I built the engine and transmission in my truck, thus needing a custom tune. I’m always second guessing if the calibration data is reliable due to “false knock” readings. Additionally, before you need to have reliable knock data from the controller. My specific controller is a Delphi P01 (0411) PCM. The P59 is very similar and these concepts likely apply to other controllers utilizing knock sensors.
What is knock
Knock is the symptom of fuel mixture igniting too early. This builds pressure too soon pushing the piston down during the upward compression stroke instead of the subsequent power stroke. As the piston fights to move upward and complete the compression stroke it rattles creating a sound referred to as knock.
Lower compression engines are more tolerant of knock, however, if there’s too much knock you can bend rods, knock holes in pistons, and blow the motor up!
Factory knock systems are calibrated to be overly safe. After all knock could blow up your motor so it’s better to have an overly safe system which reports knock when there may not be than one that misses real knock.
The problem is “false knock” can create issues when tuning things like the Torque Converter Clutch lockup and especially when calibrating timing advance.
What causes knock
Timing degrees in advance of the piston reaching the top of the cylinder (TDC) when the spark plug ignites the fuel mixture. You can adjust this on an engine, if you advance it too much you’ll create knock.
Pre-ignition gas cools better than air. Too little fuel and cylinder temperatures rise. A burr on the combustion chamber, edge of hot exhaust valve, or a hot surface can ignite the mixture prior to the spark plug firing (timing event).
Bad Gas / too low octane higher octane fuel is more tolerant of high temperature and pressure. It also creates a more consistent burn making combustion event more repeatable (e.g. peak pressure with 25 degrees every time viruses 19 to 21 degrees with low octane).
Hot spots a small burr or lip in your combustion chamber will heat soak much faster. This leads to a hot spot in the cylinder. These can ignite the fuel mixture before the spark plug fires. In my engine build I polished my combustion chambers and cleaned up all the casting irregulates such as burrs, and raised edges to mitigate this risk.
Calibrating knock sensors (never knock tune)
We’ll need a baseline to know when knock should never take place in order to desensitize the knock sensors.
Zero mechanical issues If you have loose and rattling chassis parts fix them. Leaking exhaust, fix it. Bad motor mounts, fix them. These things can trigger even the best tuned knock system.
Fuel tuning complete you need to be complete with your fuel tuning. Know your tune meters fuel correctly amount of fuel in open & closed loop as well as power enrichment. We need to know we can 100% trust the fuel mixture to be correct (or at least one the rich side).
Disable TCC Lockup the torque converter clutch is notorious for creating false knock. There’s numerous ways to do this. You can do it in the calibration and or with high end scan tool. I use HP Tuners VCM scanner.
Force Power Enrichment my power enrichment is tuned for 12.2:1 AFR. This puts extra fuel into the cylinder to make sure it stays cool and won’t pre-ignite the fuel.
Premium Fuel tuning with premium fuel adds some extra insurance against pre-detonation. Maybe a bit of overkill if your tune is for regular fuel and you’re forcing power enrichment. Some people burn off half the tank then top off the tank with say 100 octane racing fuel. (NOTE: too high and it has lead in it so be careful of what fuel you buy!)
Knock free timing tables the factory timing tables interpolate between a high octane table and low octane table. Depending on the knock detected it pulls timing from the high table until it reaches the lower table. People say the high octane table is conservative, however, I take that a step further and ONLY use the low octane table. The manufacturer deems the low octane table as a “always safe” timing table. I’ve even pulled 2 degrees from the low octane table to be extra sure. You then copy it’s values into the high octane table. This effectively disables the high octane table.
Turn off predictive knock systems burst knock is the predictive knock is one such system. I think maybe the only system in the P01 / P59 PCM(s)? I’m not convinced that’s a good long term solution but will be necessary for logging REAL knock. Be careful not to make abrupt changes in your throttle position as predicative knock protects against rapid changes in engine vacuum.
Note: I’ve never once triggered burst knock system in well over 100 logs I’m monitoring it in. I’ve seen other people who have triggered it in their logs. That said I’m keeping it in place until I hear otherwise.
Detecting False Knock
False knock tends to be random. It won’t necessarily correlate with cylinder pressure and engine RPM. You may see it once for a given condition but the next time the engine operates under those conditions it won’t be there. That’s a good indication it’s false knock.
You’ll desensitize the knock sensors within reason at this point. Reducing sensitivity by 15% may reduce false knock to point you don’t get any in the tune we described. We cannot say with certainty there’s still going to be false knock reported, nor that we’ve desensitized the sensors enough. However, this is a step in the direction towards removing some of the false knock.
I’d say there’s no way to have a 100% accurate knock detection system utilizing knock sensors. They’re basically microphones. Granted there’s probably a much better way to detect it than what the factory system does.
I believe aftermarket systems utilize an autocorrelation (similar to speech detection). That can filter out things like intake noise, transmission shifts, etc. Whereas, I assume factory system likely uses a bandpass filter (sample frequency) and signal amplitude (voltage) which can be implemented with minimal processing power. However, that has no way to filter out extraneous noise reaching the microphones (knock sensors).
Use the Torque Converter Clutch to further desensitize the knock sensors
The torque converter clutch provokes knock. It is also sensitive to engine timing. If we increase engine timing during lockup this can create real knock where there wouldn’t have been otherwise. We can use this to our advantage to learn a bit more about the knock sensors.
Reenable your TCC clutch and log some data. I’d start from your factory baseline TCC settings. Although, those many times provoke knock especially at slow speed (e.g. accelerating under 1800 RPM in 4th gear).
Step 1 Is the knock repeatable Log data and see if you can repeat the knock event given same engine conditions. If the knock repeats like in my case under 1800 RPM locked up in 4th gear then it’s real knock.
Step 2 pull timing Make note of what the timing was during knock event (say 23 degrees). How much timing did the knock event pull off the timing table? Go to your high and low timing tables. Select the cells around and including where the knock even pulled timing. Round up the timing pulled by knock event (e.g. 2.3 degrees, round up to 3.0 degrees). Then reduce the timing in and around those cells (say an extra cell on each side). Load this to your PCM.
It’s important to note, timing is calculated of numerous tables in addition to the high and low octane tables. You want to ensure your advance isn’t higher the second time we log data.
Step 3 try to repeat the knock event if the knock event cannot be repeated then it could have been real knock. Otherwise if it repeats and your advance was no more than you’d previously noted (2.3 degrees) then we’re pretty sure it was false knock. You can desensitize the knock sensors and repeat this process again.
Seat of the pants knock sensors
You can feel and hear knock if it’s drastic enough! If that’s the case lift off the throttle immediately. Depending on your build, or if it’s a factory engine it may have a reasonable amount knock tolerance, hopefully no damage was done.
When the engine knocks it’ll sound like a faint rattling. Almost like a muffled box of marbles being shook. As it increases in intensity it’s getting worse. Enough knock and the engine will shake! You can start to feel in in the seat of your pants.
I once tried to climb a hill starting out in third in my C5 Corvette. That’s DEFINATELY knock. It did it but wasn’t good. My 6.0L Tahoe build knocks if the TCC clutch is locked up while merging onto the interstate from. Specifically in 4th gear with lockup accelerating up to 2000 RPM. I’ve noticed virtually all GMT800, trucks with 3rd gen SBC & 4L60E, knock accelerating onto the interstate under similar conditions.
Once you recognize pre-detonation / knock you’ll notice it immediately. I’ve become so sensitive I can even tell when the controller pulls several degrees of timing.
The P01, P59, and earlier controllers trigger TCC lockup based off of throttle position and highway speed. I feel it should be sensitive to knock as well. I can get the controller to pull timing when in TCC lockup yet it doesn’t release the clutch. It gets to point motor feels like it’s going to stall. Likely my tune is WAY off, but I digress, would be nice if the controller released the clutch when knock reaches a given threshold.
In my opinion, knock is a limiting factor of the airflow model behind P59 PCM. The industry has since switched to a torque model. Looking at throttle position as a “torque request” from a throttle by wire setup allows the controller to avoid getting into some of the conditions these older controllers encounter.
Hope this article was helpful. Feel free to reach out to me if you have any questions.
