on bore diameter as this largely dictates the frequency of detonation vibrations in piston engines. The filter is there to send a square wave signal (I assume) to the ECM when it sees voltages above a pre-determined amplitude from the sensor. Each trailing edge from the ESC module is recorded as a knock event (again I assume this is basically what's going on), and the ECM responds accordingly by pulling timing, etc.

False knock.... well I would be putting a scope on the input to the ESC and the output from the ESC to see if there's any noise, etc that might be interpreted as knock.

The classic 3-step method for determining false knock:

1. Increase fuel in the knock area

2. Decrease timing in the knock area

3. Use high octane race fuel or other octane boosting methods to rule out actual detonation.

If doing all of these results in ZERO change to the knock behavior, you can safely assume it's false.

GD

Green89IROC305

I can and have done the first two. However I keep coming back to the question. If I have the wrong knock sensor for the ECM, can I even consider these knock counts accurate?

Regardless the correct one will go in tomorrow so we shall see what's up then.

Thanks for the tips GeneralDisorder.

GeneralDisorder

iTrader: (1)

I have been fighting some WOT knock counts also with my 700R4 and 3500 stall converter. I think I have it pretty much resolved. Some thoughts not in any particular order:

1. I found in my datalogs that there were small windows of lean readings on my narrow-band. (anywhere from 80 to 300 mV) - sometimes two or even three of the 17hz data packets showed lean. This only happened when going RAPID WOT. I had to increase the speed and volume of the AE - both TPS and MAP, but mostly TPS. I had to increase the TPS AE filter value (to about 50 at operating temp) to get AE faster (it doesn't last as long this way, but then the MAP AE takes over), and increase the AE TPS PW in the 12.5 dTPS and above by a LOT (like 2000%). Then I also increased the AE vs. RPM percentage above 2000 RPM to about a 70% increase (all the way to the end of the table). This got rid of any lean areas when going from 1000 to 3000 RPM to get up against my converter. It seems that none of the included EBL maps or factory maps have anything like what's needed for a big converter with regard to AE. Had to make BIG changes. Not surprising in retrospect.

2. Till you handle the AE and are SURE there aren't any lean areas when going to WOT with a big converter, don't drop the timing drastically. Check the logs carefully for any potential lean areas. Make sure it has all the fuel it needs first. If it's lean pulling timing probably won't help much and could lead to #3:

3. I limited the amount of timing the knock control could pull out - if you don't and you get a little detonation while tuning, and the knock retard has the authority to DUMP the timing by a crap load (like 17 degrees) then you can get a run-away detonation event where the knock retard sees a couple events and dumps your 20 degrees of timing down to 3 degrees. This can actually make the detonation worse as the extremely retarded timing causes a huge jump in cylinder temps and you get a run-away detonation effect. I limited both of the knock retard max's (PE and non-PE) to 5 degrees. Remember that a detonating cylinder tends to keep detonating so often you need to look for that first initiating event where the timing was too great or the mixture was too lean and fix that first.

GD