OK, before I give my opinions on what it is, I'm going to show you guys photos and I want what you think caused it. If you don't have experiance with this stuff, don't just post to post.

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Obviously a weak point of the piston because of the depth, and diameter of the valve relief, and it looks to have a small radius. No signs of butting.... Its hard not to say detonation, maybe combined with piston design. Pics of the plugs? Is this on corn?

 

This on E85. Street driven at 8 PSI with 23* of timing.

 

bad piston design for the application, valve relief too close to ring, relief cut too close to edge of piston. makes for a "tributary" on the crown with 3 sides getting heat. if it gets too hot a gas mixture can ignite in that corner. get a differnt brand/design piston, and make spark edges go away, same with the head. i have had both je and probe pistons hold up just fine in a 383w. i had a wastegate line come off on mine and went 21psi+ (megasquirt map only good for 21psi) afr's in the 14's and 1900 egt. the engine ran a little funny afterwards, turned out i blew the intake gasket, softened the rings and "peppered" a few pistons, they however did not come apart just left a few spots of detonation erosion on the surface. i replaced the je pistons for probe pistons, no problems with either.

 

I was blown away when I pulled my heads off. The thing ran perfect, I've never heard a hint of detonation in boost and I was very conservative with my timing, but not so much to create high EGTs.

Engine was rebuilt early last year by a mediocre shop, I think they made the ring gap too tight. There are nice shiny spots on the cylinder walls.

 

What was the top ring gapped at?

 

It's getting torn down this weekend.

 

Got the shortblock out and torn down.

Top rings were .020, 2nd ring was .024.

It's hard to tell what happened first. There are signs of detonation on the sides of the top ring lands near where they broke. So either they broke, then created hot spots in the chamber, or it started burning oil first, detonation, heat, ringlands butted then pistons broke.

I think the ring lands just butted and caused the pistons to break or created heat that caused the detonation. Either way, that gap was waaay to small for 700-800+ FWHP.

I saw some small shiny spots on the end of the rings. So yeah...

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Thanks for the follow up, I'd say the rings butted as well. Usually when the top of the ringland is damaged only that is the cause. Happened on one of my engines as well....

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I'm going with Wisco on my new pistons . Jeff recommends 6 thou times bore size on the top ring . .

mine will be .

24 top
26 second
high tension oil

 

I have Wiseco pistons in my engine (Traditional Pontiac) and have been very happy with them so far.

23 degrees of timing and 8 psi?

A friend runs a 4 cylinder Traditional Pontiac engine on E-85 at 25 psi and 29 degrees of timing and has not hurt his engine so I would say timing was not the issue.
Would agree more on the ring gap deal

JMT

Tom Vaught

 

Thanks Tom!

My new setup will have 32cc Wiseco's

 

my probes are .029 top and .032 second

 

What do the plugs look like? Only two ways to tear the top off like that, too tight of ring gap or the mors common is it was over fueled. Fat is not safe, fat puts excess fuel in the ring lands then goes of like a bomb when it gets to temp taking the top the ring land with it.

 

Hard to tell with it burning all the oil.

Afrs were Mid to low 11 on e85

 

thats more so on a nitrous setup, n/a or boosted you'd have to be pretty damn fat for anything like that to happen, especially at a lower boost setting.

 

I agree. On E85 I've been 9.5:1 at 34+lbs many times. (at 8* of timing as well) Never had an issue with fuel lighting off in ringlands.

When is the ringland area ever sealed enough to "go off like a bomb"

 

A very rich mixture on juice can do it. Can't see it happening without spray though, regardless of fuel.