Unexplained Knock.... Files and Pics Attached


Sep 29, 2008
Went to the track last weekend and thought I had the car in great shape. Since hitting the track last fall, where I was pleased with the runs, I've got a new transmission, converter, and removed most of the air conditioning components. The car is running E85 and went 11.29 at 124 last fall. Now I'm getting knock with the same tune, but is the knock real or the result of something else? Please take a look and advise. Here are two back to back runs, with the only change for the second run being adding a bit of fuel pressure to bring the injector duty cycle down. When the "knock" occurs I can feel the car slow down. Looking for advice on where to start investigating.
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Oh, to add my plugs are gapped at .030 and I plan to try new ones first, but my confidence is low on that one. Was wondering if a coil pack or ignition module going bad would cause knock like this to show up? The car is not popping and sputtering when it happens, but it does feel like the motor loads up. Some of my gauges are not perfectly in sync with what the powerlogger shows. AFR on PL is targeting 10.7 while on AEM gauge it's 11.2-11.3.
Did you hit 4th gear as it looks like you had 15 degrees of KR at the 3-4 shift. Maybe hit 4th and then lifted on the big end?
Lower boost and try it.
What was Battery Volts?
If its pulling 10 to 15 degrees of timing it will lay over.
The two gauges reading that far off is a concern IMO. I have a XFI WBO2 feeding the FAST and a PLX with gauge on pillar and they both read the same and never drift off more than a tenth.
Plus if either sensor has had oil, headgasket blown scenario, coolant, etc go across it ..replace the sensor.
Possible you are actually in the 11.3 area and thats a bit lean for corn.
Fresh plugs make a pass and then pull them right after and look.
I hope you didn't shift into 4th on that high knock run!!!

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It looks like you are running lean. Not sure why the wide band is reading richer than the NB O2's. I always run the NB O2's at .800 to .820. Anything below .790 I always experience knock. I see you have a volt booster on the car. Everything else appears to be normal.
Thanks for the replies guys. Oh, it looks like my titles for the photos aren't listed. The first run which the car felt good is listed as "File: bir1-1.dat" at the bottom of the window. The bad run is "bir1-2.dat". I'll add some more details:

- No shift to 4th on these runs. Both runs I rolled through 1st gear and then got into it once in second gear. The RPM's dropping is the motor not running well.
- My AEM gauge reads around 11.3 AFR during the run. The powerlogger gets it's AFR reading from that gauge through the analog block. I only have one wideband sensor at the bottom of the downpipe. They have never read the same, but as long as I knew the difference I figured it was ok.
- I do have a Caspers volt booster on the car and voltage stays pretty constant at 15.2V under WOT.
- Boost was 19-20 psi and timing in 3rd was 22.6*. There is no indication of timing being pulled. I assumed the Turbo Tweak chips didn't pull timing. Are they supposed to?
- The next run I pulled 2* of timing in 3rd/4th and it didn't make a difference.
- IAT sensor is just tied off in the engine bay. It is not measuring air going into the motor.

What boggles my mind is that the car has been running clean on the street this year up until now and on this same tune, but at 21 psi I made 7 clean passes at the track last August/September. Are you guys thinking this is real knock or could it be ignition related? Really appreciate the help. Thank you.
Just some observations:
-Pic #2 shows NB @ 0.788 but the graph is above the 0.800 line (?)
-Pic #4 indicates Psi5 @ 49. IDC is also at +88%. Was this @ 6 lbs boost?
-Where is the AEM grounded?
To further clarify, pic 1 & 2 go together (bad run) and pic 3 & 4 go together (good run). The black vertical marker line is in the same place in relative pictures.

Jerryl, the graphs are kinda goofy in the powerlogger. Even though the Y axis mark is at 800 the line going across the window is at 750, so I believe the numbered readout matches the graphed line. In pic 4 where the marker line is Psi5 is showing 59 and appears stable. Corresponding boost was 19.6. I believe this gauge has the same issue as my WB where it's reading doesn't match the PL reading perfectly. Knowing that I do feel that it demonstrates that fuel pressure rises and holds steady throughout the run. For the second run which went bad I increased the base FP by a few psi and the IDC did drop a bit. The AEM gauge is grounded I believe to a metal tab on the bottom of the dash which is also linked to the e-brake pedal bracket. Good point. I'll confirm tonight.

Keep the ideas coming everyone. Thanks.
Agree. Looks like FP is keeping up but strange.
Verify grounds at engine, body and battery are good, clean, tight.
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Try running the injs at 4 bar.. That should put the flow at just over 94pph. See if it knocks, again.
Ok, I'll have some time to work on the car tomorrow. Here's something I've never fully understood, so can someone please answer? This photo shows knock retard and spark advance. Is the spark advance graph only showing commanded spark, or should the retarded timing be showing up there as well? Do the aftermarket chips like this Turbo Tweak actually pull timing once knock is detected? Again, the dip in RPM is not a shift, but an effect on the motor. Much appreciated.

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The knock retard is referring to the amount of spark advance that is being retarded. The spark advance shown in PL is the commanded, and will not show the retard.
If you are showing knock retard, then timing is being pulled that amount.

What you are seeing looks a lot like false knock.
Thanks Eric, could you please share with us what clues you into it looking like false knock? My first step is to test the ethanol composition of the fuel before I go chasing anything else. However, the e85 in the car is from the same station that I've used over the past 3 years and it has tested between 85 and 90 in the past...
It's rare for knock to go from zero to 15 deg in one frame like that. Also, it pretty much stayed solid at 15 deg for an extended period. I typically see that on some cars with wheel spin, which will also cause false knock retard.
Ok, thanks for the explanation. I've seen it on wheel spin before too, but obviously this was already at speed in 3rd gear. I'll add a little more background and maybe someone will have a light bulb moment. Prior to this track experience the car had been running good this summer. The only difference compared to the track day last year was a new transmission and torque converter. Transmission shifts nicely and the spragged converter keeps temps at 165 or below. I have logs of clean highway pulls from earlier in the summer that I can share if it might help.

Recently my power steering pump began leaking, so I replaced it. While I was in there I removed the AC compressor and replaced it with one of those bypass pulleys. A washer on each side lined it up nicely with the alternator. The pulley is a little smaller in diameter, but I was already using a 63" belt and it seemed to fit well. Could it be that a 63" belt is right on the edge and after a bit of heat and under load allows the tensioner to bounce a bit? Maybe I'll consider a 62". I also removed the exhaust cutout for the track.

In short, the converter, trans, ac delete pulley, and having the cutout off are the only differences from when the car was dialed in at the end of last year.
Thanks for the suggestions everyone. This weekend I tested the fuel and it was over 85% ethanol. Also put new plugs in gapped at .028". Then I put on a 62" belt after measuring and finding that the AC delete pulley was a little over an inch shorter in circumference. It's a little on the short side, but it's working. Went on the highway and made 5 clean pulls. Here's one from today. PL shows 10.5 AFR as target, but gauge usually reads 11.0-11.1. Time for another try at the track.

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