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Knock limited engine - High boost low timing or vice versa?

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Hello everyone,

So even after watching a lot of webinars, practical standalone course and so on, I can not still decide what to go for.

I am mostly tuning audi I5 turbo engines but i think it would apply to everything else.

So here's a real example, a friend of mine had his car tuned to 1.8 bar of boost with 8 degrees of timing, the car pulled really good.

Considering that he likes to drive a lot under WOT, I have decided to lower the boost to 1.2 bars and add 5 degrees of ignition on full load/rpm (1.2 bars) and even more at cruising load, to contribute to his fuel economy and so on, which he said he already noticed.

The car drives maybe a bit worse at WOT, but as far as i know it does effect the EGT (unfortunately he doesn't have an EGT sensor), which is important for long pulls.

So he got a turbo rated at 450hp and supporting mods, obviously, we are very limited to knock as we run on RON95 fuel and the car itself weighs 1800kg's.

What approach to ignition and boost to chose?

I should mention that lambda on full load is 0.77.

Main question is :

When tuning an engine: Would you go for bigger boost and for 7-10 degrees or lower boost and 10-15 degrees at full load/rpm?

We don't have any dyno close to us, so we are tuning on the road.

Also i've noticed that his 5th cylinder is detonating a lot, while everything else is fine, so I decided to pull 2 degrees of timing for safety, can this (2 degrees) reflect negatively on the engine overall?

Thank you

From what I’ve learned lower boost and realistic timing is best.

Id like to think that lower boost puts you in a better compressor efficiency and in return have a less knock prone setup that’s also more consistent at variable temps.

I haven't tried both ways myself, but you might also consider some ECUs can detect knock and pull timing in an attempt to protect the engine. If you're already running very soft timing due to high boost, the knock feedback doesn't have much ability to help. It's pretty rare for an ECU to detect knock and reduce boost, partly because the boost solenoid / wastegate / turbocharger system responds very slowly compared to an ignition coil.

Regarding the 5th cylinder being prone to detonation, I would also look into richening up the mixture for that cylinder. Mechanically, look into the cooling system. If that cylinder is last in line for water flow, it may be wise to address that or compensate for it.

When running knock limiting fuel, I always run the lower boost level with more timing. On the dyno I have ran a car at 25psi boost with its max ign timing at that boost level before knock, and also at 27psi at max timing before knock and power output was identical, being the fuel was limiting the output. Why wait for higher boost level for the same power a lower boost level with more ignition timing can provide. It comes down to cylinder pressure anyways, not necessarily boost pressure.

Thanks guys on your opinions on this. It is reasonable to assume that higher ignition table and lower boost would be more beneficial for a driver that has a heavy foot and likes to drive 150+ mph?

Asking because this is exactly the case, and I can only imagine how his EGT was skyrocketing.

As I mentioned before, I've gone from 1.8 bar to 1.2 bar, but added 6 degrees of timing at high rpm, although I should mention that the car does not perform power-wise as it did at 1.8 bars, 100-200 km/h run got slower by nearly 2 secs.

I have been struggling to adjust my plex knock monitor, although I did install the sensor on the stock knock sensor position between 4-th and 5-th cylinder, probably I was catching a lot of noise, even though I did try different frequencies and filtering. If there are any plex experienced users, please give me a message, I am able to export the noise I'm hearing directly to you and would be happy to provide the graph I am seeing using trigger mode.

This time though, I will have a more serious approach to tuning the car, doing short runs by 500 rpm increments up to 7500 rpm.

if road tuning only, I would pull the plugs after a heavy, hard pull to your rev limit. Do this in a tall enough gear that is safe, and shut down immediately and pull plugs to take a look.

And what should i see here? If the plugs are same color?

Ir sanded due to detonation? if any present

I know you don't have dyno access which is a shame, however I'll mention what I normally see when tuning turbo engines on pump gas. Typically you'll find that the engine will show healthy increase in power as you raise the boost, albeit with a nominal drop in timing of perhaps 2 degrees per 20 kpa. You'll get to a point where you add more boost but the engine becomes very knock prone and you need to more aggressively retard timing to combat that. From here you'll tend to go around in circles adding boost, retarding timing, and making approximately the same power but with more heat and stress placed on the engine. I find that on our local 98 octane pump gas this point often (but not always) ends up being about 250 kpa (22 psi). Without a dyno I'd be looking for the point where your timing needs to fall off a cliff in order to control knock.

As to what is safer, this is a tricky question to answer and it needs a little context around it to really be able to answer properly. My personal preference is to run a little more boost and be a little more conservative on timing in order to build a buffer to the knock threshold. There's levels to this though and if your timing is severely retarded then you're going to be creating huge EGT which can in turn create other unexpected issues. For the most part I'd suggest a threshold of 2-3 degrees to the knock threshold. If you have a customer with an especially heavy foot then I'd suggest using some other safety strategies such as a speed or gear based ignition trim to pull a little more timing when the engine is in top gear and being hammered. Just be aware that retarding the timing creates more heat energy to drive the turbo which often ends up increasing the boost!

thanks Andre for such a detailed reply,

Would you call "too retarded" anything below 10 degrees? I found that the engine pulled really good at 9 degrees advance and 1.8 boost, unfortunately he doesn't have an EGT sensor to see if it becomes very hot, but it's to be installed very soon.

From your experience, is there a rule of thumb on pump gas and turbo engines which would be don't go below x number of degrees advance at full boost?

Thank you

Btw, here are the spark plugs after driving a month on 1.8 bars of boost and 1 week on 1.2 bars of boost.

Any opinions?

Thank you

1stcylinder on the left side.

Attached Files

Is that a 6 or an 8 heat range you're running? They don't look bad but it's hard to know without knowing the heat range.

What is the standard heat range for that engine? It's common to go one or two heat ranges colder (higher number is colder in NGK plugs) on a modified (increased boost) engine.

Observation is that they look very clean but that could be because of freeway driving rather than lean mixture on boost. Reading plugs is a mug's game to be honest. Too many variables.

Stock would be NGK7, and currently he's got NGK8 installed.

Stock 230hp, now approx 400.

Ah yes, can see clearly on the second from left. What mixture are you seeing WOT? I wouldn't go any leaner than lambda 0.8.

Regarding your timing, I'd say you're about at the point of diminishing returns that Andre talked about. I don't see an issue running 9 degrees advance but I wouldn't want to pull any more out.

What are your inlet charge temps looking like? People often forget about corrections, for example I usually pull about 0.25 degrees of timing per degree C of charge temp over about 40 degrees C.

Yes, intake temp correction is a good idea indeed, this might help a lot with long pulls in the 5th gear.

Currently we see about 15-20 degrees celsius outside, and his intake temps are at about 25-30 degrees.

We run 0.76 lambda on WOT.

Those plugs are really dirty, as far as trying to get a good read. I would install fresh plugs and some Wot pulls and shut down asap. Don't drive home on them. Pull over and pull them. The #3 cylinder looks suspect with oil on the base ring of the plug. This is not 100% foolproof, but you would look for stuff that doesn't look normal on a plug. The porcelin starting to get shiny vs. matte. Small flecks of black or silver. You would need an otoscope to look at the edges of the ground strap and electrode to make sure they still have their sharp edges. From there you can get an idea. Being you don't have access to a dyno, this is not a perfect science and will take a little more time. Better safe than sorry. But check that #3 cylinder also.

There are some special stands to check the spark plugs on their performance? Would you advise to do this?


@h24, also the surface of the electrode is very rough, like tarmac pavement, does it give any indications?

Thank you

I'm not familiar with the stands. But the reason I suggested using new plugs, is it looks like there is a lot of coating on them, like possibly from the fuel additives being used by your fuel stations maybe. So it is hard to get a good read. If you're at a point where you are questioning your ignition timing, get some new plugs in there and do a Wot pull and shut down immediately and pull plugs and take a look. Make sure there is no "pepper" spots, which can indicate some knock, that might be knocking carbon off the piston or domes and sticking to the spark plugs. I know it's not an exact science, so it takes a little time and trial and error if trying to do the plug read method. Also, not an exact science, but if you see the heat "ring" on the ground strap somewhere in the middle of the ground strap, You should be in the neighborhood. There are several "opinions" on reading plugs online. So you can do some research online. But since I have a dyno, I don't rely on plug reading as much to tune ignition timing, but there was a time I didn't have the dyno, so you do the best you can. I still pull plugs when doing pump gas turbo vehicles, when I get close to what I think the boost/ign timing threshold might be on pump gas.

Vlad4Md, Here is a link of many, that can give you some direction. But don't want to sound like a broken record, this is not an exact science, just some form of data so that you can make an educated decision on what is happening with your situation. Also, inspect the #3 cylinder, as is looks like you're getting oil deposits on the base circle of the plug....


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