Knock control tuning
Engine detonation, or knock, can damage or destroy an engine. This is where knock control tuning comes in. Having systems in place to control knock are necessary for modified engine and more so if pump fuel is used.
What and how knock (detonation) will not be discussed here. If you do not know about knock (detonation) phenomena, there may be value in Googling knock to find a good article. It will not be explained here.
Estimating knock control settings
In this article I will describe how to estimate your ECU knock control parameters as well as verify the settings out in the field. My ECU is a Megasquirt MS3 with a MS3X daughter board and internal knock monitoring module however the settings should be common to ECUs that offer knock control.
There are a number of ways to measure for the occurrence of knock. The most common is to listen for the ‘knock’ sound. Most ECUs will have some means of isolating the frequency to target and listen for knock. This is often called the bandpass filter however you need to estimate that frequency first.
the frequency at which knock occurs is a function of the engine piston bored. Phormula has a online calculator for estimating the first harmonic frequency here. A key note here is the frequency is the first harmonic. In some instances, such as my build, the second harmonic might be more usable to filter out other engine noise and false knock sounds.
Time integrator constant
The time integrator constant is used to ‘slow down’ inputs from the sensors and can filter out the majority of false knock sounds (eg. a valve shutting on a different cylinder in the knock observation window) which generally occur very quickly. Most ECUs likely have a default around the 100-140µs. I found that I needed to lengthen my constant as my engine is a SOHC motor which has ample engine noise. This is a parameter to fine tune during the verification process.
Knock window settings
Most ECUs with knock monitoring will have knock window settings. This allows you to target a window of crank rotation to listen for knock. If your ECU and monitor and control knock for individual cylinders, it will have this setting. Knock (detonation) typically occurs after TDC with octane based pump fuel.
Individual cylinder gain
Some ECUs (Megasquirt does) has the ability to set individual cylinder gain. This allows compensation in signals from different cylinders so that readings are similar under operation. This is verified during engine baseline tuning. I aimed to have all my cylinders reading within 5% under cruise and WOT loading conditions.
A word on knock sensor location
There is a myth that says you should mount knock sensors on ‘quiet’ parts of the engine. In reality there is no part of an engine that has location that has less engine noise. Metals transfer sound very well and you can even mount the knock sensors to manifolds or engine mounts and still take adequate readings!
My engine is a Datsun/Nissan L28 which has two factory block boses located between cylinders 2 and 3 and cylinders 4 and 5 near the top of the block. Perfect for capturing knock events which occur after TDC close to the top of the block!
The sensors I used are non-resonate two wire sensors. I purchased mine from EFI Hardware in Victoria. Links to my sensor as well as an equivalent sensor off eBay for international readers are listed below:
Verifying your knock control tuning
‘Knock (detonation) can damage engines right?’ I hear you ask. Well to verify your settings, knock needs to occur in your engine. However this can be done safely and there a number of tasks to complete to verify your settings.
Verify your engine baseline noise
Our knock control strategy uses noise right? We need to determine what the engine sounds like on an ordinary knock free day. This can be done by recording full power runs under load using ignition timing that is known to be conservative for your engine.
A knock threshold can be determined from the average data. I offset my knock threshold by 10% to what I estimated the baseline engine noise to be. Check your individual cylinder gain at this time also to even out your signals!
Knock bandpass frequency and time integrator constant
Now it is time to induce knock conditions. This is done using low load knock which is relatively safe.
For my engine I used 40° ignition advance at 20-30% TPS (I use Alpha-N algorithm) between 1400-2000rpm to induce knock. I am familiar with my engine and under these conditions I can hear the audible trademark pinging knock sound in the cabin.
Check your datalogs. You should see noticeable peaks in noise readings when knock occurs. You can adjust the time integrator constant here to remove false knock if you are observing peaks when there is likely no knock.
What happens if nothing works?
It is entirely possible and in fact happened to me. If all your seeing is high levels of noise which is difficult to seperate knock out then a tool is to alter the bandpass frequency to a second or third harmonic. This simply is multiplying the knock frequency. For my build I used the second harmonic which, by itself, cleared up false knock and engine noise.
My Megasquirt MS3 settings
I’ve shared my knock monitor settings for my Datsun/Nissan L28 as a courtesy. Ultimately you use them at your own risk. Please note while these settings work for my setup, they may not be suitable for your build.
With anything tuning an engine, IF YOU DO NOT FULLY UNDERSTAND WHAT YOU ARE DOING FIND AN EXPERT. It’s not worth damaging an engine over!
If you are a Megasquirt MS3 user using the internal knock module, I have the dialog files available for download below.
Check out my socials and please consider subscribing to my mailing list so that I can let you know when I create more guides like this.