KR – Graph 3: What the PCM Believes to be Real Knock

Posted by Alex Darmos on

The third graph below (Graph 3) shows what the PCM believes to be real knock (not just engine noise). This knock is indicated by the spikes in the signal along the graph. The largest spike by far is the one on the far left. Note the scales on BOTH axes. This is significant signal strength at VERY high frequencies. We are talking in the nanosecond range. Time separation between distinct knock pulses is about 23 microseconds.

By expanding the time base of Graph 3, we can zoom in on the large spike to the left as shown in the same graph. This is what the actual knock pulse looks like. It is a high frequency, very short burst with one occurring every 23 milliseconds as shown in the graph. Within the burst, the waveform cycle varies anywhere between 120 ns (yes NANOseconds) and 300 ns, but is consistently within those time frames in the testing that I’ve done. The actual frequency range of this corresponds from 8.3 MHz (120ns) to 3.3 MHz (300ns). The actual pulse frequency does not matter so much, except to know that it is a VERY high frequency. We aren’t talking microseconds here: we are talking NANOseconds and megahertz. Look at the voltage on that initial spike. It is nearly 16 volts!

So what’s the bottom line: why do I want to change the KR noise floor?
The point to all of this really is to show where the knock voltage is at idle, and in my car it happens to be at about 1V Peak. Prior to changing my noise floor, I was getting HEAVY KR by just accelerating away from a stop normally. It was terrible. Every time I accelerated: even lightly, I got substantial KR (10-15 degrees). I never had it that bad until right after my transmission was rebuilt. Since then the KR has been pretty intolerable. Dave asked me what my noise floor was on the knock sensors, and from my scope shots, I was able to tell him approximately 1 volt. He then raised the stock setting of 0.5 volts up to 1.0 volts. Since then, I have to REALLY work at getting KR. I have learned exactly what I have to do to get this kind of KR to occur. Thankfully, this fix in the PCM has prevented me from seeing KR under normal driving, spirited driving, and racing. Under certain circumstances, I can revert back to having KR repeatedly occur, but that is something that I have to really work at to have happen.