Those of you saying lean/detonation....what are you seeing that is leading you to that conclusion?

I've very little experience assessing 4 stroke burn-downs....but I've seen 100's of two stroke burn downs. When it's lean/detonation I see two things; a "sand-blasted" piston top, and carbon covered piston top -not enough wash. Like this:



C409's piston has neither. What are you guys seeing?

 

Look at post #26, it is not as severe as the 2-stroke piston damage that you are showing but you can see the localized signs of detonation on the edge of the piston where the valve reliefs are cut in.

 

Here are a few pictures of a melted down engine that I got to rebuild last year. The piston is out of a 2013 dodge ram 2500 6.7L cummins.

To me it looked like the top ring land failed. I wonder if there is a design issue with those pistons. Not sure if you have contacted the piston manufacturer. There is no keystone land ring area on these forged pistons. The keystone increases the strength at the top ring land area. For increased cylinder pressures.

 

I don't know... I'm not really seeing it, but a bigger/higher res pic might help. And more experience w/4 stroke burn-downs too.

 

This is my educated guess, I have attached a picture that I edited where I see that the piston got hot and started to fleck from detonation. Who knows, I could be wrong.

 

My opinion is that it's rich and and over timed.

It's loading the ring land up with fuel while on the 2 step then as it comes off the 2 step and makes it way down the track it starts detonating, lifts the top ring land cracking the crown.

After the crown is compromised it's just a matter of time before it fails spectacularly..... The piston torched completely down the side was just run longer after the top land lifted and cracked the crown.... once the top ring was exposed it didn't take long at WOT for it to torch cut down the side of that piston.

My opinion is based on the fact that it lifted adjacent to the Intake valve pocket and the fact that you have a piston showing exactly what happened before it completely torched itself to death like the other piston.

This is a common failure mode for pistons in nitrous and forced induction engines, where people are way over fueled. It also happens in N/A engines but is much less common.

Many people will take once look at the worst looking torched piston and deduce that an overly lean condition caused the failure when the real problem is in fact the opposite.
Will

 

Will,
Why in your opinion does it start to detonate as it heads down the track? The OP stated that he was running 4 degrees less timing then he had used in the past. Is it in some way related to the two step, in you opinion? I agree that I have seen this in N2O engines and also agree that a N2O engine that is too rich at the application of the N2O can fail the pistons and rings. I have delayed the fuel application .01 seconds after the N2O application to prevent this problem with port fuel injection and N2O application. I have seen this damage before in N/A applications that are too lean or too much timing, not sure about too rich. That being said I am open to the possibility that the 2-step may be playing a roll in this failure, but why did the detonation start in the first place?

 

I would say that area of the piston is just loaded up with fuel comming of the brake. All that fuel can do crazy stuff trapped in a crevice area like behind a ring once it lights off under a load. The lower than normal timing may have made this worse... we all know what retarded timing does to cyl temp under load.
Will

 

..... When my engine guy first looked at the pics of the damage he said it was because the engine was too rich ! He re-iterated that when the pistons were on the bench ... speculated that the broken piece of the piston was likely glowing at WOT and induced the detonation and subsequent melt down ... He gave me points for doing this kind of damage Naturally aspirated ... said it usually took a good bit of spray to melt one like that ! .....

 

Copy. Thanks for the "re" pics and the text. I see what you are seeing now.

 

..... New JE Pistons arrived today .....

 

..... The arc of the flaking indicates to me that it may have started AFTER that section of the piston/ring land was raised up ... I can see where that sliver of separated aluminum would get hot enough to cause detonation ... but I don't see where detonation caused the sliver of metal to raise up ! .....

 

Knocking some compression out of it I see.

Going back to pump gas or staying with E85?

Will

 

..... Yeah , the compression should come in around 12:1 now ... I haven't run the numbers yet ... I'm gonna stick with the E85 for the time being ... 12:1 is a bit steep for pump gas and I still have a few cans of moonshine on hand ... I broke a bone in my foot a couple weeks ago and sprained the hell out of my ankle so its slow going on getting things sorted out for the rebuild .....

 

****...that sucks man! I had to wear that same exact boot a few years ago. Had a snow cat tiller fall on my foot...broke 4 metatarsals. It slows you down, for sure.