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If the two cylinders 1+5 were in the combustion cycle when you went into overboost they may just by chance be the ones which took the brunt of the incident. I'm not sure what the firing order of the S2 is but that's my 2p worth. equally, It could have been the others. just sheer chance.
Faulty knock sensor?
David
If I did not misunderstand, Mark and Mihnea sayed the knocking was extensively audible up to over 5000 rpm, so it certainly had the time to happen on many cycles for each cylinder. The fact only cyl.1 and 5 showed melted edges still makes me think those cylinders could run slightly leaner then the others.
Marco
P.S. With my actual setup, when not using 98 Ron fuel (which i try to avoid) I sometimes hear some slight knock when flooring from 3K to 5K rpm. I also am wondering how come the ECU doesn't react on this....
If the two cylinders 1+5 were in the combustion cycle when you went into overboost they may just by chance be the ones which took the brunt of the incident. I'm not sure what the firing order of the S2 is
[COLOR=Red]1-3-4-5-2 Unlikely to be due to the firing order.[COLOR]
Faulty knock sensor?
Cylinders 1 and 5 are at opposite ends of the block, they both use different knock sensors.
David
It could be related to fueling or advance on 1 and 5. Hopefully Mark's modification to the pistons will solve the issue.
sorry to hear from your problem mark. And I reckon this is a good solution to the problem. I compared your pics from the piston with mine. And what I noticed was that they are both JE pistons for our cars but the are in some points different. The edge that burnt from your piston is in my piston 'bout 1 mm thick?
Cheers
Jeffer,
I just measured the last one that I havent altered yet and that area is approx 0.75mm half way down that edge and .055mm at the very edge. The difference may be because we specified the valve pockets to be made a little deeper (0.5mm I think) so more material would have been machined away.
If I did not misunderstand, Mark and Mihnea sayed the knocking was extensively audible up to over 5000 rpm, so it certainly had the time to happen on many cycles for each cylinder. The fact only cyl.1 and 5 showed melted edges still makes me think those cylinders could run slightly leaner then the others.
Marco
P.S. With my actual setup, when not using 98 Ron fuel (which i try to avoid) I sometimes hear some slight knock when flooring from 3K to 5K rpm. I also am wondering how come the ECU doesn't react on this....
Marco,
I did not hear knock, but I was not really listening for it, as I was too busy trying to keep the thing on the road and in a straight line . What I remember about the overboosting is just that the car pulled hard accelarating through fourth and that the clutch slipped and let go at around 80mph, and I of course backed off as the motor redlined. It all happened very quickly and did not last long.
I suppose the most obvious culprit is the inlet manifold. As I recall from Mark's pics, runners 1&5 are different than the others. Why the AAN manifold is designed that way is a mystery. I guess it is an attempt to increase volume or improve torque for the lardy S4/S6. Perhaps those runners are more efficient contributing to a leaner mixture (more airflow for those cyls). Unfortunately, in conjunction with the piston design this has encouraged the particular conditions necessary for the problem. I would guess that it is pre-ignition that is responsible for the severity of the damage.
If we use reverse logic and say that all cylinders have the same design of combustion chamber, pistons, ign timing and fuelling, that only leaves airflow as a variable.
As for why the "Knock" sensors didn't pick it up. Aren't they "tuned" to a certain frequency?
Would certaintly be useful to know definitively what the airflow distribution of the various manifolds are. Not sure if a flow bench could determine that. I guess that it would be impossible to duplicate boost conditions but flowing them might give as a basis for comparsion. In particular it might highlight discrepancies between certain cylinders. Whether "corrections" are possible in terms of fueling or ignition, I'll leave that to the experts.
Interestingly Jogi has a very sophisticated management system that can "Trim" individual cylinders for both those. Unfortunately it doesn't seem to have done him much use! Nevertheless the potential is there. I think Mihnea raised a good point. Without the ability to measure cylinders individually, its difficult to tell whats occuring.
The knock sensors are working fine, the problem is that the ECU takes a while to pick up wild knocking (though I keep on saying it was ping(k)ing, not knocking)...
There is no way to adjust the mixture specifically on certain cylinders, it has to be adjusted on all of them. I can do a chip for Mark with more overall fuelling, which should make it avoid this, although I'm totally positive about the fact that after we solved the overboosting issue, I've mapped it so the engine never knocks, not even a single time and the ECU doesn't pull back the timing a single degree on even 1 or 2 cylinders.
I think this has only happened because of a possible uneven airflow distribution on cylinders #1 and 5 and excessive pinging (audible) when the engine was overboosting. now it should be ok, especially with those sharp edges ironed out (good job Mark!!!)
Cheers,
Mihnea
'98 Silver Grey 2.7T S4, dual 2.75" turbo back exhaust with no cats, FMIC, BIG injectors, 10 Bar motorsport fuel pump, modded RS4 turbos, tubular exhaust manifolds, custom intakes, RS4 MAF, Bilstein PSS9, H&R ARBs, 19" BBS CH, custom remap, 511 BHP/505ft-lbs
It could be related to fueling or advance on 1 and 5. Hopefully Mark's modification to the pistons will solve the issue.
@ steve, thanks for the firing order info, I've never had reason to check it. That would totally rule out the theory.
Rover in the 1950/1960's experimented with a 5 cylinder engine. this was basically a 6 cylinder unit shortened by one cylinder. They had problems with lean running on some cylinders ( 2 x carbs shared by 5 cylinders) and also problems balancing the custom crankshaft.
Yeah but what caused the noise in the engine that forced Mark to open the engine again ?
I can buy the theory about the thin edges not being butch enough for the accidental overboost and so 'pinked' themselves into a right old mess - but why the noisy after effect - has some metal got into a nasty place elsewhere - unlikely as the bores are undamaged ?
Just confused is all !
Hope the noise miraculously disappears Mark !
Paul
Originally posted by Pisobiker
Looks excellent Mark, as your jobs always do
They will work perfectly.
I don't think you should be worried about anything else. The "pinking" ( ) was certainly caused by the sudden increase in boost you experienced. The only reason why your pistons got damaged was because of the very sharp edge (which wasn't supposed to be there ) which didn't have the chance to waiste the sudden increase in heat, due to the very small conduction area compared to its external surface. I would check carefully the head gasket, piston crowns, valves and spark plugs for any sign of knock which will give you an idea of how bad it must be before making any damage. This is not meant to say knock is acceptable (in any form), but just that if for any unpredictable reason knock occurs it should take some time before harming the engine (and make you strip down the whole engine).
Anyway, as a worning sign, since the problem occurred only on cylinders 1 and 5 it probably means those are running leaner compared to the others. If possible, then (Mihnea?) I would slightly enrichen injector's duty cycle on them, while, if overall EGT seems too conservative, slightly leaning on cylinders 2-3-4.
maybe when cold, but what happens to the pistons and bores when they heat up?? They dont stay the same size then?
The specs are for cold (well 70F) besides when they get hot they expand and that would lessen slap, wouldn't it?
It was fine and made no sounds the previous night, before and after we mapped it, but the next morning I started it and it sounded like a diesel when I revved it or when it was under load, probably drove it 15 to 20 miles or so, over the next few days trying to suss out what the noise was before giving up and stripping it down and good job I did! I did a comp test right after the last drive and that was fine too.
As I said to Steve today on the last drive the motor was hunting badly at idle, I dont know why. I haven't got around to checking the head, cams and valves yet, but other than the burnt pistons, the engine parts and clearances are fine???
I'll double check the bore/piston clearances again before I put it back together and plastigauge the bearings.....pointless really though because I know they are ok.
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. What I remember about the overboosting is just that the car pulled hard accelarating through fourth and that the clutch slipped and let go at around 80mph, and I of course backed off as the motor redlined. It all happened very quickly and did not last long.
) was certainly caused by the sudden increase in boost you experienced. The only reason why your pistons got damaged was because of the very sharp edge (which wasn't supposed to be there 
) which didn't have the chance to waiste the sudden increase in heat, due to the very small conduction area compared to its external surface. I would check carefully the head gasket, piston crowns, valves and spark plugs for any sign of knock which will give you an idea of how bad it must be before making any damage. This is not meant to say knock is acceptable (in any form), but just that if for any unpredictable reason knock occurs it should take some time before harming the engine (and make you strip down the whole engine).
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