What Caused our Engine to Blow Up? - Additional Evidence

**UPDATE** In reviewing footage from ALL WEEKEND the engine was running AFRs in the high 13s and low 14s WOT. This is difficult to see in the footage, but this was likely the cause of the piston issues. That said, I don't know why it was running so lean. New injectors, ECU, and fueling setup is on its way!
New evidence:
Trying to figure out if its:
- Fuel Starve - Heat from Lean Condition
- Fuel Starve - Knocking from Lean Condition
- Detonation - Knocking due to Timing/Tune
- Damage due to Money Shift (2 x 7700 RPM)
- Ring Stress due to Oil Starve
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[@ReneES-em1ye]

I am still going with too much internal combustion heat due to a combination of lean mixture and late timing. (Exhaust leak giving false O2 readings and the loose pulley triggering the knock sensor to pull back the timing or timing retarded too much from an incorrect tune) If the piston damage was caused by detonation then chunks of the piston probably would have been dislodged all at once then left evidence of the bits being hammered between the piston and the head. I think the pistons were progressively burnt away in short order. The molten metal from the burning piston could be the spots on the cylinder head. I believe the dark spots on the head in each cylinder indicates high heat. Any valve, valve seat, or valve guide damage could be from the excessive heat in the combustion chamber and head surface This heat could have been transferring into the intake valve seat and the intake valve itself over-riding the normal cooling affect of intake air/fuel flow and and cooling affect when it closes and contacts a cooler seat. The intake valve stem could have even increased in diameter due to over-heating and started to seize in the guide.

[@malcolmaustin2197]

I don’t see any signs of progressive failure, the broken pistons still look clean and jagged not melted. That usually indicates a sudden failure / overload. I think piston 3 really solidifies this, that break in the ring land is “clean”.

[@malcolmaustin2197]

Pre Ignition ( not Detonation ) damage, detonation occurs after the plug ignites the air and fuel. Pre Ignition happens as the piston is traveling upward and causes failure like you have experienced. That explains why you don't have damage to the rod bearings.

[@ruggedbadgerracing]

What do you think would be the cause of that

[@malcolmaustin2197]

@@ruggedbadgerracing its usually a hot spot in the combustion chamber, sometimes it could be a cracked spark plug, something with a sharp edge that can heat up to the point where it ignites the fuel mixture before the compression stroke is finished ( like WAAAY before ). Things like loose burning carbon can cause it as well as oil in the combustion chamber in the right amounts, a burnt/ burning valve, a rouge spark plug or the wrong spark plug. Maybe a injector gave up, things went way lean and EGT went too high? Could also be ignition timing too far advanced but I doubt it in your case. Lots of ways to speculate but its going to be difficult to know for sure.

[@stevec5280]

I'm still leaning towards the intermittent fuel starvation as a root cause. I think it's fantastic that y'all are tearing it down and really trying to dissect for a failure too. I enjoy the quest for answers and general knowledge. But y'all know it was starving for fuel at times. Fix the obvious problem and gather more data. DEFINITELY recommend getting a wide band and maybe EGT sensors. That way y'all can do a test day and gather data.

[@ruggedbadgerracing]

@@stevec5280 We have a wideband and I will try to get that connected to the AiM for logging. We ordered a new surge tank, pumps, plumbing a new fuel rail and we will move to return style fuel system. We also have a new conservative tuned ECU on its way from Ecotec Miata, and we will get the injectors reflowed. I will also make sure we connect the knock sensor and we will try to log that as well. We bought some plugs to block off the oiling system so we can use the OE filter on the street to see how that performs vs. the remote filter and accusump that in currently installed. That said, I am actively trying to get feedback and dial in on the exact issue because I really don't want to blow another race.

[@stevec5280]

@@ruggedbadgerracing cool beans. Sounds like all steps in the right direction. What model Ecotec/ECU is the setup btw? The standard LE5/E67 ?

[@Paul-vc4zz]

Seems like more of a tune issue than oil starve to me. Usually you spin a bearing before anything with the pistons in a low oil P situation.

[@ruggedbadgerracing]

I appreciate the feedback. I do have a new ECU on its way. Fingers crossed!

[@florenbaron7111]

If it's a Ford than failure does not surprise me.

[@jeffwasilko]

Ecotec is GM.