I took a closer look at the pistons after disassembly in my earlier engine rebuild part 1 post.

After a quick session with the brake cleaner on the piston domes there are indeed signs of detonation:

I wrote there was no sign of knock in my previous rebuild post but the pistons was simply so full of soot and carbon buildup that I missed it.  Piston #3 and #4 have the same eroded markings on the edge of intake side of the piston.
#2. was fine.

According to "MSI Motor International GmbH's" excellent information on piston damage this is a sure sign of light detonation (page 44)

Looking at the possible causes above:

  • Octane. I have been running Statoil 98 or Shell 99 on track. At the Nürburgring Aral ultimate 102 super plus. But I have tuned it on 95 to get some margin for error. All numbers are RON.
  • Diesel in the fuel ? No..
  • Oil in the combustion chamber. Absolutely. I noticed early on that there were oil seeping out the breather connection on the valve cover and later also around the VVT solenoid.  Also the intake elbow was very messy. 
  • 9.7 Compression ratio. No buildup that could have altered the compression.
  • Timing at WOT on top was 20 deg on 95 RON 8.5 psi. No knock on trackdays here in Norway with ambient 20C.  The temp compensation table in the ECU had stock values so would pull more timing with higher ambient temps.
  • AFR was on the rich side. Aimed for 11.5-12 but saw 11.2 on top. DW65C and ID1000. 
  •  IAT was 40C over ambient after 10 laps on the track. Not very good, but that is the values I tuned for.
The Oil in the combustion chamber scenario kind of explains it all. At least for me.
I never once thought it was the oil level that was wrong since I used the L5 dipstick that came with the engine. In hindsight all the symptoms with excessive crank case pressure, oil in the intake, leaks from the valve cover etc. was there from day one. Face-palm.  The only thing I haven't figured out is the white spot on the pistons.  They are the same on all of them.  No sign of any detonation or eroding in the spots.  That could have happened on the way home from the track when there was 0 compression on #1 and extreme blowby.

Some pictures of the failed ring land. It looks like it has been moving a lot: