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Knock Knowledge

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Hey Andre,

How common are Knock Detection Systems in modern vehicles? If the vehicle has an OEM Set-up, how reliable are these systems to provide accurate feedback? Does it depend on the vehicle manufacturer? When do you suggest using/not using Knock Detection? Thanks for any input!

I would expect most cars since the 90's have knock detection of some sorts, I've yet to come across one that doesn't.

Modern cars usually use the Bosch wideband knock sensor which is the same sensor that most after market systems use, these are good sensors but it depends on how the ECU has it's knock detection strategy set up as to how good it is once you start modifying the engine.

Older cars use a simpler sensor which only detects knock of a specific frequency, these become useless once you begin to modify the engine or even increase the boost in some cases

As Chris mentions, you're unlikely to come across a modern vehicle without some sort of knock detection system. As you could expect some are better than others though and the older systems struggled to deal with engine noise and distinguishing between background noise and knock.

As a rule I seldom trust the factory knock control strategy without first confirming it is accurate and effective with audio knock detection equipment. I've had examples where the factory knock system was picking up false knock but at the same time I've had them also ignore real knock so i like to know what's happening.

For me personally I won't step onto the dyno without audio knock detection - I feel that strongly about it.

Hey Andre,

If I'm tuning a built "loud" engine on an OEM ECU, using Cobb ATR for instance, what are some typical adjustments you might have to make to the OEM Knock Detection System Tables to compensate for the louder engine? Should I utilize a third party set-up to find the "knock threshold" and tune the OEM Set-up to the limits found? If I am going to tune the OEM set-up using a third party (i.e. Plex Knock Detection) how would I go about doing that? Could I just unplug the stock sensor from the ECU and use it with my Plex Handheld? If I do that, won't the ECU detect that the sensor is disconnected (DTC)? What I'm asking is, how do you tune the engine in this situation if you only have one knock sensor to use either with the ECU or the Plex? Do I have to install another knock sensor so that my ECU doesn't see the stock sensor unplugged and possibly alter the state of the tune because of the fault? Thanks for any input!

It depends a little on the tables or adjustments you have access to. The way I would suggest approaching this is as follows:

1. Install a 3rd party knock detection system such as the Plex alongside the factory knock sensors - You can't unplug the factory sensors for this testing.

2. Datalog the factory knock input on the dyno while also listening for audible knock. The aim here is to get the engine to a point where it is suffering light knock and confirm that the factory knock detection system is able to pick this up clearly. You may actually be surprised how well the OE system does even on a built engine since the knock control system is tuned to the knock frequency of the engine.

3. If you find that either the factory system is detecting knock that isn't occurring or alternatively missing knock that you can audibly hear then you're going to need to adjust the knock system's calibration and this is difficult to advise on since the actual tables you have access to as well as their function is very dependent on the particular ECU you're tuning.

I can't really give a thorough or complete method for dealing with this and in some instances it is actually easiest to disable the knock control system and rely solely on your audio knock detection to safely calibrate the base ignition advance. I know this might sound like a step backwards but this is what we were limited to in the days before aftermarket ECUs started to incorporate knock control.

If you decide to try and adjust the OE knock control strategy I suggest making moderate/large changes to the relevant tables and then testing to see what the effect is on the system's ability to detect knock. This will let you see the effect of the change easily and then once you've got a result that's heading in the right direction you can make smaller changes to fine tune the control. The tables you're looking for will likely be labelled as knock filter or something of that nature but again the tables you have available vary from manufacturer to manufacturer.

This is unfortunately one of the downfalls of reflashing an OE ECU. Some of the more complex and less used functions are not well understood even by those producing the software to allow the ECU to be reflashed.

Hey Andre,

If there is not a place to install a 3rd party Knock Sensor directly adjacent to the Stock Sensor, is there another acceptable location(s) that the sensor could be installed and still be useful?

This might help taken from the plex website

I've also had reasonable success in some engines bolting the sensor to the sump if the engine has a cast sump or sump extension. I try and stay away from the head where possible as I find you pick up a lot of noise from the valve train. In saying that though in some instances your options will be very limited as to where you can fit a sensor.

>"I would expect most cars since the 90's have knock detection of some sorts, I've yet to come across one that doesn't. "

I know I'm late to this but in the southern hemisphere knock sensors only become a common thing in the 21st century. Most engines in "family cars" were so under tuned that they didn't have knock sensors fitted. The Ford V8 as fitted to EB Falcons (as well as related Fairlanes and LTDs) and later models for a few years never had knock sensors even on the higher performance engines (and some did knock which required a rework of the distributor). HSV (Holden Special Vehicle) engines had knock sensors fitted as did V6 Holden (Buick and then Ecotec) engines but the regular 165 kw V8s certainly didn't.

I can also say that Mitsubishi, Nissan , Subaru, and Toyota, didn't fit knock sensors to the vast majority of their cars sold in Australia (and possibly NZ as well) until 2000.

In my own experience Michael I'd have to agree with Chris - By far the majority of the performance cars I have tuned even as early as the mid 80's have some kind of factory knock detection system. There are of course exceptions to every rule and knock detection systems understandably would be more likely on an engine that was considered 'performance' rather than pedestrian.

Just one example that I can come up with off the top of my head is the Nissan E15 turbo engine from the mid 80's. This was fitted with a factory knock sensor however at the time Nissan's ability to do anything useful with the sensor signal was very limited. If my sources are correct, the ECU ignored the input from the knock sensor at high load/rpm as it was impossible to separate knock from noise.

I think the difference is with the concept of "most cars" and the concept of "performance cars". Having worked through the 90s in Holden, Ford, and Toyota dealerships throughout Australia I can say "most cars" in Australia didn't come out with knock sensors and only some performance cars come out with knock sensors.

I only commented because this is a global community and I think it is important to highlight differences between regions/countries/continents if the discussion requires it.

To be fair me saying 'most cars' I was referring to performance related cars. I can't say I have much experience with 'family cars' of the 90's, in all fairness it was a total assumption as every car I deal with has them, Toyota GT-Fours, Supra's, Chasers, Nissan Skylines etc.

Any which way you look at it you should always be confirming with your own aftermarket knock detection your tuning is safe.

Chris I have no problem with what you are saying I just clarified a couple of things (and gave examples) within a context that you, personally, are not familiar with.