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Detonation Vs Oil pressure

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Just going through a datalog from when I started mapping my GTR and noticed something interesting..

I recently discovered that my timing light was giving a faulty reading (found out whilst trying to find the cause of my high EGT at Donnington track), it turned out that I was 15 degrees retarded so after correcting the timing I backed off the timing in the map and started tuning the ignition map again.

Then without realising I accidentally loaded an older map into the ecu....which was a basic timing map but obviously my engine is now 15 degrees more advanced than it was when this map was being built, on the first run I was getting knock due to engine now being 15 degrees more advanced.

Take a look at the datalog and you can clearly see the knock shown, but more interesting is the effect it has on the oil pressure reading.

I'm interested to hear peoples opinions on the reasoning for the oil pressure to be so directly affected by the knock readings?


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I would also love to hear an explanation of this.

What type of oil pressure sensor is it?


Same here, I have a few spots where the knock readings are so far below my threshold that I didn't even consider them to be knock and yet I can see a fluctuation in the oil pressure readings.

The oil pressure sensor is a standard 0-5 volt AEM 100 psi sensor remotely mounted in an oil filter relocation kit.

I have added a few more log pictures to show it happening in other places.

When there is no reading from the knock sensors the oil pressure climbs smoothly with revs.

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I can't possibly imagine that correlation to be real, however I can't offer a good explanation either. All I can say is that on our 350Z which has oil and fuel pressure logged, we don't see any variation in oil pressure when the engine knocks - That being said it's actually pretty challenging to make the VQ35 knock as hard as a turbocharged engine such as yours.

Knock is known to damage big end bearings so it's not impossible to believe there may be some impact on oil pressure but I'd be surprised to see such a dramatic and clear cause and effect. Interested to hear any other theories on this. In the mean time I may run this past Link and see if they can add anything to the conversation.

I realize that the knock sensor outputs a very small voltage....but perhaps backfeeding voltage somewhere? maybe causing interference with the voltage to the oil pressure sensor? You should calculate how much voltage it would be, given the pressure change. would be interesting to know.

Peak cylinder pressure is occurring before TDC, stalling the crank momentarily (or even driving it slightly backwards) and dropping the oil pressure.

Is this a phenomenon you've previously experienced/logged blackrex? Or are you hypothesising?


Thanks to everyone for replying, it certainly is interesting.

Andre, I sat staring and trying to work out the relationship and could only imagine that the impact on the bearings was causing the oil pressure changes but was surprised to see as much as a 2.3 bar drop in places.

This car does suffer badly with oil surge and only after fitting the oil pressure sensor and doing a lap around Donnington did this show itself, this made me wonder if brief moments of oil starvation were causing engine noise that were being picked up as knock but the oil pressure is always above 4 bar so very unlikely.

13bjunkie, the sensor is 0.5v at 0bar and 4.5v at 6.89bar. I have oil pressure steadily climbing from 6.2 bar at 3500rpm, to 6.4 bar at 5,000rpm.

The first knock sends the oil pressure reading down to 5.4 bar,and 1 second later another knock occurrence sends the oil pressure down to 4.18 bar.

Blackrex, that is interesting...I did look over the rpm data to look for signs of something there and as much the rpm does appear to fluctuate a little at the point it shows knock, unfortunately I am still running the standard CAS which suffers horribly with rpm variations anyway.

I have uploaded another picture and you can see the RPM go from very smooth to a little unstable as the knock starts to show but I would have thought the RPM changes would be more dramatic.

I am very busy at the moment but I am running a different ECU on my other Skyline, as soon as I can I will setup an oil pressure feed to the ecu and see if I get the same readings on that one.

Thanks again.

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Something that occurred to me whilst eating my porridge - could the knock be caused by the low oil pressure?

"This car does suffer badly with oil surge and only after fitting the oil pressure sensor and doing a lap around Donnington did this show itself, this made me wonder if brief moments of oil starvation were causing engine noise that were being picked up as knock but the oil pressure is always above 4 bar so very unlikely."

I am thinking that if the engine has piston cooling jets that these are being starved quiet early in a corner then maybe they are getting hot? Their early starvation MAY not initially show up on the oil pressure sensor.

Something that occurred to me whilst eating my porridge - could the knock be caused by the low oil pressure?

"This car does suffer badly with oil surge and only after fitting the oil pressure sensor and doing a lap around Donnington did this show itself, this made me wonder if brief moments of oil starvation were causing engine noise that were being picked up as knock but the oil pressure is always above 4 bar so very unlikely."

I am thinking that if the engine has piston cooling jets that these are being starved quiet early in a corner then maybe they are getting hot? Their early starvation MAY not initially show up on the oil pressure sensor.

Is this just a one off occurrence or does it happen every time it's knocking? Any chance you could post the log and let us see it all?

Hi Chris,

It seems pretty consistent in the logs.

As I said it was only after an old map was loaded and the engine had since been advanced 15 degrees that these knock readings showed up, the knock threshold and gain etc haven't been tuned to the engine so it may not be genuine knock but these 2 logs seem to have a few consistent knock readings with an apparent effect on the oil pressure/readings.



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On log 'OilVKnock1' at 2:14-2:15 you can see that my oil pressure drops rapidly which activates my RPMvsOil limit.

At that point in my limit table I have it set to activate a 1000 rpm limit (oil pressure drops below 4 bar at 4,500rpm) which could explain the slightly higher knock readings that coincide with the oil pressure.

Same applies to 'OilVKnock2' at 53 seconds.

All other oil pressure drops seem to coincide with higher knock readings.

What kind of pressure sensor are you using?

G sensor log? Oil starvation , slight bottom knock, low knock threshold curve , positive read maybe

kickerzx - The pressure sensor is a standard AEM 0-100 psi 3 wire sensor, I intend to try a different sensor and see if the results are the same.

Doing some research it seems a number of people were able to detect knock via their oil pressure sensor on a specific MX5 (uses a specific type of oil pressure sender) even down to one bloke having his oil pressure plummit everytime he came on boost but his knock readings were very low...he later found the engine had been knocking badly and the oil pressure gauge had been showing this but he didn't look into it until his engine blew from detonation.

Bimmerboy92 - Unfortunately I do not have a G sensor log, that will be something I look into, im not sure if I can tap into the original G sensor which would be nice but if not I will add one....nothing at the moment though.

In those logs my knock threshold was set low and the gain quite high but there are occasions where the threshold is at 320, the knock isn't even reaching 100 and the oil pressure shows a dip in time with the knock readings.

The oil pressure never actually dips below 2.5bar, this is thanks to the fact that I fitted an Accusump but was using the standard pressure switch with it, now that I know the pressure has been dipping so low I have wired it though the ECU so the link now references an RPM vs Oil pressure table and opens the Accusump valve based on the readings from the table.


I have seen similar issues occurring in logging, where the calculated crank speed drops at the point of knock by a large value, and then recovers quickly (the calculated crank angle values can reverse as well), but it would need to be logged at a much higher rate than 1kHz to properly catch it.

The other cause of the oil pressure drop could be that the knock is causing the rod bigends to break the hydrodynamic oil film with the crank, opening up the clearances and allowing for the oil pressure to drop.

I would have assumed given that the peak cylinder pressure spikes relating to knock still typically occurs ATDC that crankshaft related movement would be minimal. Sure it's likely to also result in more cylinder pressure in the later part of the compression stroke but I'd think the kind of movement that would need to be transferred into the oil pump drive to cause such drastic reductions in oil pressure would need to be quite dramatic.

The effect of knock on the oil film in the big end bearing feels to me like it might be more viable reason for the oil pressure drop however I still can't correlate the magnitude of the oil pressure drop with knock - It's just such a big drop in pressure. This requires more investigation/consideration on my part.

So, I have finally got the GTR out of hibernation and about to start mapping the new G4+ but thought I would go over some old logs with the newer G4+ software.

Obviously I need to make sure the timing is 100% correct this time to avoid anymore accidental over advanced timing, from going over the old logs it seems that due to my error of loading an old file after physically advancing the timing at the engine, I ended up with numerous occasions where I was at 26 degrees BTDC, 5,000-6,000rpm and .8-.9 bar boost. This caused what appears to be heavy knock on a number of occasions (on the bright side, it gives me some data regarding what timing my engine will not tolerate)

Anyway, the point of this post.... I was watching one of Andre's webinars and discovered that the Link has an RPM rate of change parameter that can be logged.

I have added a screenshot of my logged data including this RPM rate data and am still curious what is the cause of the direct relation between knock and oil pressure in my engine.

Whilst we can almost certainly say it is either-

A) Impact on the bearings from knock causing a drop in oil pressure

B) Crank reversal from knock causing a drop in oil pressure

Either scenario is bad but wondered if the RPM rate data would help with a definitive answer.

Please note that at the time I was still running the standard Nissan CAS so I was suffering badly with scatter and therefore the RPM rate is already jumping around a lot, there is however a large change at the point that I am seeing knock, does this offer anything new or would you expect to see the same change in RPM rate regardless when it is knocking?

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On my own car i log dRPM (RPM ROC). Although i always look for evidence in dRPM on every knock i cant find it for sure. Sometimes i think its there and others not. I log at a bit higher rate than you also so i have a better chance to pick it up. (I have 500hz vs your 100hz)

However on Vems ecus they were able to log dRPM PR cylinder. There they used to look at that to find knock.

However your old logs are completly useless for this purpose either way cause of your timingscatter that would overshadow any evidence anyway.

Id say your assumption that the oil pressure could drop from " crank reversal " is zero chance . There is absolutely no way the crank stops . It may twist . But man it is such a stretch to say its reversing . Also if you had a mechanical oil pressure gauge set up on your engine ( and the bearings are in good condition ) you would see when you stopped the engine oil pressure falls quickly but not immediately. So imo i dont think this theory holds up . How about oil airation ? Not a complete drop in pressure but enough to show up ? Are you doing high Gs under full power when the det is showing ? . I think if your engine was truly detonating that badly that its compressing the oil wedge it would run a bearing very quickly . Or deform the rod big end . Either way destroying the engine . An interesting discussion though .

@MobileLPG I just had a look at that screen shot and while the oil pressure trace is very erratic at the point you're seeing knock, I notice that it drops off in two other places in the log while you're at high rpm/load too with no knock present.

Out of interest what are you using for a trigger input to the ECU (I had a quick skim through the thread and couldn't see it mentioned)? The particular rpm trace where you have knock occur is quite erratic (as is the RPM ROC) which on an RB26 always makes me suspicious of trigger stability which in itself can cause knock with an otherwise safe and conservative timing map. It's not lost on me that other sections of acceleration appear much smoother but I'll ask the question anyway.

this is a very interesting issue, after giving it some thought I can only assume that there is an oiling issue with the engine possibly cavitation or aerating which is sending air or missing oil all together to the sensor, not sure if I am explaining this correctly for others to understand

Once again, thank to everyone for their input, it certainly is interesting.

Kickerzx, My logs certainly are useless for using the RPM rate to look for knock, I have posted a picture showing my RPM scatter using the standard RB26 CAS at a track day in Donnington and you can see just how bad it is.

H8LUZN, I am looking at theories put forward and then looking through the logs from my first trackday with the G4 (and therefore decent logging) and trying to see any evidence that supports one of the theories, certainly not assuming anything as my experience with finding knock is very limited, I have used my Phormula Pro to listen for knock in my R32 and I have a Phormula KS4 permanently mounted in both of my Skylines, the Neo powered R32 I have dialled in nicely but currently haven't had much time at all to work on the GTR hence still going over logs from early 2016.

Your point about crank reversal and the rate of oil pressure drop I agree with, I cannot imagine how the oil pressure could drop so rapidly from a moment of the crank 'stall' or whatever would be the best word to describe it.

Andre, Unfortunately yes, I was also suffering massive oil surge issues so there are drops in oil pressure related to high revs in low gear, braking, and heavy cornering that need attention....there is a definite trend but only seems to go erratic on the occasions when it showed knock.

The history of this car is that I have ran it on an Apexi Power FC for 5 years without issue but the data logging on the Power FC is very minimal even with the Datalogit software connected and the RPM scatter never showed itself, I fitted the Link G4 and was rushed to build a quick map before heading out on a track day (bad idea I know)

It was only after building a safe map for the track and then completing a track day that I then went over the logs and discovered how bad my oil surge was and the amount of RPM scatter, my high EGT made me suspect a timing issue and that is when I found the timing was retarded 15 degrees.

I fixed this by advancing the engine 15 degrees to get a genuine 20 degrees at the engine with the Link locked to 20 degrees, I then retarded my timing map by 15 degress to bring everything back in line, then accidentally loaded my old map and did one drive which caused the over advanced map and knock.

I was using the original RB26 CAS, I have now fitted a 24-1 trigger disc which has made a huge improvement and will be looking to fit a trigger system onto the crank, I have also changed the timing belt to a Gates Racing belt so this may also help, there is certainly a huge improvement when you look at the before and after pictures I have posted below.

It was definitely my error that caused the knock so I can deal with that in my new map, I also have flat timing in my table to avoid the timing jumping around too much (i.e at a set map row, I have timing from 5,000rpm all the way to 8,000rpm not changing as a temporary fix)

I understand the cause of the knock, I also believe it to be genuine knock as my error with loading the wrong map meant timing was advanced to 26-28 degress at .8-.9 bar...the only part I do not fully understand is why my oil pressure changes so drastically at each knock event

I beleive I can avoid it happening again but would like to be able to say "My oil pressure went erratic during knock because........."

Thanks again.

Below are my RPM with the original RB26 CAS, and now with a 24-1 trigger disc and new Gates Racing belt.

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