924Board.org

[-nick]

so... I opened up the engine over the weekend and found piston #1 has about a quarter of the crown *missing* (I can look down and see the top compression ring) and piston #4 has the entire crown missing (although the top ring is still covered by piston) and looks like it's been beaten with a hammer. Pistons #2 & #3 look just fine even though it was cylinder #2 that had the headgasket break that made me open the engine up in the first place (I should have kept it sealed up!).

Oh yeah, that hurts

For those of you who are unfamiliar- here is a pic of the JE pistons I've got:



The head looks ~ok. The valves all look fine (open/close fully and easily), but the chamber around #4 is beaten up a good bit. I'll have to get some advice from a machine shop as to whether it will be salvageable (I'm guessing it isn't that bad).

The cylinder walls also look surprisingly good. I don't know how they managed to make it through unscathed.

Where the pieces of piston are- I have no idea?! That's scary. The valve stems don't look beaten up at all. I haven't pulled the turbo completely out, but it spins smoothly. There is at least a little molten aluminum build up from the piston around the combustion chamber in the head of cylinder #1.

Boy, and I thought I spent a stupid amount of money before... If pistons #2 & #3 are truly salvageable then I suppose I should just go ahead and order up two new ones from JE rather than find a new set (might as well try to save a couple pesos if I can). Otherwise, I think I'll seach out Racing's flat-top Mahle pistons if they're available and I think might go ahead and convert to EFI & electronic ignition while I'm at it (with a knock sensor!!!). Why stop now right?!

Just when I thought I was almost finished, ha ha ha...

-nick[/img]

[Smoothie]

Ouch! That sure sucks 'n blows.
Do you think it could've developed a hot spot and then slowly melted away at that spot? -Or could a chunk have broken off and bounced around in the cylinder until it melted away?
I don't think any sizable chunk could have been blown out the exhaust without damaging the turbo - unless it by chance happened while the wastegate was open and it took that route.
_________________
"..it's made in Germany. You know the Germans always make good stuff."


'82 924T, US version, dark green metallic, 5 speed Audi 016G gearbox

[numbbers]

Well, you must have had one hell-of-a lot of detonation.
_________________
1980 924 Turbo

[jpab924]

An article I just read....a few things that I didn`t realize that could cause detonation...maybe, just maybe? Read on...I think you`ll pick up on the maybe I`m talking about. *

Causes of Detonation
Trying to isolate a detonation problem in a highly modified or even slightly modified engine requires you to determine if the combustion problem is being caused by a change you made to the engine, is inherent to a design flaw, or is derived from an external cause such as carbon deposits or poor fuel. Obviously the area with the largest variable is the fuel quality. Octane determines a fuel's ability to resist auto-ignition, or in lay terms, its ability to wait for a spark to initiate a flame. For our purposes we will explore knock independently of fuel octane rating.

When knock occurs, there is uncontrolled combustion with multiple flame fronts, with only one of them being initiated by the spark plug. During knock, cylinder pressures experience an extremely rapid rise and spontaneous burning and release of the end gases' energy. It is not uncommon to see a pressure rise of up to 8 bars per degree of crankshaft angle, or up to 50,000 bars per second. Anything that will either increase heat or retain heat in the combustion chamber will promote auto-ignition.

Common problem areas are the *sharp edges* of metal either on the piston or in the combustion chamber. For instance, if the piston has a valve relief cut into it, there is usually a very defined edge that the cutting tool leaves. This sharp edge is greatly prone to super heating and will actually retain enough heat that it will start to glow. If the fuel should hit this glowing edge either prior to the lighting of the spark plug or even after ignition, it is very likely that another flame front will initiate. If this unintended ignition occurs very early in the compression stroke, then the piston will be forced up against the increased pressure of the burning gas and will result in a form of abnormal combustion referred to as preignition. When this happens, the end result, if severe enough, is that the connecting rods bend. Turbocharged engines are specifically prone this. Carbon deposits that build up in the combustion chamber or on the piston top will have the same effect, along with the decreased volume at TDC, which in turn raises the compression ratio.

As stated earlier, anything that will raise temperatures in the cylinder will help promote detonation. This rise in temperature could occur many different ways: very lean air/fuel ratios, heating of the charge air prior to entering the combustion chamber, compression of the air molecules from forced induction, or poor heat dissipation through the cooling system. Increase of cylinder pressure will also prompt detonation. Camshaft profile, compression ratio and quench area all come into play. Design criteria as to combustion chamber shape and mixture motion, spark plug location and piston design can all make an engine more or less inherently prone to abnormal combustion. Naturally, spark timing and fuel quality round off the list.

btw, sorry to hear about that...That just plain sucks.[/b]

[jpab924]

Asides form the sharp edges leading up to detonation, I would try to figure out if maybe a lean mixture in those 2 cyls led up to this catastrophic event.

I`d really cover the bases before shelling out the bucks for some more pistons. Although if you did go with the flat tops, that would eliminate any sharp edges. Let me know what you find here. I would be interested in a set myself.[/code]

[wdb]

I'm thinking the rim on the pistons is a poor design , too thin . prone to over heating ,whether or not the AFR was right . if it melted down you would have globs of molten aluminum being batted about the comb. cham. , breaking up into smaller pieces . then when the engine cooled ,the now hard pieces beating up the cylinder ,before melting again . eventually they would be small enough to get out of the exh. valve . all depending on how long of a drive you were on at the time/each time . dam near every 924 here has run lean at 1 time or another , and didnt cause a catastrophic failure like this . I'm think you should buy different pistons , this is twice you had to rebuild because of those pistons failing . who else is running those pistons, anyone else have problems .

[924RACR]

I'm running those pistons in my race motor... not exactly the same, but JE pistons for the NA motor. They're excellent pistons, excellent quality, freakin' works of art. My motor is VERY strong, ITS 944's and RX7's can't even hardly pull away from me.

But I think this is getting back to what Racing was talking about earlier, with the flat-top VW pistons working better under higher boost conditions in the 931.
_________________
Vaughan Scott
Webmeister
'79 924 #77 SCCA H Prod racecar
'82 931 Plat. Silver
#25 Hidari Firefly P2 sports prototype

[emoore924]

Bummer

Maybe I missed it but can you post some "after" pics. I didn't see it in the shots above and I'd really like to see what the damage looked like up close -- especially because they were JE's. They are NOT known for this kind of failure and I'd like to know what happened.

I would think JE probably should be interested too. Mebbe they would want to have their metallurgist or engineer look as well. Now while this was most likely caused by a lean-induced detonation condition, there is the possibility that failure could be attributed to a defect in the billet or something that was not found during the manufacturing or q/a process. If that is the case, JE might be receptive to offering something to keep you as a customer.

Also, I have an acquaintance that is a metallurgist. He studies failures like this and offers opinions on root cause for the break. PM if you want his contact information and I can send it along.

One last thing: I call DIBBS on the broken piston(s). Don't throw them away. If you don't keep them as a topic of conversation, I'd be interested

[-nick]

Hey folks,
Thanks for the condolences! I wasn't planning on driving it much during the winter anyway, and it's only money right?

I'll try to take some pics of the damage this evening. Here's a little more info to chew on-

If anything, I think the cylinders were running a bit rich. The plugs had a little bit of soot on them (they were also pretty even in their appearance) and I somtimes had problems of having the idle hunt around a lot (running way rich). So going lean seems somewhat unlikely (although not out of the question).

Piston #1 looks fine, except for the chunk out of the rim. ie, it's not beat to death from detonation like piston #4. The chunk out of #1 also looks like a pretty clean crack/break. Much different than #4 which looks like the rim just melted down. I still can't believe how good the cylinder walls look- nor can I believe how well it was running. If the headgasket hadn't blown on the #2 piston I would still be driving this sucker!

Thinking about it... JE's design does seem strange. That rim really seems like a hot spot and it's structurally a pretty weak spot.

For the price of Racing's VW pistons (check out my other thread), I think I'm done with JE's design.

emoore- you're welcome to the remains! I still have yet to pull them out of the cylinders. I just have the head off at the moment.

-nick

[emoore924]

Broken parts! Woohoo!! Ping me offline and I'll make arrangements for shipping

[-nick]

Here are some pics-



There are a few more here:
http://www.cardomain.com/memberpage/435581/8

You can see the piece of piston crown that is gone in piston #1 and the beating that #4 took. #3 looks pretty good physically, it has a layer of baked on carbon. In a few places where I flaked the carbon off I see that the color of #3 has turned gold-ish. I don't know what that's about? Piston #2 has a few dents around the top of the piston crown- that is obviously where the detonation was happening.

-nick

[Vince Ponz]

For the chips check the fuel filter and the oil pan.
_________________
"Never let them see you sweat"
77.5 924 modified track car
79 931 Euro stock
88 924S SE
87 911 Targa stock

[Smoothie]

Holy smokes Vince - 3 of your most recent posts are showing on my 'puter as being entered at 9:24, 9:31 and 9:28... How'd you do that? -and why? (Hmm, wonder what Vince's up to this time..)
_________________
"..it's made in Germany. You know the Germans always make good stuff."


'82 924T, US version, dark green metallic, 5 speed Audi 016G gearbox