: How in the ... theorize with me here
12-22-2011, 01:30 AM
Now far as I know/knew it's more common for a ringland to break and if a piston melts the common spot is near the center because it's generally where all the heat is.
To melt a piston on the ringland is really odd, and took some talent IMO.
My best guess, being I haven't pulled them out yet, is that the rings are ms-aligned. That would mean if the car was tuned on the rich side, which I know it was, fuel could have washed down the cylinder wall and caught in the ring gap. Once the chamber got hot enough it ignited and the cylinder temps soared, melting the piston.
There are zero signs of detonation, just lots of heat.
To my understanding this engine had been on the bottle for awhile, was sold, then the new owner put a PWSC W/big boost on it and now I have it.
For how low the miles are, it's in poor shape, just over 34,000 miles.
It'll go to a better place soon enough. [thumb]
12-22-2011, 02:06 AM
First there is no way to look at a piston and say there was no detontaion , 95 times out of 100 the piston will melt around the valve reliefs and out side of the piston first
A richer engine runs much cooler so less chance of piston failure issues
12-22-2011, 02:19 AM
what the hell is with the shape of the piston? (I mean the top) It just bugs me and makes me feel weird.
Come on now, I thought you'd know that... leaner engine burns hotter.
12-22-2011, 02:31 AM
Leaner post combustion will result in higher EGT's. But by 'rich' I'm saying that this may not have happened WOT where you'd expect to be rich. More of a situation where the piston would be subjected to more burn time and be rich, such as decel after WOT, where timing is backed off and the combustion process is given more burn time. Then if there where trapped fuel the EGT's would be much higher than they should be for longer than they should be.
Remember, time is a factor here and that time hinges on how long the burn could go on for.
Not like any of it really matters though. It's toast and going on the wall of shame.
12-22-2011, 08:08 AM
I have a couple of pistons just like that........ too lean and /or detonation.
12-23-2011, 06:31 PM
Mine did the same thing. This is what mine looked like in May:
12-23-2011, 06:51 PM
I still cringe every time I see that ^^
12-23-2011, 08:08 PM
Same ^^ here:/ was it cylinder #3? It happened n/a also
12-23-2011, 09:39 PM
Cylinder #4 actually. Mine wasn't n/a I turboed it and was misled about the tune and I didn't even make it around the 2 mile circle of my neighborhood and we pushed it up my driveway lol
12-24-2011, 01:13 PM
My experience is that detonation damage is generally evident before there is catastrophic failure. If you went from A-okay to boom in a few seconds its likely pre-igition damage. Detonation doesn't always result in damage. Pre-ignition almost always will. My theory with your motor and others with melted crowns is that a piece of embered carbon lit the mixture way too btdc. The hotspot itself was likely the result of poor tuning, junk fuel, driving habits, etc. So a situation conducive to detonation actually resulted in pre-ignition damage.
12-24-2011, 05:36 PM
12-24-2011, 09:10 PM
I was taught (by a GM Field engineer) that if the prominent scratches on the piston lines up with the head bolts there was long term low grade detonation. The detonation causes the piston to expand slightly from higher than normal pressure. The cylinder walls can expand also, except where the head bolts are, those areas resist the expansion, resulting in the scratches.
12-24-2011, 09:29 PM
I can believe that.
But in this case, and with the other pistons of this engine and the other pistons I had, the scratches on the skirts are due to the extreme rod angle and piston wobbling in the bore. All the pistons look near the same as did the other ones I had. Just an inherent design flaw IMO, and why the skirts where coated to reduce galling.
12-24-2011, 11:51 PM
The lines are from the trash washing around the pistons when the damage is done
12-25-2011, 01:24 PM
I removed my head yesterday !!
12-25-2011, 09:47 PM
^^lol yay for melting pistons
12-31-2011, 06:49 PM
ooo can I join the party!
01-03-2012, 02:06 AM
FYI - "galling" of the piston skirts leaves residual aluminum which will travel to the ring lands and eventually seize the rings in the ring lands. There are many things that can cause and there are many things to reduce galling. Quality parts and quality machinist are the first things one needs.
Seizing the rings in the land compounds every problem the engine has until you break the ring land and typically it's time to pull the engine apart.