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Specks on plug

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Hi guys,

First time I get this and I have to admit, it is bugging the crap out of me. So I always tune with my knock audio detection setup and I usually look at the plugs after a few power runs on the dyno, to confirm that the tune is ok. Doing this, I never got to see detonation specks on the porcelain part of the spark plug.

Now this one, I don't hear anything with my knock detection setup, but all 4 plugs have black spots on the porcelain. I tried 2-3 places to screw my knock sensor to see if I could pick up anything but I don't. I am using new plugs everytime too. The engine is a forged 4b11T. Running it with a stock ECU and oem knock sensor too. I do get a few counts of knock once every 3-4 pulls but like I said, I don't hear anything and the knock counts aren't repeatable. I pulled a few degrees and it's still doing the same thing.

Not sure what I should trust now. With all those spark plugs articles with the word ''pre-detonation'' in it on the internet, I'm starting to second guess this method of veryfing knock. Timing mark on the ground strap is where it should be also.

hello can you post up some clear pics

Regards Ross

Here it is. Now The yellow discoloration I think comes from a little bit of C16 that was left in the tank.

Attached Files

Doesn’t look like detonation to me in this pic, I’m thinking more of a fuel deposit of some kind. Do you think it’s a possibility?

I would agree with Ross, but we could be mistaken - you can buy cheap boroscopes that work with smart phones for the display, I'd be considering buying one and having a look at the piston tops.

Thanks for the help guys. I have looked with a camera the top of the pistons and number 2 doesn’t look good. Looks melted on one edge. The other ones are ok and I have even compression on all of them.

I don’t think it’s a fuel deposit since I usually don’t see any specks with the same type of fuel.

I got to tune another vehicule after this one and I can confirm my knock audio system works like day 1 also.

Now if I look at the whole story here, the car came for a retune because it was misfiring/cutting in high rpm and high boost situation. I found out that the fuel system wasn’t enough to support his rpm/boost levels and I have lowered everything to make it safe and driveable.

It was running on C16 previously but I’m wondering if the engine was missing fuel if it could have started to melt the piston before I got to tune it. If that’s the case, will it degrade over time even though it’s not knocking ? Or the process will simply stop since the pistons no longer sees extreme temps inside the combustion chamber ?

Ah, that explains a lot. You may be OK with a lower tune level, but the piston damage is a concern as it is now compromised. Personally, I'd wind it way down and start saving for a new set of pistons - any bore damage evident - may be very light scuffing or discolouration in that area of the bore.

If you can, I would also get the injectors flow tested, that damaged cylinder may have had one a little weaker and so leaned out more than the others.

Hey Gord,

Yes we have agreed on a rebuild soon. I would like to learn as much from this experience though.

So the spark plugs porcelain specks indicates something is wrong inside the cylinder. Usually detonation but in this case, a damaged combustion chamber.

If I get this right, this engine was lean for too long which started the process of melting the edges on piston number 2. Even with race fuel, running a turbocharged engine too lean can have dangerous effects.

Once the pistons have started to melt, retuning the engine will of course help but particles from the cc can be found on the spark plugs.

Would you agree with my statements ?

Thanks again.

Yes, by 'wind it way down' I mean if the owner can't rebuild it right now, but has to use it for, for example, his/her main commuting transportation if it's their only car, it is brought down to the minimum possible boost to reduce the loading on the damaged piston(s). When you do pull it down, also have a good look at the big end bearings.