Potential Piston Problem

Potential Piston Problem

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Discussion

the_stoat

Original Poster:

504 posts

210 months

Saturday 21st March 2015
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Has anybody seen this on a piston before and what is the cause? It seems to be pitted and as if it has corroded. Think my rebuild budget is about to take another hit. For information they are Accralite forged pistons in a C20XE engined Westfield.




anonymous-user

53 months

Saturday 21st March 2015
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One two three, all together now "DETONATION"......... ;-)

the_stoat

Original Poster:

504 posts

210 months

Saturday 21st March 2015
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Max_Torque said:
One two three, all together now "DETONATION"......... ;-)
Thanks for the confirmation. Really annoyed at the amount of cash handed to an 'expert' for mapping. Had the car checked somewhere else and they did comment it was rather lean and required adjustment. Never mind this will just take me to 200% of expected budget!

stevieturbo

17,229 posts

246 months

Saturday 21st March 2015
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Looks like the piston has stood well though

Pumaracing

2,089 posts

206 months

Saturday 21st March 2015
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Max_Torque said:
One two three, all together now "DETONATION"......... ;-)
You don't think it needs stronger head bolts or the waterways moved somewhere else? Seems my expertise has finally rubbed off on you smile

the_stoat

Original Poster:

504 posts

210 months

Saturday 21st March 2015
quotequote all
stevieturbo said:
Looks like the piston has stood well though
That is what confused me as the engines I have seen with detonation before were in a much worse state.
Oh well I now have 4 interesting paper weights.

Pumaracing

2,089 posts

206 months

Saturday 21st March 2015
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the_stoat said:
Has anybody seen this on a piston before
Lol, nope. Never. Ok once or twice. Well several dozen times. I think it might be aluminium worm, otherwise known as detonation. If you get an infestation of those little buggers they eat the combustion chamber and piston crowns away. The easiest cure is a significant reduction of ignition advance or compression ratio.

the_stoat

Original Poster:

504 posts

210 months

Saturday 21st March 2015
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Pumaracing said:
Lol, nope. Never. Ok once or twice. Well several dozen times. I think it might be aluminium worm, otherwise known as detonation. If you get an infestation of those little buggers they eat the combustion chamber and piston crowns away. The easiest cure is a significant reduction of ignition advance or compression ratio.
smile The other cure is to get it set up and mapped by someone who actually knows what they are doing as opposed to claiming to know.

jontysafe

2,351 posts

177 months

Saturday 21st March 2015
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I had this aforementioned worm on my YB motor as well, it's an expensive bugger to sort. Took my VERY expensive cylinder hear with it as well as well as scoring the bore with melted piston parts and doing the big end in. I feel for you.

Was in a bloomin Westfield as well.

the_stoat

Original Poster:

504 posts

210 months

Saturday 21st March 2015
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jontysafe said:
I had this aforementioned worm on my YB motor as well, it's an expensive bugger to sort. Took my VERY expensive cylinder hear with it as well as well as scoring the bore with melted piston parts and doing the big end in. I feel for you.

Was in a bloomin Westfield as well.
Thanks, I was using a new block anyway and the head is OK so I was much luckier than you. A quick phone call and whimpering at another payment for the engine and it will all be back to a state where I can build the engine. Plan is to assemble the engine next weekend, get it run in then mapped in mid-April.

How is yours going?

jontysafe

2,351 posts

177 months

Saturday 21st March 2015
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I've decided to build what will probably be the most ridiculous engine put in a Westie and build some reliability into it as well. I haven't told my wife this yet though as anything to do with ybs now is hideously expensive. Going Borg Warner twin scroll on a custom manifold with fairly high comp engine. It's been spendy all along!

anonymous-user

53 months

Saturday 21st March 2015
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Pumaracing said:
Max_Torque said:
One two three, all together now "DETONATION"......... ;-)
You don't think it needs stronger head bolts or the waterways moved somewhere else? Seems my expertise has finally rubbed off on you smile
It kinda depends upon what the root cause of the detonation is now doesn't it...... ;-)

anonymous-user

53 months

Saturday 21st March 2015
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the_stoat said:
stevieturbo said:
Looks like the piston has stood well though
That is what confused me as the engines I have seen with detonation before were in a much worse state.
Oh well I now have 4 interesting paper weights.
Generally, what really kills pistons is actually pre-ignition, which is often a result of detonation!



(detonation is a sympathetic (ie not initiated by the spark plug) combustion of the air/fuel mixture in the endgas regions of the chamber. As the walls of the chamber are hot, and the flame front only reaches them towards the end of the burn, there is plenty of time (especially at low engine speeds) for the end gas region to become too hot / dense, and to self ignite. This results in a rapid local pressure rise and corresponding heat release, with the classic "tinkle tinkle" sound associated with detonation phenomena. In the mild cases (at low speeds, and where total chamber / piston thermal loads are low, the only real result, apart from the noise, is light pitting of the piston, generally in the most physically constrained locations, such as the top ring land, because the pressure wave and thermal energy end up being "trapped" in those locations.

If this detonation occurs at engine speeds and loads where the noise is not obvious the user, it can occur for a long time without too many issues/effects.

However, what happens is that eventually the damage to the piston becomes a hot sport, where the rough surface and thermal loads result in a sudden rise in local metal temps. Often, the extra thermal loads also result in a significant deformation of the piston profile, and that tends to lead to a lose of oil film thickness and direct metal to metal rubbing, often on the piston top land to bore area. once you get any rubbing, you get friction, and you get even more heating. This leads to a rapid chain reaction, and a runaway thermal event. At some point, the localised metal temps get so hot, that as soon as the air/fuel mixture touches that zone, it ignites. Now you are in BIG trouble, because this pre-ignition leads to an almost totally uncontrolled heat release, and massive piston damage, often to the point where the top ring land fails, often resulting in massive blowby and oil contamination (which also burns and helps to melt the piston)

At high speeds (>5000rpm) high load, you can go from light detonation to preignition (which you can't hear!) to a hole in the piston in around 5 seconds, and there will be little trace left of the early stage detonation that actually was the root cause!


For OE production engines, typically we would aim to limit the maximum knocking pressure to something like 10 bar at mid speeds, down to 3bar at high speeds. (At low speeds, we also typically use the 3bar limit to avoid audible detonation, even if that knocking pressure isn't enough to cause damage)

Engines like the old port injected inlet restricted 2.0 turbo's in WRC were massively knock limited, and i have in-cylinder traces showing cycle with 160bar peak cylinder pressure with another 40 bar of knocking pressure on top of that! It got to the point where we were "hot forming" the liners into tulip shapes at their top from all the det.......)


anonymous-user

53 months

Saturday 21st March 2015
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It's also worth noting that pre-ignition is the silent killer because of where it happens in the cycle.

Detonation can only occur late on, well after the crank angle at which ignition is normally initiated (because it is caused by the pressure wave from the normal heat release event). As such, at the point it occurs, typically around 10 to 50 deg ATDC) the chamber volume is increasing so that the piston is "removing" heat from the chamber, and cylinder pressure is falling. Because it occurs at small chamber volumes, it is a characteristically "sharp" high frequency event, with pressure oscillations around 7 to 10kHz, making the sound anomalous to normal engine frequencies and hence relatively obvious

Pre-ignition on the other hand, occurs almost immediately the fuel / air enters the chamber, and local hot spots that cause it result in a relatively slow, but continuous heat release all the way through the compression stroke, ie BEFORE TDC. This is a really issue, because we are now releasing fuel energy way too early, as chamber volume is DECREASIMG, and so there is no way for the piston to remove this heat. As a result, the pressure and temperature goes SKY HIGH, and at TDC and min chamber volume, the resulting temp is enough to simply vapourise the metal of the chamnber / piston etc! And as the heat release is smooth and gentle, it's not audible. Without crank angle synchronous in-cylinder pressure measurement it's effectively invisible other than the corresponding loss of flywheel torque as it occurs.

(remember at all times, PV = nRT, ie for a "closed" system, ignoring heat lost to the chamber walls etc, the pressure and temperature in the cylinder are proportional to it's volume.)

Pumaracing

2,089 posts

206 months

Sunday 22nd March 2015
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the_stoat said:
Thanks for the confirmation. Really annoyed at the amount of cash handed to an 'expert' for mapping. Had the car checked somewhere else and they did comment it was rather lean and required adjustment. Never mind this will just take me to 200% of expected budget!
The A/F mixture is not your problem. Or at least not the main one. Detonation is nearly always caused by excessive ignition advance, assuming the CR is not bonkers mad for the fuel octane being used. The problem with so many mapping "experts" is that although A/F ratio has well known ideal parameters the ignition advance curve is a huge variable for different types of engine build and it's harder to spot anomalies in the map. A/F is easy, at least theoretically. 12.7 for best power on pretty much any normally aspirated engine. However just a few degrees too much advance at peak torque and your wallet takes a severe hit. That ideal advance differs so much from as little as low 20s degrees on a high comp 4v engine to mid 30s or even more on a low comp 2v with a slow burning chamber (think Ford Crossflow for an example) that your average rolling road tinkerer doesn't always know where to start for a safe setting.

We see so many threads on here where a well thought out spec and carefully built engine blows up because the idiot mapper detonates the thing to death. Turbo engines are even harder to set up as the boost retard might need to be such a huge proportion of the base advance.

If the second place you went to only commented on the A/F ratio then it might actually be they don't know their arse from their elbow either.

I'm assuming from the piston pic that this is a N/A engine or I'd expect a bowl in it. You probably don't need more than 22-24 degrees full advance at WOT and it would be safer to start mapping even lower than this and then build up to see how torque alters once fueling is close to correct. How about you tell us the engine spec, CR and the WOT ignition advance you actually have?

Pumaracing

2,089 posts

206 months

Sunday 22nd March 2015
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Max, that was a very nice and well written explification of detonation and pre-ignition. I'm sure many on here will find that helpful.

Sardonicus

18,928 posts

220 months

Sunday 22nd March 2015
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Pumaracing said:
Max, that was a very nice and well written explification of detonation and pre-ignition. I'm sure many on here will find that helpful.
+1 Thats probably the most comprehensive I have read on the subject anyway.

anonymous-user

53 months

Sunday 22nd March 2015
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Pumaracing said:
We see so many threads on here where a well thought out spec and carefully built engine blows up because the idiot mapper detonates the thing to death.
I hope you'll let me add the following, by way of some defense of the "idiot mapper".

Ignition angle optimisation is not difficult to achieve given the correct resources, but it is time consuming. OEM's with access to ALL the necessary kit, including near real time crank angle synchronouos in-cylinder pressure measurement (wave goodbye to approx £200k for one of these btw!) will run for literally thousands of hours of dyno time, and 10's of thousands of engineer hrs for data reduction and analysis before the production base engine calibration is released.


For "aftermarket" mappers, they are generally never given enough time (because both their time and using a rolling road /engine dyno is expensive). Added to which is the pressure to make as much performance as possible for their client.

With ignition angle, especially on engines that are heavily knock limited (which tbh, these days is most engines due to the ever rising static CR & increased ManVolEff from complex variable intake systems etc), more advance generally equals more power, so the "temptation" to keep winding in the ignition is strong, especially as you watch the dyno torque go up every time you do so!

Without access to accurate knock determination, be that from proper in-cylinder measurement of combustion pressure, or via a secondary sensing system (eg engine block vibration sensors("knock" sensors) etc) it's difficult to know "where to stop".......

I often see N/A engines with signs of light knock damage, just as the OP has, for a couple of reasons:

1) With significantly lower maximum cylinder charge density, N/A engines often detonate at a significantly lower intensity that Forced Induction ones. Hence, it's harder to hear or sense that early onset detonation amongst all the rest of the engines mechanical noise
2) Because you can't just "turn up the boost", the mapper will be forced to tread a much closer line to MBT/BLD ignition timing in order to maximise performance.


On turbo engines, what tends to kill them is not usually the nominal ignition angle, but failure to fully and correctly compensate for those "off nominal" events like high charge temps, or poor boost control, or insufficient injector headroom, and even barometric pressure effects.

the_stoat

Original Poster:

504 posts

210 months

Sunday 22nd March 2015
quotequote all
Max / Puma - Thank you very much for the information it has been great to understand the issue in more detail.

Puma the engine will never be run in the previous configuration as I was pulling it apart to ironically build for increased reliability. The previous spec had standard rods and was running hydraulic lifters, so the standard valve train. I always had the nagging doubt that the standard vale train was too close to the reliability ceiling for comfort, as on track it would regularly see up to 7500 RPM. The future spec is as follows and any advice on questions to ask to make sure I do not get to the same position again.

Engine is a Vauxhall C20XE, it is on ITBs and will be run in a Westfield. Cam spec is as follows:

Expressed Period: 294°
Inlet Fully Open: 106° ATDC
Inlet Lift @ TDC: 0.165”
Exhaust Fully Open: 104° BTDC
Exhaust Lift @ TDC: 0.125”
Lift: 0.458”

Probably obvious to you guys but will be on solid lifters, expected rev limit to be 8500 RPM.

Not sure on the compression ratio yet as will need to decide which pistons to buy, likely to be around 11.5/6:1 (as per cam manufacturers recomendation) but will work it out properly when I get the head back from the machine shop. For info the previous compression ratio for the damaged pistons was 11.1:1. It will be 2 compression ring pistons as opposed to slippers.

Again in my quest for reliability I have worked on keeping temps and pressures stable. It is dry sumped, running a water / oil heat exchanger and a decent radiator. On track oil temps are stable at 97 degrees and water at 93 degrees. On the road both figures are about 6-7 degrees less.

Annoyingly as the car is in bits I cannot power on the ECU to get the ignition data.

PeterBurgess

775 posts

145 months

Sunday 22nd March 2015
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Most of our work is with N/A with a few SC and even fewer turbo applications. With the inertia rollers we see the power drop when there is too much advance, even before we can detect detonation. We also get a unit full of smelly fumes when the engines detonate as the 'explosion' tends to clear crap off the pistons, burn the oil off the bores and blow by into the crankcase ( this crankcase pressure can be measured during detonation conditions). Even with 'posh' fuels we can go past peak power with too much advance and no detonation!

We get a lot of folk glued to threads who are convinced big advance means big power....load of cobblers, minimum advance to make the power is what is needed'

I agree with Dave baker, too many folk not doing the job right including those setting up by ear and reading threads to tell em what advance to run. We also see most engines detonation damaged from too much advance not weak fuel unless fuel fails on one cylinder and the other cylinders drag the poorly cylinder to death.

A lot of maps are available online these days such as the megajolt ones, they are getting closer and closer as a very good starting point with so many folk posting good maps. Safety is paramount and no excuse for dynos overadvancing.

Peter